Metadata for 0.11.3 release

File listing utils and example

.github/workflows/rust.yml

@@ -1,22 +1,22 @@
 name: Rust CI
 on:
   push:
-    branches: [ master ]
+    branches: [ main ]
   pull_request:
-    branches: [ master ]
+    branches: [ main ]
 jobs:
   build:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        rust: ["1.74.0", stable, beta, nightly]
+        rust: ["1.85.0", stable, beta, nightly]
     steps:
     - uses: actions/checkout@v2
 
     - uses: dtolnay/rust-toolchain@nightly
-      if: ${{ matrix.rust == '1.74.0' }}
+      if: ${{ matrix.rust == '1.85.0' }}
     - name: Generate Cargo.lock with minimal-version dependencies
-      if: ${{ matrix.rust == '1.74.0' }}
+      if: ${{ matrix.rust == '1.85.0' }}
       run: cargo -Zminimal-versions generate-lockfile
 
     - uses: dtolnay/rust-toolchain@v1
@@ -29,7 +29,7 @@ jobs:
     - name: build
       run: cargo build -v
     - name: test
-      if: ${{ matrix.rust != '1.74.0' }}
+      if: ${{ matrix.rust != '1.85.0' }}
       run: cargo test -v && cargo doc -v
 
   features:
@@ -45,6 +45,35 @@ jobs:
     - name: Cargo check
       run: cargo check --features "${{ matrix.feature }}" --no-default-features
 
+    - name: Cargo clippy
+      if: ${{ matrix.feature != '' }}
+      run: cargo clippy --no-deps --all-features --all-targets -- -D warnings
+
+  other_archs:
+    # github actions does not support 32-bit or big endian systems directly, but
+    # it does support QEMU. so we install qemu, then build and run the tests on
+    # emulated systems. NOTE: you can also use this approach to test locally.
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        arch: [powerpc-unknown-linux-gnu, i686-unknown-linux-gnu]
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install or use cached cross-rs/cross
+        uses: baptiste0928/cargo-install@v2
+        with:
+          crate: cross
+      - name: Cache Cargo Dependencies
+        uses: Swatinem/rust-cache@v2
+        with:
+          cache-on-failure: true
+          key: ${{ matrix.arch }}
+      - name: Start Docker (required for cross-rs)
+        run: sudo systemctl start docker
+      - name: Cross-Run Tests using QEMU
+        run: cross test --target ${{ matrix.arch }}
+
   rustfmt:
     runs-on: ubuntu-latest
     steps:

CHANGES.md

@@ -1,3 +1,151 @@
+# Version 0.11.3
+
+Additions:
+- Added support for `webp` compression (as per GDAL) under the `webp` feature flag (default: off).
+- Added `Decoder::image_ifd`, creating an `IfdDecoder` of the current IFD.
+- Added `IfdDecoder::directory`, returning the `Directory` underlying it.
+- Added `LowerHex` and `UpwerHex` implementations for `IfdPointer`.
+
+Notes:
+- A new example `tiff-ls` demonstrates use of the new features.
+
+# Version 0.11.2
+
+Changes:
+- Planar files can now be read with `Decoder::read_image_to_buffer` if the
+  limits allow for such a large buffer size. Previously only the first plane
+  was read and the decoder will attempt a fallback to this behavior if the
+  limits do not allow all planes to be allocated.
+
+Fixes:
+- An off-by-one error in a `debug_assert` of `ValueBuffer` caused its use in
+  `IfdDecoder::find_tag_buf` to panic outside release mode when the buffer
+  exactly matched the required size.
+
+Additions:
+- The methods `ColorType::{num_samples, bit_depth}` are now public. This
+  promises that we only have homogeneous color depths in this major version.
+
+# Version 0.11.1
+
+Fixes:
+- The enumeration types in `tags` are now all marked with a representation of
+  their underlying TIFF type (e.g. `repr(u16)`) and variants are explicitly
+  assigned their corresponding values. That is you may _read_ the raw
+  discriminant and interpret that as the value—except for `Unknown` variants.
+- The variants `RationalBig` and `SRationalBig` of `decoder::ifd::Value` are
+  deprecated as they have no corresponding TIFF type and were not constructed
+  during decoding.
+
+Additions:
+- Types in `tags` now generally implement `TiffValue` and can be handed to the
+  `DirectoryEncoder::write_tag` method. Unlike primitive types they do _not_
+  always implement the trait for slices of themselves.
+- Added `tags::ValueBuffer`, a byte-based buffer for tags with a runtime value
+  type and count.
+- Added `IfdDecoder::find_tag_{buf,bytes}` to read tag values into a
+  `ValueBuffer` or byte slice respectively without interpreting them
+  immediately within the Rust type system.
+- Added `DirectoryEncoder::write_{data,tag}_buf` that takes a `ValueBuffer`
+  instead of a statically typed argument.
+- Added `DirectoryEncoder::write_entry{,_buf,_bytes}` that encode their
+  argument into the file and return an `Entry` for later use, without
+  immediately adding a tag-entry pair to the directory.
+- The `TiffValue` trait is now implemented for several builtin `tags::*` types
+  or slices of them as appropriate for their use as values of their intended
+  TIFF tag.
+- The `TiffValue` trait is now implemented for `[T; N]` for all types that
+  implement it for slices.
+
+# Version 0.11.0
+
+- `Directory` now implements `FromIterator<(Tag, Value)>`.
+
+Changes:
+- The decoder now interprets the `ExtraSamples` tag. The sample count must now
+  more strict match the expected value with alpha channels only allowing for
+  explicitly denoted unassociated or associated alpha. This effects the
+  indicated color type when decoding images with additional samples indicated
+  as unspecified relation.  Previously, these may have been interpreted as
+  alpha by the total sample count (e.g. RgbA if 4 samples under a photometric
+  interpretation of RGB).
+- The decoder handles planar data, current limited to non-subsampled channels.
+  The `Decoder::read_image` method return planes one-after-another depending on
+  the size of the buffer that was passed.
+- `Decoder::read_image_to_buffer` now takes `&mut DecodingResult` and resizes
+  it according to the required layout. Previously, a borrowed `DecodingBuffer`
+  was passed which can be replaced by calling `as_bytes_mut` and
+  `read_image_bytes`.
+- Several methods of tags are now `const`. Note that does not guarantee any
+  particular value when calling these methods.
+
+Fixes:
+- Fix a bug in the uncompressed encoder that could lead to short writes, i.e.
+  data silently dropped when the underlying writer did not accept all data in a
+  single write call.
+- Encoding YCbCr data now writes the `ChromaSubsampling` tag as `(1, 1)` to
+  indicate no subsampling, instead of leaving it at its default of `(2, 2)`.
+- The decoder will reject subsampled YCbCr data as there is no upsampling
+  routine, except for JPEG compressed images where the JPEG decoders handles
+  this. Since the buffer in that case indicates a full-sized plane for all
+  color samples any future support for the tag will upsample all planes (at
+  least within this major version).
+
+Additions:
+- Added support for the `CieLab` color type.
+- Added `DecodingResult::resize_to` to create a buffer with a matching sample
+  type and dimensions.
+- Added `ByteOrder::native` to access the platform's native endianness.
+- Added `ByteOrder::convert` to change the byte-order of values in a byte
+  buffer, depending on their `Type` as described at runtime.
+- Added `DirectoryOffset::new` to encode a directory whose offset is known to
+  the caller but that has not been written through the encoder itself.
+- Added `Encoder::extra_samples` to encode images with more samples than their
+  color's trait implementation would otherwise suggest.
+- Added `DirectoryEncoder::extend_from` to encode multiple tag entries from a
+  directory whose values were written to the file by means outside the
+  encoder's control.
+- Added `Decoder::read_coding_unit_bytes` to retrieve data of corresponding
+  coordinates from potentially planar data, which is encoded in multiple chunks
+  of the file.
+
+# Version 0.10.3
+
+New features:
+- Group 4 `fax` is now recognized as a compression type and decoded.
+- A crate feature `fax` controls related dependencies, on by default.
+
+Changes:
+- The presence of PhotometricInterpretation::WhiteIsZero now returns an error
+  if the color type can not be inverted by the library where they were
+  previously silently ignored. Please report any missing color types.
+
+Fixes:
+- The PhotometricInterpretation::WhiteIsZero is now also applied to 1-bit
+  images, as well as 2-bit and 4-bit sample types where previously the presence
+  of this tag was ignored.
+
+# Version 0.10.2
+
+Republished 0.10.1, fixing an accidentally removed method (`Decoder::find_tag_unsigned_vec`).
+
+# Version 0.10.1
+
+New features:
+ - Added a dedicated `Directory` type (#277)
+ - Expose methods related to `Directory` publicly (#278)
+ - Implement writing directories (#281, #283)
+ - Add ICC (color profile) to the set of known tags (#287)
+ - Added `Decoder.inner()` to access the underlying reader (#289)
+ - Added methods for reading into a pre-allocated byte buffer (#285)
+
+Changes:
+ - No longer perform color conversion when decoding embedded JPEG. This fixes decoding for JPEG images with alpha channel. (#274)
+ - More lenient LZW decoding to match libtiff (#231)
+
+Fixes:
+ - Fixed seeking multiple times in a row (#279)
+
 # Version 0.10.0
 
 New features:

Cargo.toml

@@ -1,11 +1,11 @@
 [package]
 name = "tiff"
-version = "0.10.0"
+version = "0.11.3"
 edition = "2021"
 resolver = "2"
 
 # note: when changed, also update test runner in `.github/workflows/rust.yml`
-rust-version = "1.74.0"
+rust-version = "1.85.0"
 
 license = "MIT"
 description = "TIFF decoding and encoding library in pure Rust"
@@ -14,27 +14,34 @@ authors = ["The image-rs Developers"]
 repository = "https://github.com/image-rs/image-tiff"
 categories = ["multimedia::images", "multimedia::encoding"]
 
-exclude = ["tests/images/*", "tests/fuzz_images/*"]
+exclude = ["tests/images/*", "tests/fuzz_images/*", "tests/**/*.tif"]
 
 [dependencies]
 half = { version = "2.4.1" }
 quick-error = "2.0.1"
 
+# Rename to avoid confusion with the feature name.
+fax34 = { package = "fax", version = "0.2.6", optional = true }
 flate2 = { version = "1.0.20", optional = true }
-weezl = { version = "0.1.0", optional = true }
+weezl = { version = "0.1.10", optional = true }
 zstd = { version = "0.13", optional = true }
-zune-jpeg = { version = "0.4.17", optional = true }
+zune-jpeg = { version = "0.5.1", optional = true }
+image-webp = {version = "0.2.4", optional = true }
 
 [dev-dependencies]
 criterion = "0.3.1"
+# We may already depend on this via flate2
+crc32fast = "1.5"
 
 [features]
-default = ["deflate", "jpeg", "lzw"]
+default = ["deflate", "fax", "jpeg", "lzw"]
 
 # Compression algorithms
 deflate = ["dep:flate2"]
+fax = ["dep:fax34"]
 jpeg = ["dep:zune-jpeg"]
 lzw = ["dep:weezl"]
+webp = ["dep:image-webp"]
 zstd = ["dep:zstd"]
 
 [[bench]]

README.md

@@ -1,5 +1,5 @@
 # image-tiff
-[![Build Status](https://github.com/image-rs/image-tiff/workflows/Rust%20CI/badge.svg)](https://github.com/image-rs/image-tiff/actions)
+[![Build Status](https://github.com/image-rs/image-tiff/actions/workflows/rust.yml/badge.svg?branch=main)](https://github.com/image-rs/image-tiff/actions)
 [![Documentation](https://docs.rs/tiff/badge.svg)](https://docs.rs/tiff)
 [![Further crate info](https://img.shields.io/crates/v/tiff.svg)](https://crates.io/crates/tiff)
 
@@ -36,6 +36,7 @@ This table lists photometric interpretations and sample formats which are suppor
 | LZW      | ✓        | ✓        |
 | Deflate  | ✓        | ✓        |
 | PackBits | ✓        | ✓        |
+| Fax4     | ✓        | not yet  |
 | JPEG     | ✓        | not yet  |
 | ZSTD     | ✓        | not yet  |
 

examples/decode.rs

@@ -0,0 +1,206 @@
+use tiff::decoder::{BufferLayoutPreference, Decoder, DecodingBuffer, DecodingResult};
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let Some(image) = std::env::args_os().nth(1) else {
+        eprintln!("Usage: decode FILE");
+        return Ok(());
+    };
+
+    let file = std::fs::File::open(image)?;
+    let io = std::io::BufReader::new(file);
+    let mut reader = Decoder::new(io)?;
+
+    let mut data = DecodingResult::I8(vec![]);
+
+    for i in 0u32.. {
+        let colortype = reader.colortype()?;
+        let dimensions = reader.dimensions()?;
+        let layout = reader.read_image_to_buffer(&mut data)?;
+
+        debug_planes(i, &mut data, &layout, &colortype, dimensions);
+
+        if !reader.more_images() {
+            break;
+        }
+
+        reader.next_image()?
+    }
+
+    Ok(())
+}
+
+fn debug_planes(
+    index: u32,
+    data: &mut DecodingResult,
+    layout: &BufferLayoutPreference,
+    colortype: &tiff::ColorType,
+    (width, height): (u32, u32),
+) {
+    let (depth, mut tupltype) = match colortype {
+        // Note: we will expand, so this is in fact not BLACKANDWHITE
+        tiff::ColorType::Gray(_) => (1, "GRAYSCALE"),
+        tiff::ColorType::RGB(_) => (3, "RGB"),
+        tiff::ColorType::RGBA(_) => (4, "RGB_ALPHA"),
+        tiff::ColorType::Palette(_) => (1, "PALETTE"),
+        tiff::ColorType::CMYK(_) => (4, "CMYK"),
+        tiff::ColorType::Multiband { num_samples: 2, .. } => (2, "GRAYSCALE_ALPHA"),
+        _ => {
+            eprintln!("Unsupported color type for PAM output: {:?}", colortype);
+            return;
+        }
+    };
+
+    // Number of samples in a line and number of sample lines in the image, depending on whether
+    // they are planar or not. This determines the layout of the image buffer when expanding bit
+    // packed samples to a full byte (we support power-of-two sample sizes, positive and negative,
+    // but only those).
+    let (swidth, sheight) = if layout.planes > 1 {
+        (
+            u64::from(width) * u64::from(colortype.num_samples()),
+            u64::from(height),
+        )
+    } else {
+        (
+            u64::from(width),
+            u64::from(height) * u64::from(colortype.num_samples()),
+        )
+    };
+
+    let mut fallback_buffer;
+    let maxval = match data.as_buffer(0) {
+        // `U8` is also our representation of bit-packed color. PAM uses the smallest number of
+        // *bytes* needed to represent a sample but we must also adhere to maxval. We could explode
+        // them here (e.g. represent 1-bit as `{0, 1}`) but we'd also set `maxval` accordingly and
+        // this requires tupltype to be `BLACKANDWHITE` so introducing a bunch of complexity. If
+        // you want to do that in your real code, go ahead.
+        DecodingBuffer::U8(buf) => match colortype.bit_depth() {
+            8 => u64::from(u8::MAX),
+            4 => {
+                fallback_buffer = vec![0; (swidth * sheight) as usize];
+                expand_4bit(buf, swidth, &mut fallback_buffer);
+                *data = DecodingResult::U8(fallback_buffer);
+                u64::from(u8::MAX)
+            }
+            2 => {
+                fallback_buffer = vec![0; (swidth * sheight) as usize];
+                expand_2bit(buf, swidth, &mut fallback_buffer);
+                *data = DecodingResult::U8(fallback_buffer);
+                u64::from(u8::MAX)
+            }
+            1 => {
+                fallback_buffer = vec![0; (swidth * sheight) as usize];
+                expand_1bit(buf, swidth, &mut fallback_buffer);
+                *data = DecodingResult::U8(fallback_buffer);
+                u64::from(u8::MAX)
+            }
+            _ => {
+                eprintln!(
+                    "Unsupported bit depth for U8 buffer: {}",
+                    colortype.bit_depth()
+                );
+                return;
+            }
+        },
+        DecodingBuffer::U16(_) => u64::from(u16::MAX),
+        DecodingBuffer::U32(_) => u64::from(u32::MAX),
+        DecodingBuffer::U64(_) => u64::MAX,
+        // Eh, this is a good effort I guess.
+        DecodingBuffer::I8(items) => {
+            items.iter_mut().for_each(|v| *v = 0.max(*v));
+            u64::from(i8::MAX as u8)
+        }
+        DecodingBuffer::I16(items) => {
+            items.iter_mut().for_each(|v| *v = 0.max(*v));
+            u64::from(i16::MAX as u16)
+        }
+        DecodingBuffer::I32(items) => {
+            items.iter_mut().for_each(|v| *v = 0.max(*v));
+            u64::from(i32::MAX as u32)
+        }
+        DecodingBuffer::I64(items) => {
+            items.iter_mut().for_each(|v| *v = 0.max(*v));
+            i64::MAX as u64
+        }
+        // Not supported by PAM.
+        DecodingBuffer::F16(_) | DecodingBuffer::F32(_) | DecodingBuffer::F64(_) => return,
+    };
+
+    let byte_len = if layout.planes > 1 {
+        tupltype = "GRAYSCALE";
+        layout.plane_stride.unwrap().get()
+    } else {
+        data.as_buffer(0).byte_len()
+    };
+
+    // Write out this image's planes to PAM files.
+    let header = format!("P7\nWIDTH {width}\nHEIGHT {height}\nDEPTH {depth}\nMAXVAL {maxval}\nTUPLTYPE {tupltype}\nENDHDR\n");
+
+    for (plane, plane_data) in data
+        .as_buffer(0)
+        .as_bytes_mut()
+        .chunks_exact(byte_len)
+        .enumerate()
+    {
+        let filename = format!("image_{index:03}_plane_{plane:03}.pam");
+        let mut file = std::fs::File::create(filename).expect("Failed to create output file");
+
+        use std::io::Write;
+        file.write_all(header.as_bytes())
+            .expect("Failed to write PAM header");
+        file.write_all(plane_data)
+            .expect("Failed to write PAM data");
+    }
+}
+
+/// Put exactly the ith `SCALE`-bit chunk of `val` into the least significant bits.
+#[inline(always)]
+fn bits_extract<const SCALE: u8>(val: u8, i: u8) -> u8 {
+    // Shift left to discard high bits, then right to bring desired bits down. This way also
+    // conveniently avoids all cases of overflow.
+    (val << (i * SCALE)) >> (8 - SCALE)
+}
+
+fn expand_1bit(from: &[u8], samples_per_padded: u64, into: &mut [u8]) {
+    let stride = samples_per_padded.div_ceil(8);
+
+    let from_row = from.chunks_exact(stride as usize);
+    let into_row = into.chunks_exact_mut(samples_per_padded as usize);
+
+    for (from_row, into_row) in from_row.zip(into_row) {
+        for (into, &from) in into_row.chunks_mut(8).zip(from_row) {
+            let data =
+                core::array::from_fn::<u8, 8, _>(|i| 0xff * bits_extract::<1>(from, i as u8));
+            into.copy_from_slice(&data[..into.len()]);
+        }
+    }
+}
+
+fn expand_2bit(from: &[u8], samples_per_padded: u64, into: &mut [u8]) {
+    let stride = samples_per_padded.div_ceil(4);
+
+    let from_row = from.chunks_exact(stride as usize);
+    let into_row = into.chunks_exact_mut(samples_per_padded as usize);
+
+    for (from_row, into_row) in from_row.zip(into_row) {
+        for (into, &from) in into_row.chunks_mut(4).zip(from_row) {
+            let data =
+                core::array::from_fn::<u8, 4, _>(|i| 0xaa * bits_extract::<2>(from, i as u8));
+            into.copy_from_slice(&data[..into.len()]);
+        }
+    }
+}
+
+fn expand_4bit(from: &[u8], samples_per_padded: u64, into: &mut [u8]) {
+    let stride = samples_per_padded.div_ceil(2);
+
+    let from_row = from.chunks_exact(stride as usize);
+    let into_row = into.chunks_exact_mut(samples_per_padded as usize);
+
+    for (from_row, into_row) in from_row.zip(into_row) {
+        for (into, &from) in into_row.chunks_mut(2).zip(from_row) {
+            let data =
+                core::array::from_fn::<u8, 2, _>(|i| 0x88 * bits_extract::<4>(from, i as u8));
+            into.copy_from_slice(&data[..into.len()]);
+        }
+    }
+}

examples/tiff-ls.rs

@@ -0,0 +1,124 @@
+use std::borrow::Cow;
+
+use tiff::decoder::Decoder;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let Some(image) = std::env::args_os().nth(1) else {
+        eprintln!("Usage: decode FILE");
+        return Ok(());
+    };
+
+    let file = std::fs::File::open(image)?;
+    let io = std::io::BufReader::new(file);
+    let mut reader = Decoder::new(io)?;
+
+    loop {
+        ls_dir(&mut reader);
+
+        if !reader.more_images() {
+            break;
+        }
+    }
+
+    Ok(())
+}
+
+fn ls_dir<R: std::io::Read + std::io::Seek>(reader: &mut Decoder<R>) {
+    if let Some(ifd) = reader.ifd_pointer() {
+        println!("Directory at {ifd:x}");
+    }
+
+    println!("Name\tHex\tType\tCount");
+    let ifd = reader.image_ifd();
+
+    for (tag, entry) in ifd.directory().iter() {
+        let name: &'static str = match tag {
+            tiff::tags::Tag::Artist => "Artist",
+            tiff::tags::Tag::BitsPerSample => "BitsPerSample",
+            tiff::tags::Tag::CellLength => "CellLength",
+            tiff::tags::Tag::CellWidth => "CellWidth",
+            tiff::tags::Tag::ColorMap => "ColorMap",
+            tiff::tags::Tag::Compression => "Compression",
+            tiff::tags::Tag::DateTime => "DateTime",
+            tiff::tags::Tag::ExtraSamples => "ExtraSamples",
+            tiff::tags::Tag::FillOrder => "FillOrder",
+            tiff::tags::Tag::FreeByteCounts => "FreeByteCounts",
+            tiff::tags::Tag::FreeOffsets => "FreeOffsets",
+            tiff::tags::Tag::GrayResponseCurve => "GrayResponseCurve",
+            tiff::tags::Tag::GrayResponseUnit => "GrayResponseUnit",
+            tiff::tags::Tag::HostComputer => "HostComputer",
+            tiff::tags::Tag::ImageDescription => "ImageDescription",
+            tiff::tags::Tag::ImageLength => "ImageLength",
+            tiff::tags::Tag::ImageWidth => "ImageWidth",
+            tiff::tags::Tag::Make => "Make",
+            tiff::tags::Tag::MaxSampleValue => "MaxSampleValue",
+            tiff::tags::Tag::MinSampleValue => "MinSampleValue",
+            tiff::tags::Tag::Model => "Model",
+            tiff::tags::Tag::NewSubfileType => "NewSubfileType",
+            tiff::tags::Tag::Orientation => "Orientation",
+            tiff::tags::Tag::PhotometricInterpretation => "PhotometricInterpretation",
+            tiff::tags::Tag::PlanarConfiguration => "PlanarConfiguration",
+            tiff::tags::Tag::ResolutionUnit => "ResolutionUnit",
+            tiff::tags::Tag::RowsPerStrip => "RowsPerStrip",
+            tiff::tags::Tag::SamplesPerPixel => "SamplesPerPixel",
+            tiff::tags::Tag::Software => "Software",
+            tiff::tags::Tag::StripByteCounts => "StripByteCounts",
+            tiff::tags::Tag::StripOffsets => "StripOffsets",
+            tiff::tags::Tag::SubfileType => "SubfileType",
+            tiff::tags::Tag::Threshholding => "Threshholding",
+            tiff::tags::Tag::XResolution => "XResolution",
+            tiff::tags::Tag::YResolution => "YResolution",
+            tiff::tags::Tag::Predictor => "Predictor",
+            tiff::tags::Tag::TileWidth => "TileWidth",
+            tiff::tags::Tag::TileLength => "TileLength",
+            tiff::tags::Tag::TileOffsets => "TileOffsets",
+            tiff::tags::Tag::TileByteCounts => "TileByteCounts",
+            tiff::tags::Tag::SubIfd => "SubIfd",
+            tiff::tags::Tag::SampleFormat => "SampleFormat",
+            tiff::tags::Tag::SMinSampleValue => "SMinSampleValue",
+            tiff::tags::Tag::SMaxSampleValue => "SMaxSampleValue",
+            tiff::tags::Tag::JPEGTables => "JPEGTables",
+            tiff::tags::Tag::ChromaSubsampling => "ChromaSubsampling",
+            tiff::tags::Tag::ChromaPositioning => "ChromaPositioning",
+            tiff::tags::Tag::ModelPixelScaleTag => "ModelPixelScaleTag",
+            tiff::tags::Tag::ModelTransformationTag => "ModelTransformationTag",
+            tiff::tags::Tag::ModelTiepointTag => "ModelTiepointTag",
+            tiff::tags::Tag::Copyright => "Copyright",
+            tiff::tags::Tag::ExifDirectory => "ExifDirectory",
+            tiff::tags::Tag::GpsDirectory => "GpsDirectory",
+            tiff::tags::Tag::IccProfile => "IccProfile",
+            tiff::tags::Tag::GeoKeyDirectoryTag => "GeoKeyDirectoryTag",
+            tiff::tags::Tag::GeoDoubleParamsTag => "GeoDoubleParamsTag",
+            tiff::tags::Tag::GeoAsciiParamsTag => "GeoAsciiParamsTag",
+            tiff::tags::Tag::ExifVersion => "ExifVersion",
+            tiff::tags::Tag::GdalNodata => "GdalNodata",
+            _ => "<unknown>",
+        };
+
+        let ty: Cow<'static, str> = match entry.field_type() {
+            tiff::tags::Type::BYTE => "u8".into(),
+            tiff::tags::Type::ASCII => "ascii".into(),
+            tiff::tags::Type::SHORT => "u16".into(),
+            tiff::tags::Type::LONG => "u32".into(),
+            tiff::tags::Type::RATIONAL => "r32".into(),
+            tiff::tags::Type::SBYTE => "i8".into(),
+            tiff::tags::Type::UNDEFINED => "byte".into(),
+            tiff::tags::Type::SSHORT => "s16".into(),
+            tiff::tags::Type::SLONG => "s32".into(),
+            tiff::tags::Type::SRATIONAL => "sr32".into(),
+            tiff::tags::Type::FLOAT => "f32".into(),
+            tiff::tags::Type::DOUBLE => "f64".into(),
+            tiff::tags::Type::IFD => "ifd32".into(),
+            tiff::tags::Type::LONG8 => "u64".into(),
+            tiff::tags::Type::SLONG8 => "i64".into(),
+            tiff::tags::Type::IFD8 => "ifd64".into(),
+            other => format!("{:x}", other.to_u16()).into(),
+        };
+
+        eprintln!(
+            "{name:16}\t{tag:4x}\t{ty}\t{count}",
+            tag = tag.to_u16(),
+            count = entry.count(),
+        );
+    }
+}

src/bytecast.rs

@@ -10,6 +10,9 @@
 //! the unsafe code guidelines).
 //!
 //! TODO: Would like to use std-lib here.
+// Until we implement predictors for f16, we don't need f16_as_ne_bytes. (Due to the macro we do
+// not apply this directly to the functions). And rust-version is not new enough for `expect`.
+#![allow(dead_code)]
 use std::{mem, slice};
 
 use half::f16;

src/decoder/cycles.rs

@@ -0,0 +1,169 @@
+use crate::{tags::IfdPointer, TiffError, TiffFormatError};
+use std::collections::HashMap;
+
+/// The IFD structure of a TIFF file should be limited to a forest of entries, and a tree when we
+/// only consider having a primary IFD and its children. There is up to one primary child of a node
+/// that is the `next` entry in an IFD itself. Since we offer iteration over this chain we want to
+/// detect when a new edge would introduce a cycle, such that callers can be guaranteed to
+/// terminate iteration at some point even in malicious images.
+///
+/// However, we are not necessarily visiting the IFDs in their topological order.
+///
+/// Since we only consider one child, we run union find to assign each pointer to a known chain.
+/// Then, when we insert a new edge we check if they belong to the same component.
+#[derive(Default, Debug)]
+pub struct IfdCycles {
+    /// The root of each component union.
+    component_union: HashMap<ComponentId, ComponentId>,
+    links: HashMap<IfdPointer, u64>,
+    chains: HashMap<IfdPointer, ComponentId>,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+struct ComponentId(u64);
+
+impl IfdCycles {
+    pub fn new() -> Self {
+        IfdCycles::default()
+    }
+
+    pub fn insert_next(
+        &mut self,
+        from: IfdPointer,
+        to: Option<IfdPointer>,
+    ) -> Result<bool, TiffError> {
+        let to_offset = to.map_or(0, |p| p.0);
+
+        // For some reason we got
+        match self.links.get(&from) {
+            Some(existing) if *existing == to_offset => return Ok(false),
+            // We got here twice with two different reads of the same IFD. That is .. unusual and
+            // we interpret it as a cycle.
+            Some(_) => return Err(TiffError::FormatError(TiffFormatError::CycleInOffsets)),
+            None => self.links.insert(from, to_offset),
+        };
+
+        self.ensure_node(from);
+
+        if let Some(to) = to {
+            self.ensure_node(to);
+
+            let parent = self.nominal_component(from);
+            let child = self.nominal_component(to);
+
+            if parent == child {
+                return Err(TiffError::FormatError(TiffFormatError::CycleInOffsets));
+            }
+
+            self.component_union.insert(child, parent);
+        }
+
+        Ok(true)
+    }
+
+    fn ensure_node(&mut self, ifd: IfdPointer) {
+        self.chains.entry(ifd).or_insert_with(|| {
+            let id = ComponentId(self.component_union.len() as u64);
+            self.component_union.insert(id, id);
+            id
+        });
+    }
+
+    fn nominal_component(&mut self, node: IfdPointer) -> ComponentId {
+        let id = self.chains[&node];
+
+        let nomimal = {
+            let mut iter = id;
+
+            loop {
+                let parent = self.component_union[&iter];
+
+                if parent == iter {
+                    break parent;
+                }
+
+                iter = parent;
+            }
+        };
+
+        if nomimal != id {
+            // Compress.
+            let mut iter = id;
+
+            loop {
+                let parent = self.component_union[&iter];
+
+                if parent == iter {
+                    break;
+                }
+
+                self.component_union.insert(iter, nomimal);
+                iter = parent;
+            }
+        }
+
+        nomimal
+    }
+}
+
+#[test]
+fn cycles_are_detected() {
+    let mut cycles = IfdCycles::new();
+
+    cycles
+        .insert_next(IfdPointer(0x20), Some(IfdPointer(0x800)))
+        .expect("non-existing link is valid");
+
+    cycles
+        .insert_next(IfdPointer(0x800), Some(IfdPointer(0x20)))
+        .expect_err("cycle must be detected");
+}
+
+#[test]
+fn reflective_cycle() {
+    let mut cycles = IfdCycles::new();
+
+    cycles
+        .insert_next(IfdPointer(0x20), Some(IfdPointer(0x20)))
+        .expect_err("self-referential cycle must be detected");
+}
+
+#[test]
+fn late_cycle() {
+    let mut cycles = IfdCycles::new();
+
+    cycles
+        .insert_next(IfdPointer(0x20), Some(IfdPointer(0x40)))
+        .expect("non-existing link is valid");
+
+    cycles
+        .insert_next(IfdPointer(0x60), Some(IfdPointer(0x80)))
+        .expect("non-existing link is valid");
+    cycles
+        .insert_next(IfdPointer(0x80), Some(IfdPointer(0x20)))
+        .expect("non-existing link is valid");
+
+    cycles
+        .insert_next(IfdPointer(0x40), Some(IfdPointer(0x60)))
+        .expect_err("non-existing link is valid");
+}
+
+#[test]
+fn odd_cycle() {
+    let mut cycles = IfdCycles::new();
+
+    cycles
+        .insert_next(IfdPointer(0x20), Some(IfdPointer(0x40)))
+        .expect("non-existing link is valid");
+
+    cycles
+        .insert_next(IfdPointer(0x60), Some(IfdPointer(0x80)))
+        .expect("non-existing link is valid");
+    cycles
+        .insert_next(IfdPointer(0x80), Some(IfdPointer(0x20)))
+        .expect("non-existing link is valid");
+
+    cycles
+        .insert_next(IfdPointer(0x40), Some(IfdPointer(0x80)))
+        .expect_err("non-existing link is valid");
+}

src/decoder/ifd.rs

@@ -6,12 +6,12 @@ use std::mem;
 use std::str;
 
 use super::stream::{ByteOrder, EndianReader};
-use crate::tags::{Tag, Type};
+use crate::tags::{IfdPointer, Tag, Type, ValueBuffer};
 use crate::{TiffError, TiffFormatError, TiffResult};
 
 use self::Value::{
-    Ascii, Byte, Double, Float, Ifd, IfdBig, List, Rational, RationalBig, SRational, SRationalBig,
-    Short, Signed, SignedBig, SignedByte, SignedShort, Unsigned, UnsignedBig,
+    Ascii, Byte, Double, Float, Ifd, IfdBig, List, Rational, SRational, Short, Signed, SignedBig,
+    SignedByte, SignedShort, Unsigned, UnsignedBig,
 };
 
 #[allow(unused_qualifications)]
@@ -30,8 +30,16 @@ pub enum Value {
     Double(f64),
     List(Vec<Value>),
     Rational(u32, u32),
+    #[deprecated(
+        note = "Not implemented in BigTIFF with a standard tag value",
+        since = "0.11.1"
+    )]
     RationalBig(u64, u64),
     SRational(i32, i32),
+    #[deprecated(
+        note = "Not implemented in BigTIFF with a standard tag value",
+        since = "0.11.1"
+    )]
     SRationalBig(i64, i64),
     Ascii(String),
     Ifd(u32),
@@ -130,6 +138,25 @@ impl Value {
         }
     }
 
+    /// Turn this value into an `IfdPointer`.
+    ///
+    /// Notice that this does not take an argument, a 64-bit IFD is always allowed. If the
+    /// difference is crucial and you do not want to be permissive you're expected to filter this
+    /// out before.
+    ///
+    /// For compatibility the smaller sized tags should always be allowed i.e. you might use a
+    /// non-bigtiff's directory and its tag types and move it straight to a bigtiff. For instance
+    /// the SubIFD tag is defined as `LONG or IFD`:
+    ///
+    /// <https://web.archive.org/web/20181105221012/https://www.awaresystems.be/imaging/tiff/tifftags/subifds.html>
+    pub fn into_ifd_pointer(self) -> TiffResult<IfdPointer> {
+        match self {
+            Unsigned(val) | Ifd(val) => Ok(IfdPointer(val.into())),
+            IfdBig(val) => Ok(IfdPointer(val)),
+            _ => Err(TiffError::FormatError(TiffFormatError::InvalidTypeForTag)),
+        }
+    }
+
     pub fn into_string(self) -> TiffResult<String> {
         match self {
             Ascii(val) => Ok(val),
@@ -151,7 +178,8 @@ impl Value {
             Unsigned(val) => Ok(vec![val]),
             UnsignedBig(val) => Ok(vec![u32::try_from(val)?]),
             Rational(numerator, denominator) => Ok(vec![numerator, denominator]),
-            RationalBig(numerator, denominator) => {
+            #[expect(deprecated)]
+            Value::RationalBig(numerator, denominator) => {
                 Ok(vec![u32::try_from(numerator)?, u32::try_from(denominator)?])
             }
             Ifd(val) => Ok(vec![val]),
@@ -201,7 +229,8 @@ impl Value {
                             new_vec.push(numerator);
                             new_vec.push(denominator);
                         }
-                        SRationalBig(numerator, denominator) => {
+                        #[expect(deprecated)]
+                        Value::SRationalBig(numerator, denominator) => {
                             new_vec.push(i32::try_from(numerator)?);
                             new_vec.push(i32::try_from(denominator)?);
                         }
@@ -215,7 +244,8 @@ impl Value {
             Signed(val) => Ok(vec![val]),
             SignedBig(val) => Ok(vec![i32::try_from(val)?]),
             SRational(numerator, denominator) => Ok(vec![numerator, denominator]),
-            SRationalBig(numerator, denominator) => {
+            #[expect(deprecated)]
+            Value::SRationalBig(numerator, denominator) => {
                 Ok(vec![i32::try_from(numerator)?, i32::try_from(denominator)?])
             }
             _ => Err(TiffError::FormatError(TiffFormatError::InvalidTypeForTag)),
@@ -264,7 +294,8 @@ impl Value {
             Unsigned(val) => Ok(vec![val.into()]),
             UnsignedBig(val) => Ok(vec![val]),
             Rational(numerator, denominator) => Ok(vec![numerator.into(), denominator.into()]),
-            RationalBig(numerator, denominator) => Ok(vec![numerator, denominator]),
+            #[expect(deprecated)]
+            Value::RationalBig(numerator, denominator) => Ok(vec![numerator, denominator]),
             Ifd(val) => Ok(vec![val.into()]),
             IfdBig(val) => Ok(vec![val]),
             Ascii(val) => Ok(val.chars().map(u32::from).map(u64::from).collect()),
@@ -282,7 +313,8 @@ impl Value {
                             new_vec.push(numerator.into());
                             new_vec.push(denominator.into());
                         }
-                        SRationalBig(numerator, denominator) => {
+                        #[expect(deprecated)]
+                        Value::SRationalBig(numerator, denominator) => {
                             new_vec.push(numerator);
                             new_vec.push(denominator);
                         }
@@ -296,12 +328,30 @@ impl Value {
             Signed(val) => Ok(vec![val.into()]),
             SignedBig(val) => Ok(vec![val]),
             SRational(numerator, denominator) => Ok(vec![numerator.into(), denominator.into()]),
-            SRationalBig(numerator, denominator) => Ok(vec![numerator, denominator]),
+            #[expect(deprecated)]
+            Value::SRationalBig(numerator, denominator) => Ok(vec![numerator, denominator]),
             _ => Err(TiffError::FormatError(TiffFormatError::InvalidTypeForTag)),
         }
     }
+
+    pub fn into_ifd_vec(self) -> TiffResult<Vec<IfdPointer>> {
+        let vec = match self {
+            Unsigned(val) | Ifd(val) => return Ok(vec![IfdPointer(val.into())]),
+            IfdBig(val) => return Ok(vec![IfdPointer(val)]),
+            List(vec) => vec,
+            _ => return Err(TiffError::FormatError(TiffFormatError::InvalidTypeForTag)),
+        };
+
+        vec.into_iter().map(Self::into_ifd_pointer).collect()
+    }
 }
 
+/// A combination of type, count, and offset.
+///
+/// In a TIFF the data offset portion of an entry is used for inline data in case the length of the
+/// encoded value does not exceed the size of the offset field. Since the size of the offset field
+/// depends on the file kind (4 bytes for standard TIFF, 8 bytes for BigTIFF) the interpretation of
+/// this struct is only complete in combination with file metadata.
 #[derive(Clone)]
 pub struct Entry {
     type_: Type,
@@ -319,12 +369,14 @@ impl ::std::fmt::Debug for Entry {
 }
 
 impl Entry {
+    /// Create a new entry fit to be added to a standard TIFF IFD.
     pub fn new(type_: Type, count: u32, offset: [u8; 4]) -> Entry {
-        let mut offset = offset.to_vec();
-        offset.append(&mut vec![0; 4]);
-        Entry::new_u64(type_, count.into(), offset[..].try_into().unwrap())
+        let mut entry_off = [0u8; 8];
+        entry_off[..4].copy_from_slice(&offset);
+        Entry::new_u64(type_, count.into(), entry_off)
     }
 
+    /// Create a new entry with data for a Big TIFF IFD.
     pub fn new_u64(type_: Type, count: u64, offset: [u8; 8]) -> Entry {
         Entry {
             type_,
@@ -333,8 +385,23 @@ impl Entry {
         }
     }
 
+    pub fn field_type(&self) -> Type {
+        self.type_
+    }
+
+    pub fn count(&self) -> u64 {
+        self.count
+    }
+
+    pub(crate) fn offset(&self) -> &[u8] {
+        &self.offset
+    }
+
     /// Returns a mem_reader for the offset/value field
-    fn r(&self, byte_order: ByteOrder) -> EndianReader<io::Cursor<Vec<u8>>> {
+    pub(crate) fn offset_field_reader(
+        &self,
+        byte_order: ByteOrder,
+    ) -> EndianReader<io::Cursor<Vec<u8>>> {
         EndianReader::new(io::Cursor::new(self.offset.to_vec()), byte_order)
     }
 
@@ -350,43 +417,25 @@ impl Entry {
         }
 
         let bo = reader.byte_order;
-
-        let tag_size = match self.type_ {
-            Type::BYTE | Type::SBYTE | Type::ASCII | Type::UNDEFINED => 1,
-            Type::SHORT | Type::SSHORT => 2,
-            Type::LONG | Type::SLONG | Type::FLOAT | Type::IFD => 4,
-            Type::LONG8
-            | Type::SLONG8
-            | Type::DOUBLE
-            | Type::RATIONAL
-            | Type::SRATIONAL
-            | Type::IFD8 => 8,
-        };
-
-        let value_bytes = match self.count.checked_mul(tag_size) {
-            Some(n) => n,
-            None => {
-                return Err(TiffError::LimitsExceeded);
-            }
-        };
+        let value_bytes = self.type_.value_bytes(self.count)?;
 
         // Case 2: there is one value.
         if self.count == 1 {
             // 2a: the value is 5-8 bytes and we're in BigTiff mode.
             if bigtiff && value_bytes > 4 && value_bytes <= 8 {
                 return Ok(match self.type_ {
-                    Type::LONG8 => UnsignedBig(self.r(bo).read_u64()?),
-                    Type::SLONG8 => SignedBig(self.r(bo).read_i64()?),
-                    Type::DOUBLE => Double(self.r(bo).read_f64()?),
+                    Type::LONG8 => UnsignedBig(self.offset_field_reader(bo).read_u64()?),
+                    Type::SLONG8 => SignedBig(self.offset_field_reader(bo).read_i64()?),
+                    Type::DOUBLE => Double(self.offset_field_reader(bo).read_f64()?),
                     Type::RATIONAL => {
-                        let mut r = self.r(bo);
+                        let mut r = self.offset_field_reader(bo);
                         Rational(r.read_u32()?, r.read_u32()?)
                     }
                     Type::SRATIONAL => {
-                        let mut r = self.r(bo);
+                        let mut r = self.offset_field_reader(bo);
                         SRational(r.read_i32()?, r.read_i32()?)
                     }
-                    Type::IFD8 => IfdBig(self.r(bo).read_u64()?),
+                    Type::IFD8 => IfdBig(self.offset_field_reader(bo).read_u64()?),
                     Type::BYTE
                     | Type::SBYTE
                     | Type::ASCII
@@ -403,13 +452,13 @@ impl Entry {
             // 2b: the value is at most 4 bytes or doesn't fit in the offset field.
             return Ok(match self.type_ {
                 Type::BYTE => Byte(self.offset[0]),
-                Type::SBYTE => SignedByte(i8::from(self.offset[0] as i8)),
+                Type::SBYTE => SignedByte(self.offset[0] as i8),
                 Type::UNDEFINED => Byte(self.offset[0]),
-                Type::SHORT => Short(self.r(bo).read_u16()?),
-                Type::SSHORT => SignedShort(self.r(bo).read_i16()?),
-                Type::LONG => Unsigned(self.r(bo).read_u32()?),
-                Type::SLONG => Signed(self.r(bo).read_i32()?),
-                Type::FLOAT => Float(self.r(bo).read_f32()?),
+                Type::SHORT => Short(self.offset_field_reader(bo).read_u16()?),
+                Type::SSHORT => SignedShort(self.offset_field_reader(bo).read_i16()?),
+                Type::LONG => Unsigned(self.offset_field_reader(bo).read_u32()?),
+                Type::SLONG => Signed(self.offset_field_reader(bo).read_i32()?),
+                Type::FLOAT => Float(self.offset_field_reader(bo).read_f32()?),
                 Type::ASCII => {
                     if self.offset[0] == 0 {
                         Ascii("".to_string())
@@ -418,28 +467,28 @@ impl Entry {
                     }
                 }
                 Type::LONG8 => {
-                    reader.goto_offset(self.r(bo).read_u32()?.into())?;
+                    reader.goto_offset(self.offset_field_reader(bo).read_u32()?.into())?;
                     UnsignedBig(reader.read_u64()?)
                 }
                 Type::SLONG8 => {
-                    reader.goto_offset(self.r(bo).read_u32()?.into())?;
+                    reader.goto_offset(self.offset_field_reader(bo).read_u32()?.into())?;
                     SignedBig(reader.read_i64()?)
                 }
                 Type::DOUBLE => {
-                    reader.goto_offset(self.r(bo).read_u32()?.into())?;
+                    reader.goto_offset(self.offset_field_reader(bo).read_u32()?.into())?;
                     Double(reader.read_f64()?)
                 }
                 Type::RATIONAL => {
-                    reader.goto_offset(self.r(bo).read_u32()?.into())?;
+                    reader.goto_offset(self.offset_field_reader(bo).read_u32()?.into())?;
                     Rational(reader.read_u32()?, reader.read_u32()?)
                 }
                 Type::SRATIONAL => {
-                    reader.goto_offset(self.r(bo).read_u32()?.into())?;
+                    reader.goto_offset(self.offset_field_reader(bo).read_u32()?.into())?;
                     SRational(reader.read_i32()?, reader.read_i32()?)
                 }
-                Type::IFD => Ifd(self.r(bo).read_u32()?),
+                Type::IFD => Ifd(self.offset_field_reader(bo).read_u32()?),
                 Type::IFD8 => {
-                    reader.goto_offset(self.r(bo).read_u32()?.into())?;
+                    reader.goto_offset(self.offset_field_reader(bo).read_u32()?.into())?;
                     IfdBig(reader.read_u64()?)
                 }
             });
@@ -470,7 +519,7 @@ impl Entry {
                     ));
                 }
                 Type::SHORT => {
-                    let mut r = self.r(bo);
+                    let mut r = self.offset_field_reader(bo);
                     let mut v = Vec::new();
                     for _ in 0..self.count {
                         v.push(Short(r.read_u16()?));
@@ -478,7 +527,7 @@ impl Entry {
                     return Ok(List(v));
                 }
                 Type::SSHORT => {
-                    let mut r = self.r(bo);
+                    let mut r = self.offset_field_reader(bo);
                     let mut v = Vec::new();
                     for _ in 0..self.count {
                         v.push(SignedShort(r.read_i16()?));
@@ -486,7 +535,7 @@ impl Entry {
                     return Ok(List(v));
                 }
                 Type::LONG => {
-                    let mut r = self.r(bo);
+                    let mut r = self.offset_field_reader(bo);
                     let mut v = Vec::new();
                     for _ in 0..self.count {
                         v.push(Unsigned(r.read_u32()?));
@@ -494,7 +543,7 @@ impl Entry {
                     return Ok(List(v));
                 }
                 Type::SLONG => {
-                    let mut r = self.r(bo);
+                    let mut r = self.offset_field_reader(bo);
                     let mut v = Vec::new();
                     for _ in 0..self.count {
                         v.push(Signed(r.read_i32()?));
@@ -502,7 +551,7 @@ impl Entry {
                     return Ok(List(v));
                 }
                 Type::FLOAT => {
-                    let mut r = self.r(bo);
+                    let mut r = self.offset_field_reader(bo);
                     let mut v = Vec::new();
                     for _ in 0..self.count {
                         v.push(Float(r.read_f32()?));
@@ -510,7 +559,7 @@ impl Entry {
                     return Ok(List(v));
                 }
                 Type::IFD => {
-                    let mut r = self.r(bo);
+                    let mut r = self.offset_field_reader(bo);
                     let mut v = Vec::new();
                     for _ in 0..self.count {
                         v.push(Ifd(r.read_u32()?));
@@ -529,74 +578,149 @@ impl Entry {
         }
 
         // Case 4: there is more than one value, and it doesn't fit in the offset field.
+        let mut v;
+        self.set_reader_offset_relative(bigtiff, reader, 0)?;
+
         match self.type_ {
-            // TODO check if this could give wrong results
-            // at a different endianess of file/computer.
-            Type::BYTE => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                let mut buf = [0; 1];
-                reader.inner().read_exact(&mut buf)?;
-                Ok(UnsignedBig(u64::from(buf[0])))
-            }),
-            Type::SBYTE => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                Ok(SignedBig(i64::from(reader.read_i8()?)))
-            }),
-            Type::SHORT => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                Ok(Short(reader.read_u16()?))
-            }),
-            Type::SSHORT => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                Ok(SignedShort(reader.read_i16()?))
-            }),
-            Type::LONG => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                Ok(Unsigned(reader.read_u32()?))
-            }),
-            Type::SLONG => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                Ok(Signed(reader.read_i32()?))
-            }),
-            Type::FLOAT => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                Ok(Float(reader.read_f32()?))
-            }),
-            Type::DOUBLE => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                Ok(Double(reader.read_f64()?))
-            }),
+            Type::BYTE | Type::UNDEFINED => {
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(bytes.iter().copied().map(Byte))
+                })
+            }
+            Type::SBYTE => {
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(bytes.iter().copied().map(|v| SignedByte(v as i8)))
+                })
+            }
+            Type::SHORT => {
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(
+                        bytes
+                            .chunks_exact(2)
+                            .map(|ch| Short(u16::from_ne_bytes(ch.try_into().unwrap()))),
+                    )
+                })
+            }
+            Type::SSHORT => {
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(
+                        bytes
+                            .chunks_exact(2)
+                            .map(|ch| SignedShort(i16::from_ne_bytes(ch.try_into().unwrap()))),
+                    )
+                })
+            }
+            Type::LONG => {
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(
+                        bytes
+                            .chunks_exact(4)
+                            .map(|ch| Unsigned(u32::from_ne_bytes(ch.try_into().unwrap()))),
+                    )
+                })
+            }
+            Type::SLONG => {
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(
+                        bytes
+                            .chunks_exact(4)
+                            .map(|ch| Signed(i32::from_ne_bytes(ch.try_into().unwrap()))),
+                    )
+                })
+            }
+            Type::FLOAT => {
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(
+                        bytes
+                            .chunks_exact(4)
+                            .map(|ch| Float(f32::from_ne_bytes(ch.try_into().unwrap()))),
+                    )
+                })
+            }
+            Type::DOUBLE => {
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(
+                        bytes
+                            .chunks_exact(8)
+                            .map(|ch| Double(f64::from_ne_bytes(ch.try_into().unwrap()))),
+                    )
+                })
+            }
             Type::RATIONAL => {
-                self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                    Ok(Rational(reader.read_u32()?, reader.read_u32()?))
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(bytes.chunks_exact(8).map(|ch| {
+                        Rational(
+                            u32::from_ne_bytes(ch[..4].try_into().unwrap()),
+                            u32::from_ne_bytes(ch[4..].try_into().unwrap()),
+                        )
+                    }))
                 })
             }
             Type::SRATIONAL => {
-                self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                    Ok(SRational(reader.read_i32()?, reader.read_i32()?))
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(bytes.chunks_exact(8).map(|ch| {
+                        SRational(
+                            i32::from_ne_bytes(ch[..4].try_into().unwrap()),
+                            i32::from_ne_bytes(ch[4..].try_into().unwrap()),
+                        )
+                    }))
                 })
             }
-            Type::LONG8 => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                Ok(UnsignedBig(reader.read_u64()?))
-            }),
-            Type::SLONG8 => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                Ok(SignedBig(reader.read_i64()?))
-            }),
-            Type::IFD => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                Ok(Ifd(reader.read_u32()?))
-            }),
-            Type::IFD8 => self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                Ok(IfdBig(reader.read_u64()?))
-            }),
-            Type::UNDEFINED => {
-                self.decode_offset(self.count, bo, bigtiff, limits, reader, |reader| {
-                    let mut buf = [0; 1];
-                    reader.inner().read_exact(&mut buf)?;
-                    Ok(Byte(buf[0]))
+            Type::LONG8 => {
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(
+                        bytes
+                            .chunks_exact(8)
+                            .map(|ch| UnsignedBig(u64::from_ne_bytes(ch.try_into().unwrap()))),
+                    )
+                })
+            }
+            Type::SLONG8 => {
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(
+                        bytes
+                            .chunks_exact(8)
+                            .map(|ch| SignedBig(i64::from_ne_bytes(ch.try_into().unwrap()))),
+                    )
+                })
+            }
+            Type::IFD => {
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(
+                        bytes
+                            .chunks_exact(4)
+                            .map(|ch| Ifd(u32::from_ne_bytes(ch.try_into().unwrap()))),
+                    )
+                })
+            }
+            Type::IFD8 => {
+                v = Self::vec_with_capacity(self.count, limits)?;
+                self.decode_values(self.count, self.type_, reader, |bytes| {
+                    v.extend(
+                        bytes
+                            .chunks_exact(8)
+                            .map(|ch| IfdBig(u64::from_ne_bytes(ch.try_into().unwrap()))),
+                    )
                 })
             }
             Type::ASCII => {
                 let n = usize::try_from(self.count)?;
-                if n > limits.decoding_buffer_size {
-                    return Err(TiffError::LimitsExceeded);
-                }
 
-                if bigtiff {
-                    reader.goto_offset(self.r(bo).read_u64()?)?
-                } else {
-                    reader.goto_offset(self.r(bo).read_u32()?.into())?
+                if n > limits.decoding_buffer_size {
+                    return Err(dbg!(TiffError::LimitsExceeded));
                 }
 
                 let mut out = vec![0; n];
@@ -605,43 +729,199 @@ impl Entry {
                 if let Some(first) = out.iter().position(|&b| b == 0) {
                     out.truncate(first);
                 }
-                Ok(Ascii(String::from_utf8(out)?))
-            }
+
+                return Ok(Ascii(String::from_utf8(out)?));
             }
+        }?;
+
+        Ok(List(v))
     }
 
-    #[inline]
-    fn decode_offset<R, F>(
+    pub(crate) fn buffered_value<R: Read + Seek>(
         &self,
-        value_count: u64,
-        bo: ByteOrder,
-        bigtiff: bool,
+        buf: &mut ValueBuffer,
         limits: &super::Limits,
+        bigtiff: bool,
         reader: &mut EndianReader<R>,
-        decode_fn: F,
-    ) -> TiffResult<Value>
-    where
-        R: Read + Seek,
-        F: Fn(&mut EndianReader<R>) -> TiffResult<Value>,
-    {
+    ) -> TiffResult<()> {
+        if self.count == 0 {
+            buf.assume_type(self.type_, 0, reader.byte_order);
+            return Ok(());
+        }
+
+        let value_bytes = self.buffer_with_capacity(buf, limits)?;
+
+        // Case 1: the value fits in the offset field.
+        if value_bytes <= 4 || bigtiff && value_bytes <= 8 {
+            let src = &self.offset[..value_bytes];
+            buf.raw_bytes_mut()[..value_bytes].copy_from_slice(src);
+            buf.assume_type(self.type_, self.count, reader.byte_order);
+
+            return Ok(());
+        }
+
+        // Case 2: the value is stored in the reader at an offset.
+        self.set_reader_offset_relative(bigtiff, reader, 0)?;
+
+        // In case of an error we set the type and endianess.
+        buf.assume_type(self.type_, 0, reader.byte_order);
+        let target = &mut buf.raw_bytes_mut()[..value_bytes];
+        // FIXME: if the read fails we have already grown to full size, which is not great.
+        reader.inner().read_exact(target)?;
+        buf.assume_type(self.type_, self.count, reader.byte_order);
+
+        Ok(())
+    }
+
+    pub(crate) fn raw_value_at<R: Read + Seek>(
+        &self,
+        buf: &mut [u8],
+        bigtiff: bool,
+        reader: &mut EndianReader<R>,
+        at: u64,
+    ) -> TiffResult<usize> {
+        if self.count == 0 {
+            return Ok(0);
+        }
+
+        // We have no limits to handle, we do not allocate.
+        let value_bytes = self.type_.value_bytes(self.count)?;
+
+        // No bytes to fill into the buffer.
+        if at >= value_bytes {
+            return Ok(0);
+        }
+
+        // Case 1: the value fits in the offset field.
+        if value_bytes <= 4 || bigtiff && value_bytes <= 8 {
+            // `at < value_bytes` and `value_bytes <= 8` so casting is mathematical
+            let src = &self.offset[..value_bytes as usize][at as usize..];
+            let len = src.len().min(buf.len());
+            buf[..len].copy_from_slice(&src[..len]);
+            return Ok(value_bytes as usize);
+        }
+
+        // Case 2: the value is stored in the reader at an offset. We will find the offset
+        // encoded in the entry, apply the relative start position and seek there.
+        self.set_reader_offset_relative(bigtiff, reader, at)?;
+
+        let remainder = value_bytes - at;
+        let len = usize::try_from(remainder)
+            .unwrap_or(usize::MAX)
+            .min(buf.len());
+
+        let target = &mut buf[..len];
+        reader.inner().read_exact(target)?;
+
+        // Design note: in a previous draft we would consume the rest of the bytes of this value
+        // here (into a stack buffer if need be) to verify the stream itself. But in the end we
+        // have `Seek` so we better verify this by seeking over the rest of the bytes, finding if
+        // the stream continues that far. Even that is maybe bad if we wanted to provide a
+        // async-adaptor that `WouldBlock` errors to fill back a read window then the seek is
+        // poison to that, too.
+
+        // So a really simple choice: The caller is responsible for handling the fact that this did
+        // not verify the whole value. Attempt a 1-byte read at the end of the value instead?
+        Ok(len)
+    }
+
+    // Returns `Ok(bytes)` if our value's bytes through type and count fit into `usize` and are
+    // within the limits. Extends the buffer to that many bytes.
+    fn buffer_with_capacity(
+        &self,
+        buf: &mut ValueBuffer,
+        limits: &super::Limits,
+    ) -> TiffResult<usize> {
+        let bytes = self.type_.value_bytes(self.count())?;
+
+        let allowed_length = usize::try_from(bytes)
+            .ok()
+            .filter(|&n| n <= limits.decoding_buffer_size)
+            .ok_or(TiffError::LimitsExceeded)?;
+
+        buf.prepare_length(allowed_length);
+
+        Ok(allowed_length)
+    }
+
+    fn vec_with_capacity(
+        value_count: u64,
+        limits: &super::Limits,
+    ) -> Result<Vec<Value>, TiffError> {
         let value_count = usize::try_from(value_count)?;
+
         if value_count > limits.decoding_buffer_size / mem::size_of::<Value>() {
             return Err(TiffError::LimitsExceeded);
         }
 
-        let mut v = Vec::with_capacity(value_count);
+        Ok(Vec::with_capacity(value_count))
+    }
+
+    /// Seek to an offset within a value stored in the offset defined by this entry.
+    fn set_reader_offset_relative<R>(
+        &self,
+        bigtiff: bool,
+        reader: &mut EndianReader<R>,
+        at: u64,
+    ) -> TiffResult<()>
+    where
+        R: Read + Seek,
+    {
+        let bo = reader.byte_order;
 
         let offset = if bigtiff {
-            self.r(bo).read_u64()?
+            self.offset_field_reader(bo).read_u64()?
         } else {
-            self.r(bo).read_u32()?.into()
+            self.offset_field_reader(bo).read_u32()?.into()
         };
+
+        // FIXME: `at` should be within `self.type_.value_bytes(self.count)` and that itself should
+        // be within the bounds of the stream. But we do not check this eagerly so this below will
+        // fail sometimes differently for exotic streams, depending on the method by which we read
+        // (at once or through multiple raw into-byte-slice reads).
+        let offset = offset.checked_add(at).ok_or(TiffError::FormatError(
+            TiffFormatError::InconsistentSizesEncountered,
+        ))?;
+
         reader.goto_offset(offset)?;
 
-        for _ in 0..value_count {
-            v.push(decode_fn(reader)?)
+        Ok(())
     }
-        Ok(List(v))
+
+    #[inline]
+    fn decode_values<R, F>(
+        &self,
+        value_count: u64,
+        type_: Type,
+        reader: &mut EndianReader<R>,
+        mut collect: F,
+    ) -> TiffResult<()>
+    where
+        R: Read + Seek,
+        F: FnMut(&[u8]),
+    {
+        let mut total_bytes = type_.value_bytes(value_count)?;
+        let mut buffer = [0u8; 512];
+
+        let buf_unit = usize::from(type_.byte_len());
+        let mul_of_ty = buffer.len() / buf_unit * buf_unit;
+
+        let cls = type_.endian_bytes();
+        let native = ByteOrder::native();
+
+        while total_bytes > 0 {
+            // `now <= mul_of_ty < 512` so casting is mathematical
+            let now = total_bytes.min(mul_of_ty as u64);
+            total_bytes -= now;
+
+            let buffer = &mut buffer[..now as usize];
+            reader.inner().read_exact(buffer)?;
+
+            reader.byte_order.convert_endian_bytes(cls, buffer, native);
+            collect(buffer);
+        }
+
+        Ok(())
     }
 }
 
@@ -668,4 +948,6 @@ fn offset_to_sbytes(n: usize, entry: &Entry) -> TiffResult<Value> {
 }
 
 /// Type representing an Image File Directory
+#[doc(hidden)]
+#[deprecated = "Use struct `tiff::Directory` instead which contains all fields relevant to an Image File Directory, including the offset to the next directory"]
 pub type Directory = HashMap<Tag, Entry>;

src/decoder/image.rs

@@ -1,12 +1,16 @@
-use super::ifd::{Directory, Value};
-use super::stream::{ByteOrder, PackBitsReader};
+use super::ifd::Value;
+use super::stream::PackBitsReader;
 use super::tag_reader::TagReader;
-use super::{predict_f16, predict_f32, predict_f64, Limits};
-use super::{stream::EndianReader, ChunkType};
+use super::ChunkType;
+use super::{predict_f16, predict_f32, predict_f64, ValueReader};
 use crate::tags::{
-    CompressionMethod, PhotometricInterpretation, PlanarConfiguration, Predictor, SampleFormat, Tag,
+    CompressionMethod, ExtraSamples, PhotometricInterpretation, PlanarConfiguration, Predictor,
+    SampleFormat, Tag,
 };
-use crate::{ColorType, TiffError, TiffFormatError, TiffResult, TiffUnsupportedError, UsageError};
+use crate::{
+    ColorType, Directory, TiffError, TiffFormatError, TiffResult, TiffUnsupportedError, UsageError,
+};
+
 use std::io::{self, Cursor, Read, Seek};
 use std::sync::Arc;
 
@@ -27,10 +31,10 @@ pub(crate) struct TileAttributes {
 
 impl TileAttributes {
     pub fn tiles_across(&self) -> usize {
-        (self.image_width + self.tile_width - 1) / self.tile_width
+        self.image_width.div_ceil(self.tile_width)
     }
     pub fn tiles_down(&self) -> usize {
-        (self.image_height + self.tile_length - 1) / self.tile_length
+        self.image_height.div_ceil(self.tile_length)
     }
     fn padding_right(&self) -> usize {
         (self.tile_width - self.image_width % self.tile_width) % self.tile_width
@@ -65,6 +69,11 @@ pub(crate) struct Image {
     pub height: u32,
     pub bits_per_sample: u8,
     pub samples: u16,
+    /// The `ExtraSamples`, defaulting to empty if not given.
+    pub extra_samples: Vec<ExtraSamples>,
+    /// Number of samples that belong to the photometric interpretation, samples except
+    /// `ExtraSamples` (338, 0x0152) tag.
+    pub photometric_samples: u16,
     pub sample_format: SampleFormat,
     pub photometric_interpretation: PhotometricInterpretation,
     pub compression_method: CompressionMethod,
@@ -76,21 +85,66 @@ pub(crate) struct Image {
     pub tile_attributes: Option<TileAttributes>,
     pub chunk_offsets: Vec<u64>,
     pub chunk_bytes: Vec<u64>,
+    pub chroma_subsampling: (u16, u16),
+}
+
+/// Describes how to read a tile-aligned portion of the image.
+#[derive(Clone)]
+pub(crate) struct ReadoutLayout {
+    /// The planar configuration, which applies to both the underlying image and the output buffer.
+    /// This may be relaxed if we find a clean enough way to provide it.
+    pub planar_config: PlanarConfiguration,
+
+    /// The sample interpretation (interpret with planar_config).
+    ///
+    /// FIXME: we should not require this here. The ability to turn out the raw bytes from the
+    /// sample arrays is very different from turning out interpretable color. Firstly we can always
+    /// readout `Multiband` but currently only use that ColorType in special circumstances (it must
+    /// not overlap cases where actually want to use a ColorType).
+    ///
+    /// And then we have CIE Lab, which uses a tuple of `(u8, i8, i8)`, that is still filterable
+    /// but still not represented by any of our `DecoderResult` variants. Other color variants
+    /// depend on extra tags (YCbCrCoefficients/0x0211) and we don't have a good side channel to
+    /// tag the output with all that TIFF specific information, so arguably we should process and
+    /// apply those to the data so it becomes a self-contained representation.
+    ///
+    /// This should be computed at a higher level, in `Decoder`, instead.
+    pub color: ColorType,
+    /// The number of bytes from one row to another.
+    pub minimum_row_stride: usize,
+    /// The format of samples (assumed uniform for now, same with depth of `ColorType`).
+    pub sample_format: SampleFormat,
+
+    /// Number of bytes to advance in output per row.
+    pub row_stride: usize,
+    /// Number of bytes to advance in output per chunk in width.
+    pub chunk_row_stride: usize,
+    /// Number of bytes to advance in output per chunk in height.
+    pub chunk_col_stride: usize,
+    /// Number of bytes in output from one plane to another.
+    pub plane_stride: usize,
+
+    /// Bits per sample in the encoded data.
+    pub tiff_bits_per_sample: u8,
+    /// Number of samples in the encoded data.
+    pub tiff_samples: u16,
+    /// Dimensions of the underlying rectangular chunks (tile or strips).
+    pub tiff_chunk_dimensions: (u32, u32),
+    /// Number of bytes in the underlying data with all samples per row of chunks.
+    pub tiff_row_bytes: usize,
+
+    /// Chunks until wrapping to the next row of chunks.
+    pub chunks_across: u32,
+    /// Chunks to advance to get to the next plane of chunks.
+    pub chunks_per_plane: u32,
 }
 
 impl Image {
     pub fn from_reader<R: Read + Seek>(
-        reader: &mut EndianReader<R>,
+        decoder: &mut ValueReader<R>,
         ifd: Directory,
-        limits: &Limits,
-        bigtiff: bool,
     ) -> TiffResult<Image> {
-        let mut tag_reader = TagReader {
-            reader,
-            limits,
-            ifd: &ifd,
-            bigtiff,
-        };
+        let mut tag_reader = TagReader { decoder, ifd: &ifd };
 
         let width = tag_reader.require_tag(Tag::ImageWidth)?.into_u32()?;
         let height = tag_reader.require_tag(Tag::ImageLength)?.into_u32()?;
@@ -115,7 +169,7 @@ impl Image {
         };
 
         let jpeg_tables = if compression_method == CompressionMethod::ModernJPEG
-            && ifd.contains_key(&Tag::JPEGTables)
+            && ifd.contains(Tag::JPEGTables)
         {
             let vec = tag_reader
                 .find_tag(Tag::JPEGTables)?
@@ -137,10 +191,30 @@ impl Image {
             .map(Value::into_u16)
             .transpose()?
             .unwrap_or(1);
+
         if samples == 0 {
             return Err(TiffFormatError::SamplesPerPixelIsZero.into());
         }
 
+        let extra_samples = match tag_reader.find_tag(Tag::ExtraSamples)? {
+            Some(n) => n.into_u16_vec()?,
+            None => vec![],
+        };
+
+        let extra_samples = extra_samples
+            .into_iter()
+            .map(|x| ExtraSamples::from_u16(x).unwrap_or(ExtraSamples::Unspecified))
+            .collect::<Vec<_>>();
+
+        let photometric_samples = match usize::from(samples).checked_sub(extra_samples.len()) {
+            None => {
+                return Err(TiffError::FormatError(
+                    TiffFormatError::InconsistentSizesEncountered,
+                ));
+            }
+            Some(n) => n as u16,
+        };
+
         let sample_format = match tag_reader.find_tag_uint_vec(Tag::SampleFormat)? {
             Some(vals) => {
                 let sample_format: Vec<_> = vals
@@ -148,12 +222,15 @@ impl Image {
                     .map(SampleFormat::from_u16_exhaustive)
                     .collect();
 
+                let Some(format) = sample_format.first().copied() else {
+                    // Reject empty sample formats
+                    return Err(TiffFormatError::InvalidTagValueType(Tag::SampleFormat).into());
+                };
                 // TODO: for now, only homogenous formats across samples are supported.
-                if !sample_format.windows(2).all(|s| s[0] == s[1]) {
+                if !sample_format.iter().all(|&s| s == format) {
                     return Err(TiffUnsupportedError::UnsupportedSampleFormat(sample_format).into());
                 }
-
-                sample_format[0]
+                format
             }
             None => SampleFormat::Uint,
         };
@@ -164,7 +241,7 @@ impl Image {
 
         // Technically bits_per_sample.len() should be *equal* to samples, but libtiff also allows
         // it to be a single value that applies to all samples.
-        if bits_per_sample.len() != samples.into() && bits_per_sample.len() != 1 {
+        if bits_per_sample.len() != usize::from(samples) && bits_per_sample.len() != 1 {
             return Err(TiffError::FormatError(
                 TiffFormatError::InconsistentSizesEncountered,
             ));
@@ -199,6 +276,25 @@ impl Image {
             .transpose()?
             .unwrap_or(PlanarConfiguration::Chunky);
 
+        let ycbcr_subsampling = tag_reader.find_tag_uint_vec::<u16>(Tag::ChromaSubsampling)?;
+
+        let chroma_subsampling = if let Some(subsamples) = &ycbcr_subsampling {
+            let [a, b] = subsamples.as_slice() else {
+                return Err(TiffError::FormatError(TiffFormatError::InvalidCountForTag(
+                    Tag::ChromaSubsampling,
+                    subsamples.len(),
+                )));
+            };
+
+            // ImageWidth and ImageLength are constrained to be integer multiples of
+            // YCbCrSubsampleHoriz and YCbCrSubsampleVert respectively. TileWidth and TileLength
+            // have the same constraints. RowsPerStrip must be an integer multiple of
+            // YCbCrSubsampleVert.
+            (*a, *b)
+        } else {
+            (2, 2)
+        };
+
         let planes = match planar_config {
             PlanarConfiguration::Chunky => 1,
             PlanarConfiguration::Planar => samples,
@@ -210,10 +306,10 @@ impl Image {
         let strip_decoder;
         let tile_attributes;
         match (
-            ifd.contains_key(&Tag::StripByteCounts),
-            ifd.contains_key(&Tag::StripOffsets),
-            ifd.contains_key(&Tag::TileByteCounts),
-            ifd.contains_key(&Tag::TileOffsets),
+            ifd.contains(Tag::StripByteCounts),
+            ifd.contains(Tag::StripOffsets),
+            ifd.contains(Tag::TileByteCounts),
+            ifd.contains(Tag::TileOffsets),
         ) {
             (true, true, false, false) => {
                 chunk_type = ChunkType::Strip;
@@ -297,6 +393,8 @@ impl Image {
             height,
             bits_per_sample: bits_per_sample[0],
             samples,
+            extra_samples,
+            photometric_samples,
             sample_format,
             photometric_interpretation,
             compression_method,
@@ -308,19 +406,24 @@ impl Image {
             tile_attributes,
             chunk_offsets,
             chunk_bytes,
+            chroma_subsampling,
         })
     }
 
     pub(crate) fn colortype(&self) -> TiffResult<ColorType> {
+        let is_alpha_extra_samples = matches!(
+            self.extra_samples.as_slice(),
+            [ExtraSamples::AssociatedAlpha, ..] | [ExtraSamples::UnassociatedAlpha, ..]
+        );
+
         match self.photometric_interpretation {
-            PhotometricInterpretation::RGB => match self.samples {
-                3 => Ok(ColorType::RGB(self.bits_per_sample)),
+            PhotometricInterpretation::RGB => match self.photometric_samples {
+                3 => Ok(if is_alpha_extra_samples {
+                    ColorType::RGBA(self.bits_per_sample)
+                } else {
+                    ColorType::RGB(self.bits_per_sample)
+                }),
                 4 => Ok(ColorType::RGBA(self.bits_per_sample)),
-                // FIXME: We should _ignore_ other components. In particular:
-                // > Beware of extra components. Some TIFF files may have more components per pixel
-                // than you think. A Baseline TIFF reader must skip over them gracefully,using the
-                // values of the SamplesPerPixel and BitsPerSample fields.
-                // > -- TIFF 6.0 Specification, Section 7, Additional Baseline requirements.
                 _ => Err(TiffError::UnsupportedError(
                     TiffUnsupportedError::InterpretationWithBits(
                         self.photometric_interpretation,
@@ -328,8 +431,12 @@ impl Image {
                     ),
                 )),
             },
-            PhotometricInterpretation::CMYK => match self.samples {
-                4 => Ok(ColorType::CMYK(self.bits_per_sample)),
+            PhotometricInterpretation::CMYK => match self.photometric_samples {
+                4 => Ok(if is_alpha_extra_samples {
+                    ColorType::CMYKA(self.bits_per_sample)
+                } else {
+                    ColorType::CMYK(self.bits_per_sample)
+                }),
                 5 => Ok(ColorType::CMYKA(self.bits_per_sample)),
                 _ => Err(TiffError::UnsupportedError(
                     TiffUnsupportedError::InterpretationWithBits(
@@ -338,7 +445,7 @@ impl Image {
                     ),
                 )),
             },
-            PhotometricInterpretation::YCbCr => match self.samples {
+            PhotometricInterpretation::YCbCr => match self.photometric_samples {
                 3 => Ok(ColorType::YCbCr(self.bits_per_sample)),
                 _ => Err(TiffError::UnsupportedError(
                     TiffUnsupportedError::InterpretationWithBits(
@@ -347,7 +454,13 @@ impl Image {
                     ),
                 )),
             },
+            // TODO: treatment of WhiteIsZero is not quite consistent with `invert_colors` that is
+            // later called when that interpretation is read. That function does not support
+            // Multiband as a color type and will error. It's unclear how to resolve that exactly.
             PhotometricInterpretation::BlackIsZero | PhotometricInterpretation::WhiteIsZero => {
+                // Note: compatibility with previous implementation requires us to return extra
+                // samples as `Multiband`. For gray images however the better choice would be
+                // returning a `Gray` color, i.e. matching on `photometric_samples` instead.
                 match self.samples {
                     1 => Ok(ColorType::Gray(self.bits_per_sample)),
                     _ => Ok(ColorType::Multiband {
@@ -356,24 +469,71 @@ impl Image {
                     }),
                 }
             }
-            // TODO: this is bad we should not fail at this point
-            PhotometricInterpretation::RGBPalette
-            | PhotometricInterpretation::TransparencyMask
-            | PhotometricInterpretation::CIELab => Err(TiffError::UnsupportedError(
+            // ```
+            // struct IccLab /* Interpretation 9* {
+            //     pub L: u8, // SampleFormat::Uint
+            //     pub a: u8, // SampleFormat::Uint, defined as TiffLab::a + 128
+            //     pub b: u8, // SampleFormat::Uint, defined as TiffLab::b + 128
+            // }
+            // ```
+            PhotometricInterpretation::IccLab => match self.photometric_samples {
+                3 if matches!(self.sample_format, SampleFormat::Uint) => {
+                    Ok(ColorType::Lab(self.bits_per_sample))
+                }
+                _ => Err(TiffError::UnsupportedError(
                     TiffUnsupportedError::InterpretationWithBits(
                         self.photometric_interpretation,
                         vec![self.bits_per_sample; self.samples as usize],
                     ),
                 )),
+            },
+            // Unsupported due to inherently heterogeneous sample types. This is represented as:
+            // ```
+            // struct TiffLab /* Interpretation 8* {
+            //     pub L: u8, // SampleFormat::Uint
+            //     pub a: i8, // SampleFormat::Int
+            //     pub b: i8, // SampleFormat::Int
+            // }
+            // ```
+            PhotometricInterpretation::CIELab => Err(TiffError::UnsupportedError(
+                TiffUnsupportedError::InterpretationWithBits(
+                    PhotometricInterpretation::CIELab,
+                    vec![self.bits_per_sample; self.samples as usize],
+                ),
+            )),
+            // Unsupported due to extra unfiltering and conversion steps. We need to find the
+            // Decode tag (SRATIONAL; 2 * SamplesPerPixel) and apply the following conversion:
+            //
+            // L* = Decode[0] + Lsample x (Decode[1] - Decode[0]) / (2^n -1)
+            // …
+            //
+            // So we'll have a larger depth in the output and either worry about reducing fractions
+            // or turn everything into floats. That's a lot of decisions.
+            PhotometricInterpretation::ItuLab => Err(TiffError::UnsupportedError(
+                TiffUnsupportedError::InterpretationWithBits(
+                    PhotometricInterpretation::CIELab,
+                    vec![self.bits_per_sample; self.samples as usize],
+                ),
+            )),
+            PhotometricInterpretation::RGBPalette | PhotometricInterpretation::TransparencyMask => {
+                Err(TiffError::UnsupportedError(
+                    TiffUnsupportedError::InterpretationWithBits(
+                        self.photometric_interpretation,
+                        vec![self.bits_per_sample; self.samples as usize],
+                    ),
+                ))
+            }
         }
     }
 
-    fn create_reader<'r, R: 'r + Read>(
+    fn create_reader<'r, R: 'r + Read + Seek>(
         reader: R,
-        #[allow(unused_variables)] photometric_interpretation: PhotometricInterpretation,
         compression_method: CompressionMethod,
         compressed_length: u64,
-        #[allow(unused_variables)] jpeg_tables: Option<&[u8]>,
+        // FIXME: these should be `expect` attributes or we choose another way of passing them.
+        #[cfg_attr(not(feature = "jpeg"), allow(unused_variables))] jpeg_tables: Option<&[u8]>,
+        #[cfg_attr(not(feature = "fax"), allow(unused_variables))] dimensions: (u32, u32),
+        #[cfg_attr(not(feature = "webp"), allow(unused_variables))] samples: u16,
     ) -> TiffResult<Box<dyn Read + 'r>> {
         Ok(match compression_method {
             CompressionMethod::None => Box::new(reader),
@@ -425,38 +585,36 @@ impl Image {
                 let mut jpeg_data = Vec::new();
                 jpeg_reader.read_to_end(&mut jpeg_data)?;
 
-                let mut decoder = zune_jpeg::JpegDecoder::new(jpeg_data);
+                let mut decoder =
+                    zune_jpeg::JpegDecoder::new(zune_core::bytestream::ZCursor::new(jpeg_data));
                 let mut options: zune_core::options::DecoderOptions = Default::default();
-                match photometric_interpretation {
-                    PhotometricInterpretation::RGB => {
-                        options =
-                            options.jpeg_set_out_colorspace(zune_core::colorspace::ColorSpace::RGB);
-                    }
-                    PhotometricInterpretation::CMYK => {
-                        options = options
-                            .jpeg_set_out_colorspace(zune_core::colorspace::ColorSpace::CMYK);
-                    }
-                    PhotometricInterpretation::YCbCr => {
-                        options = options
-                            .jpeg_set_out_colorspace(zune_core::colorspace::ColorSpace::YCbCr);
-                    }
-                    PhotometricInterpretation::WhiteIsZero
-                    | PhotometricInterpretation::BlackIsZero
-                    | PhotometricInterpretation::TransparencyMask => {}
-                    PhotometricInterpretation::RGBPalette | PhotometricInterpretation::CIELab => {
-                        return Err(TiffError::UnsupportedError(
-                            TiffUnsupportedError::UnsupportedInterpretation(
-                                photometric_interpretation,
-                            ),
-                        ));
-                    }
+
+                // Disable color conversion by setting the output colorspace to the input
+                // colorspace.
+                decoder.decode_headers()?;
+                if let Some(colorspace) = decoder.input_colorspace() {
+                    options = options.jpeg_set_out_colorspace(colorspace);
                 }
+
                 decoder.set_options(options);
 
                 let data = decoder.decode()?;
 
                 Box::new(Cursor::new(data))
             }
+            #[cfg(feature = "fax")]
+            CompressionMethod::Fax4 => Box::new(super::stream::Group4Reader::new(
+                dimensions,
+                reader,
+                compressed_length,
+            )?),
+            #[cfg(feature = "webp")]
+            CompressionMethod::WebP => Box::new(super::stream::WebPReader::new(
+                reader,
+                compressed_length,
+                samples,
+            )?),
+
             method => {
                 return Err(TiffError::UnsupportedError(
                     TiffUnsupportedError::UnsupportedCompressionMethod(method),
@@ -472,18 +630,25 @@ impl Image {
     /// Example with `bits_per_sample = [8, 8, 8]` and `PhotometricInterpretation::RGB`:
     /// * `PlanarConfiguration::Chunky` -> 3 (RGBRGBRGB...)
     /// * `PlanarConfiguration::Planar` -> 1 (RRR...) (GGG...) (BBB...)
-    pub(crate) fn samples_per_pixel(&self) -> usize {
+    pub(crate) fn samples_per_pixel(&self) -> u16 {
         match self.planar_config {
-            PlanarConfiguration::Chunky => self.samples.into(),
+            PlanarConfiguration::Chunky => self.samples,
+            PlanarConfiguration::Planar => 1,
+        }
+    }
+
+    pub(crate) fn samples_per_out_texel(&self, color: ColorType) -> u16 {
+        match self.planar_config {
+            PlanarConfiguration::Chunky => color.num_samples(),
             PlanarConfiguration::Planar => 1,
         }
     }
 
     /// Number of strips per pixel.
-    pub(crate) fn strips_per_pixel(&self) -> usize {
+    pub(crate) fn strips_per_pixel(&self) -> u16 {
         match self.planar_config {
             PlanarConfiguration::Chunky => 1,
-            PlanarConfiguration::Planar => self.samples.into(),
+            PlanarConfiguration::Planar => self.samples,
         }
     }
 
@@ -521,14 +686,107 @@ impl Image {
         }
     }
 
+    pub(crate) fn readout_for_image(&self) -> TiffResult<ReadoutLayout> {
+        let Image { width, height, .. } = *self;
+        self.readout_for_size(width, height)
+    }
+
+    /// Get the layout for reading out a tile-aligned portion of the image.
+    ///
+    /// The provided width and height should be less than or equal to the image dimensions.
+    pub(crate) fn readout_for_size(&self, width: u32, height: u32) -> TiffResult<ReadoutLayout> {
+        let color = self.colortype()?;
+
+        let tiff_samples = self.samples_per_pixel();
+        let tiff_bits_per_sample = self.bits_per_sample;
+        let data_samples = self.samples_per_out_texel(color);
+        let tiff_chunk_dimensions = self.chunk_dimensions()?;
+        let strips_per_pixel = self.strips_per_pixel();
+
+        let data_dimensions = (width, height);
+
+        let tiff_row_bits = (u64::from(tiff_chunk_dimensions.0) * u64::from(tiff_bits_per_sample))
+            .checked_mul(u64::from(tiff_samples))
+            .ok_or(TiffError::LimitsExceeded)?;
+        let tiff_row_bytes: usize = tiff_row_bits.div_ceil(8).try_into()?;
+
+        let chunk_row_bits = (u64::from(tiff_chunk_dimensions.0) * u64::from(tiff_bits_per_sample))
+            .checked_mul(u64::from(data_samples))
+            .ok_or(TiffError::LimitsExceeded)?;
+        let chunk_row_bytes: usize = chunk_row_bits.div_ceil(8).try_into()?;
+
+        let data_row_bits = (u64::from(data_dimensions.0) * u64::from(tiff_bits_per_sample))
+            .checked_mul(u64::from(data_samples))
+            .ok_or(TiffError::LimitsExceeded)?;
+        let data_row_bytes: usize = data_row_bits.div_ceil(8).try_into()?;
+
+        let chunk_col_stride: usize = data_row_bits
+            .div_ceil(8)
+            .checked_mul(u64::from(tiff_chunk_dimensions.1))
+            .ok_or(TiffError::LimitsExceeded)?
+            .try_into()?;
+
+        let plane_stride: usize = data_row_bits
+            .div_ceil(8)
+            .checked_mul(u64::from(data_dimensions.1))
+            .ok_or(TiffError::LimitsExceeded)?
+            .try_into()?;
+
+        let minimum_row_stride = data_row_bytes;
+
+        let chunks_across: u32 = data_dimensions.0.div_ceil(tiff_chunk_dimensions.0);
+        let chunks_per_plane = (self.chunk_offsets.len() as u32) / u32::from(strips_per_pixel);
+
+        // We would not get an offset in byte units, sorry, no bit interleaving in the output.
+        if chunks_across > 1 && chunk_row_bits % 8 != 0 {
+            return Err(TiffError::UnsupportedError(
+                TiffUnsupportedError::MisalignedTileBoundaries,
+            ));
+        }
+
+        // Only this color type interprets the tag, which is defined with a default of (2, 2)
+        if matches!(color, ColorType::YCbCr(_)) && self.chroma_subsampling != (1, 1) {
+            // The JPEG library does upsampling for us and defines its buffers correctly
+            // (presumably). All other compression schemes are not supported..
+            //
+            // NOTE: as explained in <fa225e820b96bef35f01bf4685654beeb4a8df0c> we may be better
+            // off supporting this tag by consistently upsampling, not by adjusting the buffer
+            // size. At least as a default this makes more sense and is much more permissive in
+            // case the compression stream disagrees with the tags (we would not have enough / or
+            // the wrong buffer layout if we only asked for subsampled planes in a planar layout).
+            if !matches!(self.compression_method, CompressionMethod::ModernJPEG) {
+                return Err(TiffError::UnsupportedError(
+                    TiffUnsupportedError::ChromaSubsampling,
+                ));
+            }
+        }
+
+        Ok(ReadoutLayout {
+            planar_config: self.planar_config,
+            color,
+            minimum_row_stride,
+            sample_format: self.sample_format,
+            row_stride: data_row_bytes,
+            chunk_row_stride: chunk_row_bytes,
+            chunk_col_stride,
+            plane_stride,
+            tiff_bits_per_sample,
+            tiff_samples,
+            tiff_chunk_dimensions,
+            tiff_row_bytes,
+            chunks_across,
+            chunks_per_plane,
+        })
+    }
+
     pub(crate) fn chunk_data_dimensions(&self, chunk_index: u32) -> TiffResult<(u32, u32)> {
         let dims = self.chunk_dimensions()?;
 
         match self.chunk_type {
             ChunkType::Strip => {
-                let strip_attrs = self.strip_decoder.as_ref().unwrap();
-                let strips_per_band =
-                    self.height.saturating_sub(1) / strip_attrs.rows_per_strip + 1;
+                let rows_per_strip = dims.1;
+                let strips_per_band = self.height.div_ceil(rows_per_strip);
+
                 let strip_height_without_padding = (chunk_index % strips_per_band)
                     .checked_mul(dims.1)
                     .and_then(|x| self.height.checked_sub(x))
@@ -555,15 +813,22 @@ impl Image {
 
     pub(crate) fn expand_chunk(
         &self,
-        reader: impl Read,
+        reader: &mut ValueReader<impl Read + Seek>,
         buf: &mut [u8],
-        output_row_stride: usize,
-        byte_order: ByteOrder,
+        layout: &ReadoutLayout,
         chunk_index: u32,
-        limits: &Limits,
     ) -> TiffResult<()> {
+        let ValueReader {
+            reader,
+            bigtiff: _,
+            limits,
+        } = reader;
+
+        let byte_order = reader.byte_order;
+
         // Validate that the color type is supported.
-        let color_type = self.colortype()?;
+        let color_type = layout.color;
+
         match color_type {
             ColorType::RGB(n)
             | ColorType::RGBA(n)
@@ -625,6 +890,7 @@ impl Image {
                 .ok_or(TiffError::FormatError(
                     TiffFormatError::InconsistentSizesEncountered,
                 ))?;
+
         if *compressed_bytes > limits.intermediate_buffer_size as u64 {
             return Err(TiffError::LimitsExceeded);
         }
@@ -632,38 +898,44 @@ impl Image {
         let compression_method = self.compression_method;
         let photometric_interpretation = self.photometric_interpretation;
         let predictor = self.predictor;
-        let samples = self.samples_per_pixel();
 
+        let samples = layout.tiff_samples;
+        let data_samples = layout.samples_per_out_texel();
+
+        // We have two dimensions: the 2d rectangle of encoded data and the 2d rectangle this
+        // takes up in the output. Each has an associated count of bits per pixel. The first
+        // dimension, i.e. a ''row'', is the number of pixels that are encoded with bit packing
+        // while the second is the byte-padded array of each so encoded slices.
+        //
+        // During decoding we map the relevant bits from one to the other.
         let chunk_dims = self.chunk_dimensions()?;
         let data_dims = self.chunk_data_dimensions(chunk_index)?;
 
-        let chunk_row_bits = (u64::from(chunk_dims.0) * u64::from(self.bits_per_sample))
-            .checked_mul(samples as u64)
-            .ok_or(TiffError::LimitsExceeded)?;
-        let chunk_row_bytes: usize = ((chunk_row_bits + 7) / 8).try_into()?;
-
-        let data_row_bits = (u64::from(data_dims.0) * u64::from(self.bits_per_sample))
-            .checked_mul(samples as u64)
-            .ok_or(TiffError::LimitsExceeded)?;
-        let data_row_bytes: usize = ((data_row_bits + 7) / 8).try_into()?;
+        let chunk_row_bytes: usize = layout.tiff_row_bytes;
+        let data_row_bytes: usize = layout.chunk_row_bytes(data_dims.0)?;
 
         // TODO: Should these return errors instead?
-        assert!(output_row_stride >= data_row_bytes);
-        assert!(buf.len() >= output_row_stride * (data_dims.1 as usize - 1) + data_row_bytes);
+        assert!(layout.minimum_row_stride >= data_row_bytes);
+        assert!(buf.len() >= layout.row_stride * (data_dims.1 as usize - 1) + data_row_bytes);
+
+        let is_all_bits = samples == data_samples;
+        let is_output_chunk_rows = layout.row_stride == chunk_row_bytes;
 
         let mut reader = Self::create_reader(
-            reader,
-            photometric_interpretation,
+            reader.inner(),
             compression_method,
             *compressed_bytes,
             self.jpeg_tables.as_deref().map(|a| &**a),
+            chunk_dims,
+            self.samples,
         )?;
 
-        if output_row_stride == chunk_row_bytes as usize {
+        if is_output_chunk_rows && is_all_bits {
+            // Here we can read directly into the output buffer itself.
             let tile = &mut buf[..chunk_row_bytes * data_dims.1 as usize];
             reader.read_exact(tile)?;
 
-            for row in tile.chunks_mut(chunk_row_bytes as usize) {
+            for row in tile.chunks_mut(chunk_row_bytes) {
                 super::fix_endianness_and_predict(
                     row,
                     color_type.bit_depth(),
@@ -672,14 +944,15 @@ impl Image {
                     predictor,
                 );
             }
+
             if photometric_interpretation == PhotometricInterpretation::WhiteIsZero {
-                super::invert_colors(tile, color_type, self.sample_format);
+                super::invert_colors(tile, color_type, self.sample_format)?;
             }
         } else if chunk_row_bytes > data_row_bytes && self.predictor == Predictor::FloatingPoint {
             // The floating point predictor shuffles the padding bytes into the encoded output, so
             // this case is handled specially when needed.
             let mut encoded = vec![0u8; chunk_row_bytes];
-            for row in buf.chunks_mut(output_row_stride).take(data_dims.1 as usize) {
+            for row in buf.chunks_mut(layout.row_stride).take(data_dims.1 as usize) {
                 reader.read_exact(&mut encoded)?;
 
                 let row = &mut row[..data_row_bytes];
@@ -690,18 +963,19 @@ impl Image {
                     _ => unreachable!(),
                 }
                 if photometric_interpretation == PhotometricInterpretation::WhiteIsZero {
-                    super::invert_colors(row, color_type, self.sample_format);
+                    super::invert_colors(row, color_type, self.sample_format)?;
                 }
             }
-        } else {
-            for (i, row) in buf
-                .chunks_mut(output_row_stride)
-                .take(data_dims.1 as usize)
-                .enumerate()
-            {
+        } else if is_all_bits {
+            // We read row-by-row but each row fits in its output buffer.
+            for row in buf.chunks_mut(layout.row_stride).take(data_dims.1 as usize) {
                 let row = &mut row[..data_row_bytes];
-                reader.read_exact(row)?;
+                let used = data_row_bytes.min(chunk_row_bytes);
 
+                // Two ways how we get here: we have more bytes in our chunk data than in the image
+                // we are to read. Then we need to skip the rest of the data. Or we have a bigger
+                // row stride than the chunk contains data, then we  need to fill only the front.
+                reader.read_exact(&mut row[..used])?;
                 // Skip horizontal padding
                 if chunk_row_bytes > data_row_bytes {
                     let len = u64::try_from(chunk_row_bytes - data_row_bytes)?;
@@ -715,12 +989,183 @@ impl Image {
                     byte_order,
                     predictor,
                 );
+
                 if photometric_interpretation == PhotometricInterpretation::WhiteIsZero {
-                    super::invert_colors(row, color_type, self.sample_format);
+                    super::invert_colors(row, color_type, self.sample_format)?;
+                }
+            }
+        } else {
+            // The encoded data potentially takes up more space than the output data so we must be
+            // prepared to discard some of it. That decision is bit-by-bit.
+            let bits_per_pixel = u32::from(self.bits_per_sample) * u32::from(self.samples);
+            // Assumes the photometric samples are always the start.. This is slightly problematic.
+            // To expand spport we should instead have different methods of transforming the read
+            // buffer data, not only the `compact_photometric_bytes` method below and then choose
+            // from the right one with supplied parameters. Then we can also bit-for-bit copy with
+            // a selection for better performance.
+            let photometric_bit_end = u32::from(self.bits_per_sample) * data_samples as u32;
+
+            debug_assert!(bits_per_pixel >= photometric_bit_end);
+
+            if bits_per_pixel % 8 != 0 || photometric_bit_end % 8 != 0 {
+                return Err(TiffError::UnsupportedError(
+                    TiffUnsupportedError::InterpretationWithBits(
+                        self.photometric_interpretation,
+                        vec![self.bits_per_sample; self.samples as usize],
+                    ),
+                ));
+            }
+
+            let photo_range = photometric_bit_end / 8..bits_per_pixel / 8;
+            let mut encoded = vec![0u8; chunk_row_bytes];
+            for row in buf.chunks_mut(layout.row_stride).take(data_dims.1 as usize) {
+                reader.read_exact(&mut encoded)?;
+
+                Self::compact_photometric_bytes(&mut encoded, row, &photo_range);
+
+                super::fix_endianness_and_predict(
+                    row,
+                    color_type.bit_depth(),
+                    samples,
+                    byte_order,
+                    predictor,
+                );
+
+                if photometric_interpretation == PhotometricInterpretation::WhiteIsZero {
+                    super::invert_colors(row, color_type, self.sample_format)?;
                 }
             }
         }
 
         Ok(())
     }
+
+    /// Turn a contiguous buffer of a whole number of raw sample arrays into a whole number of
+    /// photometric sample arrays by removing the extra samples in-between.
+    fn compact_photometric_bytes(
+        raw: &mut [u8],
+        row: &mut [u8],
+        photo_range: &std::ops::Range<u32>,
+    ) {
+        raw.chunks_exact_mut(photo_range.end as usize)
+            .zip(row.chunks_exact_mut(photo_range.start as usize))
+            .for_each(|(src, dst)| {
+                dst.copy_from_slice(&src[..photo_range.start as usize]);
+            });
+    }
+}
+
+impl ReadoutLayout {
+    pub(crate) fn samples_per_out_texel(&self) -> u16 {
+        match self.planar_config {
+            PlanarConfiguration::Chunky => self.color.num_samples(),
+            PlanarConfiguration::Planar => 1,
+        }
+    }
+
+    // For a concrete chunk, which may be a partial border chunk, the byte length of one row of its
+    // pixel data.
+    pub(crate) fn chunk_row_bytes(&self, width: u32) -> TiffResult<usize> {
+        let data_samples = self.samples_per_out_texel();
+        let data_row_bits = (u64::from(width) * u64::from(self.tiff_bits_per_sample))
+            .checked_mul(u64::from(data_samples))
+            .ok_or(TiffError::LimitsExceeded)?;
+        Ok(data_row_bits.div_ceil(8).try_into()?)
+    }
+
+    pub(crate) fn set_row_stride(&mut self, row_stride: usize) -> Result<(), TiffError> {
+        if row_stride < self.minimum_row_stride {
+            return Err(TiffError::UsageError(
+                UsageError::InsufficientOutputRowStride {
+                    needed: self.minimum_row_stride,
+                    requested: row_stride,
+                },
+            ));
+        }
+
+        let data_row_bytes = u64::try_from(row_stride)?;
+
+        let chunk_col_stride = data_row_bytes
+            .checked_mul(u64::from(self.tiff_chunk_dimensions.1))
+            .ok_or(TiffError::LimitsExceeded)?
+            .try_into()?;
+
+        let height = self.plane_stride.checked_div(self.row_stride);
+
+        let plane_stride = height
+            .and_then(|h| data_row_bytes.checked_mul(h as u64))
+            // If height was zero, or the previous stride was zero, there are no bytes in a plane
+            .unwrap_or(0)
+            .try_into()?;
+
+        self.row_stride = row_stride;
+        self.chunk_col_stride = chunk_col_stride;
+        self.plane_stride = plane_stride;
+
+        Ok(())
+    }
+
+    /// Reduce this down to the layout of the output planes.
+    pub(crate) fn to_plane_layout(&self) -> Result<PlaneLayout, TiffError> {
+        let num_planes = self.color.num_samples() / self.samples_per_out_texel();
+
+        // Using the standard range iterator as checked_add on steroids.
+        //
+        // Note: for supporting subsampling, adjust as required.
+        let mut offset = (0..=usize::MAX).step_by(self.plane_stride);
+
+        let plane_offsets = offset.by_ref().take(usize::from(num_planes)).collect();
+
+        // Get the past-the-end of the last plane.
+        //
+        // This also verifies the `take` above was not short.
+        let Some(total_bytes) = offset.next() else {
+            return Err(TiffError::LimitsExceeded);
+        };
+
+        Ok(PlaneLayout {
+            plane_offsets,
+            total_bytes,
+            readout: self.clone(),
+        })
+    }
+}
+
+/// A `ReadoutLayout` with pre-calculated plane information.
+pub(crate) struct PlaneLayout {
+    /// The underlying readout layout.
+    pub readout: ReadoutLayout,
+    /// Buffer offset from one plane of output to the next.
+    pub plane_offsets: Vec<usize>,
+    /// Total number of bytes for all planes in given order.
+    pub total_bytes: usize,
+}
+
+impl PlaneLayout {
+    /// Return the number of planes to extract into the provided buffer.
+    pub(crate) fn used_planes(&self, buffer: &[impl Sized]) -> TiffResult<u16> {
+        self.readout.assert_min_layout(buffer)?;
+        let buffer_len = core::mem::size_of_val(buffer);
+
+        // Note: with differently sized planes this is dependent on the plane.
+        let last_plane_start = buffer_len.checked_sub(self.readout.plane_stride);
+
+        // Find how many planes fit into the output buffer.
+        let used_plane_offsets = self
+            .plane_offsets
+            .iter()
+            .enumerate()
+            // Find the first plane that would not fit completely at its offset.
+            .skip_while(|(_, &offset)| last_plane_start >= Some(offset))
+            .nth(0)
+            // If all planes fit, use all of them.
+            .map_or(self.plane_offsets.len(), |(idx, _)| idx);
+
+        debug_assert!(
+            used_plane_offsets <= usize::from(u16::MAX),
+            "Planes limited by number of samples, which is encoded as u16"
+        );
+
+        Ok(used_plane_offsets as u16)
+    }
 }

src/decoder/stream.rs

@@ -1,15 +1,9 @@
 //! All IO functionality needed for TIFF decoding
-
+#[cfg(feature = "webp")]
+use std::io::Cursor;
 use std::io::{self, BufRead, BufReader, Read, Seek, Take};
 
-/// Byte order of the TIFF file.
-#[derive(Clone, Copy, Debug)]
-pub enum ByteOrder {
-    /// little endian byte order
-    LittleEndian,
-    /// big endian byte order
-    BigEndian,
-}
+pub use crate::tags::ByteOrder;
 
 /// Reader that is aware of the byte order.
 #[derive(Debug)]
@@ -18,7 +12,7 @@ pub struct EndianReader<R> {
     pub(crate) byte_order: ByteOrder,
 }
 
-impl<R: Read + Seek> EndianReader<R> {
+impl<R: Read> EndianReader<R> {
     pub fn new(reader: R, byte_order: ByteOrder) -> Self {
         Self { reader, byte_order }
     }
@@ -27,7 +21,10 @@ impl<R: Read + Seek> EndianReader<R> {
         &mut self.reader
     }
 
-    pub fn goto_offset(&mut self, offset: u64) -> io::Result<()> {
+    pub fn goto_offset(&mut self, offset: u64) -> io::Result<()>
+    where
+        R: Seek,
+    {
         self.reader.seek(io::SeekFrom::Start(offset))?;
         Ok(())
     }
@@ -43,17 +40,6 @@ impl<R: Read + Seek> EndianReader<R> {
         })
     }
 
-    /// Reads an i8
-    #[inline(always)]
-    pub fn read_i8(&mut self) -> Result<i8, io::Error> {
-        let mut n = [0u8; 1];
-        self.reader.read_exact(&mut n)?;
-        Ok(match self.byte_order {
-            ByteOrder::LittleEndian => i8::from_le_bytes(n),
-            ByteOrder::BigEndian => i8::from_be_bytes(n),
-        })
-    }
-
     /// Reads an i16
     #[inline(always)]
     pub fn read_i16(&mut self) -> Result<i16, io::Error> {
@@ -132,20 +118,17 @@ impl<R: Read + Seek> EndianReader<R> {
     }
 }
 
-///
-/// # READERS
-///
-
-///
-/// ## Deflate Reader
-///
+//
+// # READERS
+//
 
+/// Type alias for the deflate Reader
 #[cfg(feature = "deflate")]
 pub type DeflateReader<R> = flate2::read::ZlibDecoder<R>;
 
-///
-/// ## LZW Reader
-///
+//
+// ## LZW Reader
+//
 
 /// Reader that decompresses LZW streams
 #[cfg(feature = "lzw")]
@@ -158,12 +141,15 @@ pub struct LZWReader<R: Read> {
 impl<R: Read> LZWReader<R> {
     /// Wraps a reader
     pub fn new(reader: R, compressed_length: usize) -> LZWReader<R> {
+        let configuration =
+            weezl::decode::Configuration::with_tiff_size_switch(weezl::BitOrder::Msb, 8)
+                .with_yield_on_full_buffer(true);
         Self {
             reader: BufReader::with_capacity(
                 (32 * 1024).min(compressed_length),
                 reader.take(u64::try_from(compressed_length).unwrap()),
             ),
-            decoder: weezl::decode::Decoder::with_tiff_size_switch(weezl::BitOrder::Msb, 8),
+            decoder: configuration.build(),
         }
     }
 }
@@ -184,9 +170,6 @@ impl<R: Read> Read for LZWReader<R> {
                     }
                 }
                 Ok(weezl::LzwStatus::NoProgress) => {
-                    assert_eq!(result.consumed_in, 0);
-                    assert_eq!(result.consumed_out, 0);
-                    assert!(self.reader.buffer().is_empty());
                     return Err(io::Error::new(
                         io::ErrorKind::UnexpectedEof,
                         "no lzw end code found",
@@ -201,10 +184,11 @@ impl<R: Read> Read for LZWReader<R> {
     }
 }
 
-///
-/// ## PackBits Reader
-///
+//
+// ## PackBits Reader
+//
 
+/// Internal state machine for the PackBitsReader.
 enum PackBitsReaderState {
     Header,
     Literal,
@@ -272,6 +256,161 @@ impl<R: Read> Read for PackBitsReader<R> {
     }
 }
 
+#[cfg(feature = "fax")]
+pub struct Group4Reader<R: Read> {
+    decoder: fax34::decoder::Group4Decoder<io::Bytes<io::BufReader<io::Take<R>>>>,
+    line_buf: io::Cursor<Vec<u8>>,
+    height: u16,
+    width: u16,
+    y: u16,
+}
+
+#[cfg(feature = "fax")]
+impl<R: Read> Group4Reader<R> {
+    pub fn new(
+        dimensions: (u32, u32),
+        reader: R,
+        compressed_length: u64,
+    ) -> crate::TiffResult<Self> {
+        let width = u16::try_from(dimensions.0)?;
+        let height = u16::try_from(dimensions.1)?;
+
+        Ok(Self {
+            decoder: fax34::decoder::Group4Decoder::new(
+                io::BufReader::new(reader.take(compressed_length)).bytes(),
+                width,
+            )?,
+            line_buf: io::Cursor::new(Vec::with_capacity(width.into())),
+            width,
+            height,
+            y: 0,
+        })
+    }
+}
+
+#[cfg(feature = "fax")]
+impl<R: Read> Read for Group4Reader<R> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        // Either we have not read any line or we are at the end of a line.
+        if self.line_buf.position() as usize == self.line_buf.get_ref().len()
+            && self.y < self.height
+        {
+            let next = self.decoder.advance().map_err(std::io::Error::other)?;
+
+            match next {
+                fax34::decoder::DecodeStatus::End => (),
+                fax34::decoder::DecodeStatus::Incomplete => {
+                    self.y += 1;
+
+                    // We known `transitions` yields exactly `self.width` items (per doc).
+                    // FIXME: performance. We do not need an individual pixel iterator, filling
+                    // memory especially for long runs can be much quicker. The `transitions` are
+                    // the positions at which each run-length ends, i.e. the prefix sum of run
+                    // lengths. Runs in fax4 start with white.
+                    let transitions = fax34::decoder::pels(self.decoder.transition(), self.width);
+
+                    let buffer = self.line_buf.get_mut();
+                    buffer.resize(usize::from(self.width).div_ceil(8), 0u8);
+
+                    let target = &mut buffer[..];
+
+                    // Note: it may seem strange to treat black as 0b1 and white as 0b0 despite all
+                    // our streams by default decoding as-if PhotometricInterpretation::BlackIsMin.
+                    // This is however consistent with libtiff. It seems that fax4's "White"
+                    // differs from what libtiff thinks of as "White". For content, a line of data
+                    // is in runlength encoding of white-black-white-black always starting with
+                    // white. In libtiff, the loop always does both colors in one go and the
+                    // structure is:
+                    //
+                    // ```
+                    // for (; runs < erun; runs += 2)
+                    //   // white run
+                    //       do { *lp++ = 0L; } while (…)
+                    //   // black run
+                    //       do { *lp++ = -1L; } while (…)
+                    // ```
+                    //
+                    // So indeed the Fax4::White run is implemented by filling with zeros.
+                    let mut bits = transitions.map(|c| match c {
+                        fax34::Color::Black => true,
+                        fax34::Color::White => false,
+                    });
+
+                    // Assemble bits in MSB as per our library representation for buffer.
+                    for byte in target {
+                        let mut val = 0;
+
+                        for (idx, bit) in bits.by_ref().take(8).enumerate() {
+                            val |= u8::from(bit) << (7 - idx % 8);
+                        }
+
+                        *byte = val;
+                    }
+
+                    self.line_buf.set_position(0);
+                }
+            }
+        }
+
+        self.line_buf.read(buf)
+    }
+}
+
+#[cfg(feature = "webp")]
+pub struct WebPReader {
+    inner: Cursor<Vec<u8>>,
+}
+
+#[cfg(feature = "webp")]
+impl WebPReader {
+    pub fn new<R: Read + Seek>(
+        reader: R,
+        compressed_length: u64,
+        samples: u16,
+    ) -> crate::TiffResult<Self> {
+        let mut decoder =
+            image_webp::WebPDecoder::new(io::BufReader::new(reader.take(compressed_length)))
+                .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;
+
+        if !(samples == 4 || (samples == 3 && !decoder.has_alpha())) {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidData,
+                "bad sample count for WebP compressed data",
+            )
+            .into());
+        }
+
+        let total_bytes =
+            samples as usize * decoder.dimensions().0 as usize * decoder.dimensions().1 as usize;
+        let mut data = vec![0; total_bytes];
+
+        decoder
+            .read_image(&mut data)
+            .map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;
+
+        // Add a fully opaque alpha channel if needed
+        if samples == 4 && !decoder.has_alpha() {
+            for i in (0..(total_bytes / 4)).rev() {
+                data[i * 4 + 3] = 255;
+                data[i * 4 + 2] = data[i * 3 + 2];
+                data[i * 4 + 1] = data[i * 3 + 1];
+                data[i * 4] = data[i * 3];
+            }
+        }
+
+        Ok(Self {
+            inner: Cursor::new(data),
+        })
+    }
+}
+
+#[cfg(feature = "webp")]
+impl Read for WebPReader {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        self.inner.read(buf)
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;

src/decoder/tag_reader.rs

@@ -1,25 +1,85 @@
 use std::io::{Read, Seek};
 
-use crate::tags::Tag;
-use crate::{TiffError, TiffFormatError, TiffResult};
+use crate::{
+    tags::{Tag, ValueBuffer},
+    Directory, TiffError, TiffFormatError, TiffResult,
+};
 
-use super::ifd::{Directory, Value};
-use super::stream::EndianReader;
-use super::Limits;
+use super::ifd::{Entry, Value};
+use super::ValueReader;
 
-pub(crate) struct TagReader<'a, R: Read + Seek> {
-    pub reader: &'a mut EndianReader<R>,
-    pub ifd: &'a Directory,
-    pub limits: &'a Limits,
-    pub bigtiff: bool,
+pub(crate) struct TagReader<'lt, R: EntryDecoder + ?Sized> {
+    pub(crate) decoder: &'lt mut R,
+    pub(crate) ifd: &'lt Directory,
 }
-impl<'a, R: Read + Seek> TagReader<'a, R> {
-    pub(crate) fn find_tag(&mut self, tag: Tag) -> TiffResult<Option<Value>> {
-        Ok(match self.ifd.get(&tag) {
-            Some(entry) => Some(entry.clone().val(self.limits, self.bigtiff, self.reader)?),
-            None => None,
-        })
+
+pub(crate) trait EntryDecoder {
+    /// Turn an entry into a value by fetching the required bytes in the stream.
+    fn entry_val(&mut self, entry: &Entry) -> TiffResult<Value>;
+    /// Fill a  `ValueBuffer` by fetching the required bytes.
+    fn entry_buf(&mut self, entry: &Entry, _: &mut ValueBuffer) -> TiffResult<()>;
+    /// Fill the prefix of the buffer and return the actual bytes. Start interpreting the inner
+    /// value at `offset`.
+    fn entry_raw(&mut self, entry: &Entry, _: &mut [u8], offset: u64) -> TiffResult<usize>;
+}
+
+impl<R: Seek + Read> EntryDecoder for ValueReader<R> {
+    fn entry_val(&mut self, entry: &Entry) -> TiffResult<Value> {
+        entry.val(&self.limits, self.bigtiff, &mut self.reader)
     }
+
+    fn entry_buf(&mut self, entry: &Entry, buf: &mut ValueBuffer) -> TiffResult<()> {
+        entry.buffered_value(buf, &self.limits, self.bigtiff, &mut self.reader)
+    }
+
+    fn entry_raw(&mut self, entry: &Entry, buf: &mut [u8], offset: u64) -> TiffResult<usize> {
+        entry.raw_value_at(buf, self.bigtiff, &mut self.reader, offset)
+    }
+}
+
+impl<'a, R: EntryDecoder + ?Sized> TagReader<'a, R> {
+    pub(crate) fn find_tag(&mut self, tag: Tag) -> TiffResult<Option<Value>> {
+        let entry = match self.ifd.get(tag) {
+            None => return Ok(None),
+            Some(entry) => entry.clone(),
+        };
+
+        self.decoder.entry_val(&entry).map(Some)
+    }
+
+    /// Find a tag and fill the provided buffer with its raw data.
+    ///
+    /// Returns the entry of the tag if found. This signals that the buffer was overwritten.
+    /// Otherwise the buffer is preserved as-is.
+    pub(crate) fn find_tag_buf(
+        &mut self,
+        tag: Tag,
+        buf: &mut ValueBuffer,
+    ) -> TiffResult<Option<Entry>> {
+        let entry = match self.ifd.get(tag) {
+            None => return Ok(None),
+            Some(entry) => entry.clone(),
+        };
+
+        self.decoder.entry_buf(&entry, buf)?;
+        Ok(Some(entry))
+    }
+
+    pub(crate) fn find_tag_raw(
+        &mut self,
+        tag: Tag,
+        buf: &mut [u8],
+        at: u64,
+    ) -> TiffResult<Option<usize>> {
+        let entry = match self.ifd.get(tag) {
+            None => return Ok(None),
+            Some(entry) => entry.clone(),
+        };
+
+        let len = self.decoder.entry_raw(&entry, buf, at)?;
+        Ok(Some(len))
+    }
+
     pub(crate) fn require_tag(&mut self, tag: Tag) -> TiffResult<Value> {
         match self.find_tag(tag)? {
             Some(val) => Ok(val),
@@ -28,6 +88,7 @@ impl<'a, R: Read + Seek> TagReader<'a, R> {
             )),
         }
     }
+
     pub fn find_tag_uint_vec<T: TryFrom<u64>>(&mut self, tag: Tag) -> TiffResult<Option<Vec<T>>> {
         self.find_tag(tag)?
             .map(|v| v.into_u64_vec())

src/directory.rs

@@ -0,0 +1,196 @@
+use core::fmt;
+use std::{collections::BTreeMap, num::NonZeroU64};
+
+use crate::{
+    decoder::ifd::Entry,
+    tags::{IfdPointer, Tag},
+};
+
+/// An Image File Directory (IFD).
+///
+/// A directory is a map of [`Tag`]s to [`Value`](crate::decoder::ifd::Value)s. The values are
+/// stored anywhere in the file, with the directory containing the offsets and length of the
+/// associated values for each tag present in the directory.
+///
+/// A directory can be created with with
+/// [`Decoder::read_directory`](crate::decoder::Decoder::read_directory) or as an empty directory
+/// to be extended with entries. A directory may be used with
+/// [`Decoder::read_directory_tags`](crate::decoder::Decoder::read_directory_tags) to read the
+/// values associated with tags from an underlying file.
+#[doc(alias = "IFD")]
+pub struct Directory {
+    /// There are at most `u16::MAX` entries in any single directory, the count is stored as a
+    /// 2-byte value. The order in the file is implied to be ascending by tag value (the decoder
+    /// does not mind unordered entries).
+    pub(crate) entries: BTreeMap<u16, Entry>,
+    pub(crate) next_ifd: Option<NonZeroU64>,
+}
+
+impl Directory {
+    /// Create a directory in an initial state without entries. Note that an empty directory can
+    /// not be encoded in a file, it must contain at least one entry.
+    pub fn empty() -> Self {
+        Directory {
+            entries: BTreeMap::new(),
+            next_ifd: None,
+        }
+    }
+
+    /// Retrieve the value associated with a tag.
+    pub fn get(&self, tag: Tag) -> Option<&Entry> {
+        self.entries.get(&tag.to_u16())
+    }
+
+    /// Check if the directory contains a specified tag.
+    pub fn contains(&self, tag: Tag) -> bool {
+        self.entries.contains_key(&tag.to_u16())
+    }
+
+    /// Iterate over all known and unknown tags in this directory.
+    pub fn iter(&self) -> impl Iterator<Item = (Tag, &Entry)> + '_ {
+        self.entries
+            .iter()
+            .map(|(k, v)| (Tag::from_u16_exhaustive(*k), v))
+    }
+
+    /// Insert additional entries into the directory.
+    ///
+    /// Note that a directory can contain at most `u16::MAX` values. There may be one entry that
+    /// does not fit into the directory. This entry is silently ignored (please check [`Self::len`]
+    /// to detect the condition). Providing a tag multiple times or a tag that already exists
+    /// within this directory overwrites the entry.
+    pub fn extend(&mut self, iter: impl IntoIterator<Item = (Tag, Entry)>) {
+        // Code size conscious extension, avoid monomorphic extensions with the assumption of these
+        // not being performance sensitive in practice. (Maybe we have a polymorphic interface for
+        // the crate usage in the future.
+        self.extend_inner(iter.into_iter().by_ref())
+    }
+
+    /// Get the number of entries.
+    pub fn len(&self) -> usize {
+        // The keys are `u16`. Since IFDs are required to have at least one entry this would have
+        // been a trivial thing to do in the specification by storing it minus one but alas. In
+        // BigTIFF the count is stored as 8-bit anyways.
+        self.entries.len()
+    }
+
+    /// Check if there are any entries in this directory. Note that an empty directory can not be
+    /// encoded in the file, it must contain at least one entry.
+    pub fn is_empty(&self) -> bool {
+        self.entries.is_empty()
+    }
+
+    /// Get the pointer to the next IFD, if it was defined.
+    pub fn next(&self) -> Option<IfdPointer> {
+        self.next_ifd.map(|n| IfdPointer(n.get()))
+    }
+
+    pub fn set_next(&mut self, next: Option<IfdPointer>) {
+        self.next_ifd = next.and_then(|n| NonZeroU64::new(n.0));
+    }
+
+    fn extend_inner(&mut self, iter: &mut dyn Iterator<Item = (Tag, Entry)>) {
+        for (tag, entry) in iter {
+            // If the tag is already present, it will be overwritten.
+            let map_entry = self.entries.entry(tag.to_u16());
+
+            match map_entry {
+                std::collections::btree_map::Entry::Vacant(vacant_entry) => {
+                    vacant_entry.insert(entry);
+                }
+                std::collections::btree_map::Entry::Occupied(mut occupied_entry) => {
+                    occupied_entry.insert(entry);
+                }
+            }
+        }
+    }
+
+    pub(crate) fn encoded_len<K: crate::encoder::TiffKind>(&self) -> u64 {
+        let (len_count, len_offset);
+
+        match std::mem::size_of::<K::OffsetType>() {
+            offset @ 4 => {
+                // non-BigTiff uses a `u16` here.
+                len_count = 2;
+                len_offset = offset as u64;
+            }
+            offset => {
+                let offset = offset as u64;
+                // assuming BigTIFF
+                len_count = offset;
+                len_offset = offset;
+            }
+        }
+
+        // Each entry is 12 bytes in classic TIFF, 20 bytes in BigTIFF.
+        let entry_bytes = 4 + 2 * len_offset;
+        len_count + entry_bytes * self.entries.len() as u64 + len_offset
+    }
+}
+
+impl fmt::Debug for Directory {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("Directory")
+            .field(
+                "entries",
+                &self.entries.iter().map(|(k, v)| (Tag::from_u16(*k), v)),
+            )
+            .field("next_ifd", &self.next_ifd)
+            .finish()
+    }
+}
+
+impl core::iter::FromIterator<(Tag, Entry)> for Directory {
+    fn from_iter<T: IntoIterator<Item = (Tag, Entry)>>(iter: T) -> Self {
+        let mut dir = Directory::empty();
+        dir.extend(iter);
+        dir
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Directory;
+    use crate::{decoder::ifd::Entry, tags::Tag};
+
+    #[test]
+    fn directory_multiple_entries() {
+        let mut dir = Directory::empty();
+        assert_eq!(dir.len(), 0);
+
+        dir.extend((0..=u16::MAX).map(|i| {
+            let tag = Tag::Unknown(1);
+            let entry = Entry::new_u64(crate::tags::Type::BYTE, i.into(), [0; 8]);
+            (tag, entry)
+        }));
+
+        assert_eq!(dir.len(), 1, "Only one tag was ever modified");
+
+        assert_eq!(
+            dir.get(Tag::Unknown(1))
+                .expect("tag 1 should be present after this chain")
+                .count(),
+            u16::MAX.into()
+        );
+    }
+
+    #[test]
+    fn iteration_order() {
+        let mut dir = Directory::empty();
+        assert_eq!(dir.len(), 0);
+
+        let fake_entry = Entry::new_u64(crate::tags::Type::BYTE, 0, [0; 8]);
+        dir.extend((0..32).map(|i| {
+            let tag = Tag::Unknown(i);
+            let entry = fake_entry.clone();
+            (tag, entry)
+        }));
+
+        let iter_order: Vec<u16> = dir.iter().map(|(tag, _e)| tag.to_u16()).collect();
+        assert_eq!(
+            iter_order,
+            (0..32).collect::<Vec<_>>(),
+            "Tags must be in ascending order according to the specification"
+        );
+    }
+}

src/encoder/compression/deflate.rs

@@ -11,21 +11,17 @@ pub struct Deflate {
 /// It allows trading compression ratio for compression speed.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 #[non_exhaustive]
+#[derive(Default)]
 pub enum DeflateLevel {
     /// The fastest possible compression mode.
     Fast = 1,
     /// The conserative choice between speed and ratio.
+    #[default]
     Balanced = 6,
     /// The best compression available with Deflate.
     Best = 9,
 }
 
-impl Default for DeflateLevel {
-    fn default() -> Self {
-        DeflateLevel::Balanced
-    }
-}
-
 impl Deflate {
     /// Create a new deflate compressor with a specific level of compression.
     pub fn with_level(level: DeflateLevel) -> Self {

src/encoder/compression/uncompressed.rs

@@ -14,7 +14,7 @@ impl Compression for Uncompressed {
 
 impl CompressionAlgorithm for Uncompressed {
     fn write_to<W: Write>(&mut self, writer: &mut W, bytes: &[u8]) -> Result<u64, io::Error> {
-        writer.write(bytes).map(|byte_count| byte_count as u64)
+        writer.write_all(bytes).map(|()| bytes.len() as u64)
     }
 }
 

src/encoder/mod.rs

@@ -2,17 +2,20 @@ pub use tiff_value::*;
 
 use std::{
     cmp,
-    collections::BTreeMap,
     io::{self, Seek, Write},
     marker::PhantomData,
     mem,
-    num::TryFromIntError,
+    num::{NonZeroU64, TryFromIntError},
 };
 
 use crate::{
+    decoder::ifd::Entry,
     error::{TiffResult, UsageError},
-    tags::{CompressionMethod, ResolutionUnit, SampleFormat, Tag},
-    TiffError, TiffFormatError,
+    tags::{
+        ByteOrder, CompressionMethod, ExtraSamples, IfdPointer, PhotometricInterpretation,
+        ResolutionUnit, SampleFormat, Tag, Type, ValueBuffer,
+    },
+    Directory, TiffError, TiffFormatError,
 };
 
 pub mod colortype;
@@ -37,8 +40,9 @@ pub type Predictor = crate::tags::Predictor;
 #[cfg(feature = "deflate")]
 pub type DeflateLevel = compression::DeflateLevel;
 
-#[derive(Clone, Copy, PartialEq)]
+#[derive(Clone, Copy, PartialEq, Default)]
 pub enum Compression {
+    #[default]
     Uncompressed,
     #[cfg(feature = "lzw")]
     Lzw,
@@ -47,12 +51,6 @@ pub enum Compression {
     Packbits,
 }
 
-impl Default for Compression {
-    fn default() -> Self {
-        Self::Uncompressed
-    }
-}
-
 impl Compression {
     fn tag(&self) -> CompressionMethod {
         match self {
@@ -79,10 +77,10 @@ impl Compression {
 
 /// Encoder for Tiff and BigTiff files.
 ///
-/// With this type you can get a `DirectoryEncoder` or a `ImageEncoder`
+/// With this type you can get a [`DirectoryEncoder`] or a [`ImageEncoder`]
 /// to encode Tiff/BigTiff ifd directories with images.
 ///
-/// See `DirectoryEncoder` and `ImageEncoder`.
+/// See [`DirectoryEncoder`] and [`ImageEncoder`].
 ///
 /// # Examples
 /// ```
@@ -107,6 +105,8 @@ pub struct TiffEncoder<W, K: TiffKind = TiffKindStandard> {
     kind: PhantomData<K>,
     predictor: Predictor,
     compression: Compression,
+    /// The offset of the last main image directory's `next` field.
+    last_ifd_chain: NonZeroU64,
 }
 
 /// Constructor functions to create standard Tiff files.
@@ -135,16 +135,19 @@ impl<W: Write + Seek> TiffEncoder<W, TiffKindBig> {
 impl<W: Write + Seek, K: TiffKind> TiffEncoder<W, K> {
     /// Creates a new Tiff or BigTiff encoder, inferred from the return type.
     pub fn new_generic(writer: W) -> TiffResult<Self> {
-        let mut encoder = TiffEncoder {
-            writer: TiffWriter::new(writer),
+        let mut writer = TiffWriter::new(writer);
+        K::write_header(&mut writer)?;
+
+        let last_ifd_chain = NonZeroU64::new(writer.previous_ifd_pointer::<K>())
+            .expect("Header is at a non-zero offset");
+
+        Ok(TiffEncoder {
+            writer,
             kind: PhantomData,
             predictor: Predictor::None,
             compression: Compression::Uncompressed,
-        };
-
-        K::write_header(&mut encoder.writer)?;
-
-        Ok(encoder)
+            last_ifd_chain,
+        })
     }
 
     /// Set the predictor to use
@@ -165,8 +168,27 @@ impl<W: Write + Seek, K: TiffKind> TiffEncoder<W, K> {
     }
 
     /// Create a [`DirectoryEncoder`] to encode an ifd directory.
-    pub fn new_directory(&mut self) -> TiffResult<DirectoryEncoder<W, K>> {
-        DirectoryEncoder::new(&mut self.writer)
+    #[deprecated = "`image_directory` replaced the old behavior and clarifies the intent"]
+    #[doc(hidden)]
+    pub fn new_directory(&mut self) -> TiffResult<DirectoryEncoder<'_, W, K>> {
+        Self::chain_directory(&mut self.writer, &mut self.last_ifd_chain)
+    }
+
+    /// Create a [`DirectoryEncoder`] to encode an ifd (directory).
+    ///
+    /// The caller is responsible for ensuring that the directory is a valid image in the main TIFF
+    /// IFD sequence. To encode additional directories that are not linked into the sequence, use
+    /// [`Self::extra_directory`][TiffEncoder::extra_directory].
+    pub fn image_directory(&mut self) -> TiffResult<DirectoryEncoder<'_, W, K>> {
+        Self::chain_directory(&mut self.writer, &mut self.last_ifd_chain)
+    }
+
+    /// Create a [`DirectoryEncoder`] to encode a non-image directory.
+    ///
+    /// The directory is not linked into the sequence of directories. For instance, encode Exif
+    /// directories or SubIfd directories with this method.
+    pub fn extra_directory(&mut self) -> TiffResult<DirectoryEncoder<'_, W, K>> {
+        Self::unchained_directory(&mut self.writer)
     }
 
     /// Create an [`ImageEncoder`] to encode an image one slice at a time.
@@ -174,8 +196,8 @@ impl<W: Write + Seek, K: TiffKind> TiffEncoder<W, K> {
         &mut self,
         width: u32,
         height: u32,
-    ) -> TiffResult<ImageEncoder<W, C, K>> {
-        let encoder = DirectoryEncoder::new(&mut self.writer)?;
+    ) -> TiffResult<ImageEncoder<'_, W, C, K>> {
+        let encoder = Self::chain_directory(&mut self.writer, &mut self.last_ifd_chain)?;
         ImageEncoder::new(encoder, width, height, self.compression, self.predictor)
     }
 
@@ -189,11 +211,23 @@ impl<W: Write + Seek, K: TiffKind> TiffEncoder<W, K> {
     where
         [C::Inner]: TiffValue,
     {
-        let encoder = DirectoryEncoder::new(&mut self.writer)?;
+        let encoder = Self::chain_directory(&mut self.writer, &mut self.last_ifd_chain)?;
         let image: ImageEncoder<W, C, K> =
             ImageEncoder::new(encoder, width, height, self.compression, self.predictor)?;
         image.write_data(data)
     }
+
+    fn chain_directory<'lt>(
+        writer: &'lt mut TiffWriter<W>,
+        last_ifd_chain: &'lt mut NonZeroU64,
+    ) -> TiffResult<DirectoryEncoder<'lt, W, K>> {
+        let last_ifd = *last_ifd_chain;
+        DirectoryEncoder::new(writer, Some(last_ifd), Some(last_ifd_chain))
+    }
+
+    fn unchained_directory(writer: &mut TiffWriter<W>) -> TiffResult<DirectoryEncoder<'_, W, K>> {
+        DirectoryEncoder::new(writer, None, None)
+    }
 }
 
 /// Low level interface to encode ifd directories.
@@ -202,119 +236,341 @@ impl<W: Write + Seek, K: TiffKind> TiffEncoder<W, K> {
 /// Encoding can silently fail while this is dropping.
 pub struct DirectoryEncoder<'a, W: 'a + Write + Seek, K: TiffKind> {
     writer: &'a mut TiffWriter<W>,
-    dropped: bool,
+    /// The position of the previous directory's `next` field, if any.
+    chained_ifd_pos: Option<NonZeroU64>,
+    /// An output to write the `next` field offset on completion.
+    write_chain: Option<&'a mut NonZeroU64>,
+    kind: PhantomData<K>,
     // We use BTreeMap to make sure tags are written in correct order
-    ifd_pointer_pos: u64,
-    ifd: BTreeMap<u16, DirectoryEntry<K::OffsetType>>,
+    directory: Directory,
+    dropped: bool,
+}
+
+/// The offset of an encoded directory in the file.
+///
+/// Both the `offset` and `pointer` field are a complete description of the start offset of the
+/// dictionary which is used to point to it. The struct also describes the extent covered by the
+/// encoding of the directory. The constructors [`new`][DirectoryOffset::new] allow the checked
+/// conversion. Note that even though the offsets are public do not expect their information to
+/// propagate when modified.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub struct DirectoryOffset<K: TiffKind> {
+    /// The start of the directory as a Tiff value.
+    ///
+    /// This is a bit of a wart in the strongly typed design of the encoder. The value _type_ must
+    /// itself know how to be represented in the Tiff file but that may differ based on endianess
+    /// as well as the offset size (`u32` or BigTIFF's `u64`). Thankfully we're allowed to
+    /// represent offsets with `LONG` or `IFD` in the usual case.
+    pub offset: K::OffsetType,
+    /// The start of the directory as a pure offset.
+    pub pointer: IfdPointer,
+    /// The offset of its sequence link field, in our private representation. Before exposing this
+    /// make sure that we don't want any invariants (e.g. pointer being smaller than this). The
+    /// type should be fine.
+    ifd_chain: NonZeroU64,
+    /// The kind of Tiff file the offset is for.
+    kind: PhantomData<K>,
 }
 
 impl<'a, W: 'a + Write + Seek, K: TiffKind> DirectoryEncoder<'a, W, K> {
-    fn new(writer: &'a mut TiffWriter<W>) -> TiffResult<Self> {
-        // the previous word is the IFD offset position
-        let ifd_pointer_pos = writer.offset() - mem::size_of::<K::OffsetType>() as u64;
+    /// Construct a directory writer appending to data, assuming the writer is currently positioned
+    /// immediately after a previously written IFD to which to append.
+    fn new(
+        writer: &'a mut TiffWriter<W>,
+        chained_ifd_pos: Option<NonZeroU64>,
+        chain_into: Option<&'a mut NonZeroU64>,
+    ) -> TiffResult<Self> {
         writer.pad_word_boundary()?; // TODO: Do we need to adjust this for BigTiff?
-        Ok(DirectoryEncoder {
+        Ok(Self {
             writer,
+            chained_ifd_pos,
+            write_chain: chain_into,
+            kind: PhantomData,
+            directory: Directory::empty(),
             dropped: false,
-            ifd_pointer_pos,
-            ifd: BTreeMap::new(),
         })
     }
 
     /// Write a single ifd tag.
     pub fn write_tag<T: TiffValue>(&mut self, tag: Tag, value: T) -> TiffResult<()> {
+        // Encodes the value if necessary. In the current bytes all integers are taken as native
+        // endian and thus transparent but this keeps the interface generic.
         let mut bytes = Vec::with_capacity(value.bytes());
         {
             let mut writer = TiffWriter::new(&mut bytes);
             value.write(&mut writer)?;
         }
 
-        self.ifd.insert(
-            tag.to_u16(),
+        let entry = Self::write_entry_inner(
+            self.writer,
             DirectoryEntry {
-                data_type: <T>::FIELD_TYPE.to_u16(),
+                data_type: <T>::FIELD_TYPE,
                 count: value.count().try_into()?,
-                data: bytes,
+                data: bytes.into(),
             },
-        );
+        )?;
+
+        self.directory.extend([(tag, entry)]);
 
         Ok(())
     }
 
-    fn write_directory(&mut self) -> TiffResult<u64> {
-        // Start by writing out all values
-        for &mut DirectoryEntry {
-            data: ref mut bytes,
-            ..
-        } in self.ifd.values_mut()
-        {
-            let data_bytes = mem::size_of::<K::OffsetType>();
+    /// Write a tag-value pair with prepared byte data.
+    ///
+    /// Note that the library will _not_ attempt to verify that the data type or the count of the
+    /// buffered value is permissible for the given tag. The data will be associated with the tag
+    /// exactly as-is.
+    ///
+    /// An error is returned if the byte order of the presented value does not match the byte order
+    /// of the underlying file (i.e. the [native byte order](`crate::tags::ByteOrder::native`)).
+    pub fn write_tag_buf(&mut self, tag: Tag, value: &ValueBuffer) -> TiffResult<()> {
+        let entry = self.write_entry_buf(value)?;
+        self.directory.extend([(tag, entry)]);
 
-            if bytes.len() > data_bytes {
-                let offset = self.writer.offset();
-                self.writer.write_bytes(bytes)?;
-                *bytes = vec![0; data_bytes];
-                let mut writer = TiffWriter::new(bytes as &mut [u8]);
-                K::write_offset(&mut writer, offset)?;
-            } else {
-                while bytes.len() < data_bytes {
-                    bytes.push(0);
-                }
-            }
-        }
-
-        let offset = self.writer.offset();
-
-        K::write_entry_count(self.writer, self.ifd.len())?;
-        for (
-            tag,
-            DirectoryEntry {
-                data_type: field_type,
-                count,
-                data: offset,
-            },
-        ) in self.ifd.iter()
-        {
-            self.writer.write_u16(*tag)?;
-            self.writer.write_u16(*field_type)?;
-            (*count).write(self.writer)?;
-            self.writer.write_bytes(offset)?;
-        }
-
-        Ok(offset)
+        Ok(())
     }
 
     /// Write some data to the tiff file, the offset of the data is returned.
     ///
-    /// This could be used to write tiff strips.
+    /// This could be used to write tiff strips. All of the data will be written into the file as a
+    /// slice of bytes. It can not be used as an entry in a directory directly, where very short
+    /// values are instead represented in the offset field directly. Use [`Self::write_entry`]
+    /// instead.
     pub fn write_data<T: TiffValue>(&mut self, value: T) -> TiffResult<u64> {
         let offset = self.writer.offset();
         value.write(self.writer)?;
         Ok(offset)
     }
 
+    /// Write some data directly to the tiff file, the offset of the data is returned.
+    ///
+    /// All of the data will be written into the file as a slice of bytes. It can not be used as an
+    /// entry in a directory directly, where very short values are instead represented in the
+    /// offset field directly. Use [`Self::write_entry_buf`] instead.
+    ///
+    /// An error is returned if the byte order of the presented value does not match the byte order
+    /// of the underlying file (i.e. the [native byte order](`crate::tags::ByteOrder::native`)).
+    pub fn write_data_buf(&mut self, value: &ValueBuffer) -> TiffResult<u64> {
+        self.check_value_byteorder(value)?;
+        let offset = self.writer.offset();
+        self.writer.write_bytes(value.as_bytes())?;
+        Ok(offset)
+    }
+
+    /// Write a directory value into the file.
+    ///
+    /// Returns an [`Entry`]. If the value is short enough it will *not* be written in the file but
+    /// stored in the entry's offset field.
+    pub fn write_entry<T: TiffValue>(&mut self, value: T) -> TiffResult<Entry> {
+        Self::write_entry_inner(
+            self.writer,
+            DirectoryEntry {
+                data_type: T::FIELD_TYPE,
+                count: K::convert_offset(value.count() as u64)?,
+                // FIXME: not optimal we always allocate here. But the only other method we have
+                // available is `T::write(&mut TiffWriter<W>)` and we must pass that into
+                // `write_entry_inner` but then have a combinatorial explosion of instantiations
+                // which is absurd.
+                data: value.data(),
+            },
+        )
+    }
+
+    /// Write a directory value into the file.
+    ///
+    /// An error is returned if the byte order of the presented value does not match the byte order
+    /// of the underlying file (i.e. the [native byte order](`crate::tags::ByteOrder::native`)).
+    ///
+    /// Returns an [`Entry`]. If the value is short enough it will *not* be written in the file but
+    /// stored in the entry's offset field.
+    pub fn write_entry_buf(&mut self, value: &ValueBuffer) -> TiffResult<Entry> {
+        self.check_value_byteorder(value)?;
+
+        Self::write_entry_inner(
+            self.writer,
+            DirectoryEntry {
+                data_type: value.data_type(),
+                count: K::convert_offset(value.count())?,
+                data: value.as_bytes().into(),
+            },
+        )
+    }
+
+    /// Write a directory entry by its byte data.
+    ///
+    /// The data must be pre-formatted to the byte order expected by the file.
+    ///
+    /// Returns an [`Entry`]. If the value is short enough it will *not* be written in the file but
+    /// stored in the entry's offset field.
+    pub fn write_entry_bytes(&mut self, ty: Type, data: &[u8]) -> TiffResult<Entry> {
+        if data.len() % usize::from(ty.byte_len()) != 0 {
+            return Err(TiffError::UsageError(UsageError::MismatchedEntryLength {
+                ty,
+                found: data.len(),
+            }));
+        }
+
+        let count = data.len() / usize::from(ty.byte_len());
+        let count = u64::try_from(count)?;
+
+        Self::write_entry_inner(
+            self.writer,
+            DirectoryEntry {
+                data_type: ty,
+                count: K::convert_offset(count)?,
+                data: data.into(),
+            },
+        )
+    }
+
+    /// Insert previously written key-value entries.
+    ///
+    /// It is the caller's responsibility to ensure that the data referred to by the entries is
+    /// actually (or will be) written to the file. Note that small values are necessarily inline in
+    /// the entry, the size limit depends on the Tiff kind. So do not confuse a directory from a
+    /// BigTiff in a standard tiff or the other way around.
+    pub fn extend_from(&mut self, dir: &Directory) {
+        let entries = dir.iter().map(|(tag, val)| (tag, val.clone()));
+        self.directory.extend(entries);
+    }
+
+    /// Define the parent directory.
+    ///
+    /// Each directory has an offset based link to its successor, forming a linked list in the
+    /// file. The encoder writes its own offset into the parent's field when [`Self::finish`] is
+    /// called or it is dropped. This redefines the parent. An offset representation of the parent
+    /// must be acquired by finishing it with [`Self::finish_with_offsets`].
+    pub fn set_parent(&mut self, offset: &DirectoryOffset<K>) {
+        self.chained_ifd_pos = Some(offset.ifd_chain);
+    }
+
+    /// Write out the ifd directory itself.
+    pub fn finish(mut self) -> TiffResult<()> {
+        self.finish_internal()?;
+        Ok(())
+    }
+
+    /// Write the IFD to the file and return the offset it is written to.
+    ///
+    /// The offset can be used as the value, or as part of a list of values, in the entry of
+    /// another directory. If you're constructing the entry directly you'll want to use an
+    /// appropriate type variant for this such as [`Type::IFD`].
+    pub fn finish_with_offsets(mut self) -> TiffResult<DirectoryOffset<K>> {
+        self.finish_internal()
+    }
+
+    fn write_directory(&mut self) -> TiffResult<u64> {
+        // Start by turning all buffered unwritten values into entries.
+        let offset = self.writer.offset();
+        K::write_entry_count(self.writer, self.directory.len())?;
+
+        let offset_bytes = mem::size_of::<K::OffsetType>();
+        for (tag, entry) in self.directory.iter() {
+            self.writer.write_u16(tag.to_u16())?;
+            self.writer.write_u16(entry.field_type().to_u16())?;
+            let count = K::convert_offset(entry.count())?;
+            count.write(self.writer)?;
+            self.writer.write_bytes(&entry.offset()[..offset_bytes])?;
+        }
+
+        Ok(offset)
+    }
+
+    // Does it really make sense that this would be a (potentially) compressible write? For actual
+    // directory values their size must match the data written so there's going to be a silent
+    // problem if that actually occurs.
+    fn write_entry_inner(
+        writer: &mut TiffWriter<W>,
+        value: DirectoryEntry<K::OffsetType>,
+    ) -> TiffResult<Entry> {
+        let DirectoryEntry {
+            data: ref bytes,
+            ref count,
+            data_type,
+        } = value;
+
+        let in_entry_bytes = mem::size_of::<K::OffsetType>();
+
+        let mut offset_bytes = [0; 8];
+
+        if bytes.len() > in_entry_bytes {
+            let offset = writer.offset();
+            writer.write_bytes(bytes)?;
+
+            let offset = K::convert_offset(offset)?;
+            offset_bytes[..offset.bytes()].copy_from_slice(&offset.data());
+        } else {
+            // Note: we have indicated our native byte order in the header, hence this
+            // corresponds to our byte order no matter the value type.
+            offset_bytes[..bytes.len()].copy_from_slice(bytes);
+        }
+
+        // Undoing some hidden type. Offset is either u32 or u64. Due to the trait API being public
+        // and some oversight, we can not clone the `count: K::OffsetType` and thus not convert it.
+        // Instead, write it to a buffer...
+        let mut count_bytes = [0; 8];
+        debug_assert!(in_entry_bytes == 4 || in_entry_bytes == 8);
+        // Nominally Cow but we only expect `Cow::Borrowed`.
+        count_bytes[..count.bytes()].copy_from_slice(&count.data());
+
+        Ok(if in_entry_bytes == 4 {
+            let count = u32::from_ne_bytes(count_bytes[..4].try_into().unwrap());
+            Entry::new(data_type, count, offset_bytes[..4].try_into().unwrap())
+        } else {
+            debug_assert_eq!(in_entry_bytes, 8);
+            let count = u64::from_ne_bytes(count_bytes);
+            Entry::new_u64(data_type, count, offset_bytes)
+        })
+    }
+
+    fn check_value_byteorder(&self, value: &ValueBuffer) -> TiffResult<()> {
+        // FIXME: we should enable writing files with any chosen byte order.
+        if value.byte_order() != ByteOrder::native() {
+            Err(TiffError::UsageError(UsageError::ByteOrderMismatch))
+        } else {
+            Ok(())
+        }
+    }
+
     /// Provides the number of bytes written by the underlying TiffWriter during the last call.
     fn last_written(&self) -> u64 {
         self.writer.last_written()
     }
 
-    fn finish_internal(&mut self) -> TiffResult<()> {
+    fn finish_internal(&mut self) -> TiffResult<DirectoryOffset<K>> {
         let ifd_pointer = self.write_directory()?;
+        let offset = K::convert_offset(ifd_pointer)?;
+
+        if let Some(prior) = self.chained_ifd_pos {
             let curr_pos = self.writer.offset();
 
-        self.writer.goto_offset(self.ifd_pointer_pos)?;
+            self.writer.goto_offset(prior.get())?;
+            // Note how we are not writing the `offset` type itself here as doing so would need to
+            // go through the `TiffValue` trait—the type is not constrained by much. But for the
+            // trait we need a `TiffWriter` which comes with a bunch of additional state such as
+            // compressor etc. that we have no need for.
             K::write_offset(self.writer, ifd_pointer)?;
+
             self.writer.goto_offset(curr_pos)?;
+        }
+
         K::write_offset(self.writer, 0)?;
 
+        let ifd_chain = NonZeroU64::new(self.writer.previous_ifd_pointer::<K>())
+            .expect("IFD chain field is at a non-zero offset");
+
+        if let Some(prior) = self.write_chain.take() {
+            *prior = ifd_chain;
+        }
+
         self.dropped = true;
 
-        Ok(())
-    }
-
-    /// Write out the ifd directory.
-    pub fn finish(mut self) -> TiffResult<()> {
-        self.finish_internal()
+        Ok(DirectoryOffset {
+            pointer: IfdPointer(ifd_pointer),
+            offset,
+            ifd_chain,
+            kind: PhantomData,
+        })
     }
 }
 
@@ -366,6 +622,7 @@ pub struct ImageEncoder<'a, W: 'a + Write + Seek, C: ColorType, K: TiffKind> {
     row_samples: u64,
     width: u32,
     height: u32,
+    extra_samples: Vec<u16>,
     rows_per_strip: u64,
     strip_offsets: Vec<K::OffsetType>,
     strip_byte_count: Vec<K::OffsetType>,
@@ -411,22 +668,25 @@ impl<'a, W: 'a + Write + Seek, T: ColorType, K: TiffKind> ImageEncoder<'a, W, T,
         let rows_per_strip = {
             match compression.tag() {
                 CompressionMethod::PackBits => 1, // Each row must be packed separately. Do not compress across row boundaries
-                _ => (1_000_000 + row_bytes - 1) / row_bytes,
+                _ => 1_000_000_u64.div_ceil(row_bytes),
             }
         };
 
-        let strip_count = (u64::from(height) + rows_per_strip - 1) / rows_per_strip;
+        let strip_count = u64::from(height).div_ceil(rows_per_strip);
 
         encoder.write_tag(Tag::ImageWidth, width)?;
         encoder.write_tag(Tag::ImageLength, height)?;
         encoder.write_tag(Tag::Compression, compression.tag().to_u16())?;
         encoder.write_tag(Tag::Predictor, predictor.to_u16())?;
 
-        encoder.write_tag(Tag::BitsPerSample, <T>::BITS_PER_SAMPLE)?;
-        let sample_format: Vec<_> = <T>::SAMPLE_FORMAT.iter().map(|s| s.to_u16()).collect();
-        encoder.write_tag(Tag::SampleFormat, &sample_format[..])?;
         encoder.write_tag(Tag::PhotometricInterpretation, <T>::TIFF_VALUE.to_u16())?;
 
+        if matches!(<T>::TIFF_VALUE, PhotometricInterpretation::YCbCr) {
+            // The default for this tag is 2,2 for subsampling but we do not support such a
+            // transformation. Instead all samples must be provided.
+            encoder.write_tag(Tag::ChromaSubsampling, &[1u16, 1u16][..])?;
+        }
+
         encoder.write_tag(Tag::RowsPerStrip, u32::try_from(rows_per_strip)?)?;
 
         encoder.write_tag(
@@ -435,7 +695,7 @@ impl<'a, W: 'a + Write + Seek, T: ColorType, K: TiffKind> ImageEncoder<'a, W, T,
         )?;
         encoder.write_tag(Tag::XResolution, Rational { n: 1, d: 1 })?;
         encoder.write_tag(Tag::YResolution, Rational { n: 1, d: 1 })?;
-        encoder.write_tag(Tag::ResolutionUnit, ResolutionUnit::None.to_u16())?;
+        encoder.write_tag(Tag::ResolutionUnit, ResolutionUnit::None)?;
 
         Ok(ImageEncoder {
             encoder,
@@ -443,6 +703,7 @@ impl<'a, W: 'a + Write + Seek, T: ColorType, K: TiffKind> ImageEncoder<'a, W, T,
             strip_idx: 0,
             row_samples,
             rows_per_strip,
+            extra_samples: vec![],
             width,
             height,
             strip_offsets: Vec::new(),
@@ -454,6 +715,27 @@ impl<'a, W: 'a + Write + Seek, T: ColorType, K: TiffKind> ImageEncoder<'a, W, T,
         })
     }
 
+    pub fn extra_samples(&mut self, extra: &[ExtraSamples]) -> Result<(), TiffError> {
+        if self.strip_idx != 0 {
+            return Err(TiffError::UsageError(
+                UsageError::ReconfiguredAfterImageWrite,
+            ));
+        }
+
+        let samples = self.extra_samples.len() + extra.len() + <T>::BITS_PER_SAMPLE.len();
+        let row_samples = u64::from(self.width) * u64::try_from(samples)?;
+
+        self.extra_samples
+            .extend(extra.iter().map(ExtraSamples::to_u16));
+
+        self.encoder
+            .write_tag(Tag::ExtraSamples, &self.extra_samples[..])?;
+
+        self.row_samples = row_samples;
+
+        Ok(())
+    }
+
     /// Number of samples the next strip should have.
     pub fn next_strip_sample_count(&self) -> u64 {
         if self.strip_idx >= self.strip_count {
@@ -584,13 +866,46 @@ impl<'a, W: 'a + Write + Seek, T: ColorType, K: TiffKind> ImageEncoder<'a, W, T,
         self.encoder.write_tag(Tag::RowsPerStrip, value)?;
 
         let value: u64 = value as u64;
-        self.strip_count = (self.height as u64 + value - 1) / value;
+        self.strip_count = (self.height as u64).div_ceil(value);
         self.rows_per_strip = value;
 
         Ok(())
     }
 
-    fn finish_internal(&mut self) -> TiffResult<()> {
+    fn finish_internal(&mut self) -> TiffResult<DirectoryOffset<K>> {
+        if self.extra_samples.is_empty() {
+            self.encoder
+                .write_tag(Tag::BitsPerSample, <T>::BITS_PER_SAMPLE)?;
+        } else {
+            let mut sample_format: Vec<_> = <T>::BITS_PER_SAMPLE.to_vec();
+            let replicated =
+                core::iter::repeat_n(<T>::BITS_PER_SAMPLE[0], self.extra_samples.len());
+            sample_format.extend(replicated);
+
+            self.encoder
+                .write_tag(Tag::BitsPerSample, &sample_format[..])?;
+
+            self.encoder.write_tag(
+                Tag::SamplesPerPixel,
+                u16::try_from(<T>::BITS_PER_SAMPLE.len() + self.extra_samples.len())?,
+            )?;
+        }
+
+        let mut sample_format: Vec<_> = <T>::SAMPLE_FORMAT.iter().map(|s| s.to_u16()).collect();
+        let extra_format = sample_format
+            .first()
+            .copied()
+            // Frankly should not occur, we have no sample format without at least one entry. We
+            // would need an interface upgrade to handle this however. Either decide to support
+            // heterogeneous sample formats or a way to provide fallback that is used for purely
+            // extra samples and not provided via a slice used for samples themselves.
+            .unwrap_or(SampleFormat::Void.to_u16());
+
+        sample_format.extend(core::iter::repeat_n(extra_format, self.extra_samples.len()));
+
+        self.encoder
+            .write_tag(Tag::SampleFormat, &sample_format[..])?;
+
         self.encoder
             .write_tag(Tag::StripOffsets, K::convert_slice(&self.strip_offsets))?;
         self.encoder.write_tag(
@@ -609,7 +924,8 @@ impl<'a, W: 'a + Write + Seek, T: ColorType, K: TiffKind> ImageEncoder<'a, W, T,
 
     /// Write out image and ifd directory.
     pub fn finish(mut self) -> TiffResult<()> {
-        self.finish_internal()
+        self.finish_internal()?;
+        Ok(())
     }
 }
 
@@ -621,10 +937,10 @@ impl<'a, W: Write + Seek, C: ColorType, K: TiffKind> Drop for ImageEncoder<'a, W
     }
 }
 
-struct DirectoryEntry<S> {
-    data_type: u16,
+struct DirectoryEntry<'data, S> {
+    data_type: Type,
     count: S,
-    data: Vec<u8>,
+    data: std::borrow::Cow<'data, [u8]>,
 }
 
 /// Trait to abstract over Tiff/BigTiff differences.
@@ -669,6 +985,71 @@ pub trait TiffKind {
     fn convert_slice(slice: &[Self::OffsetType]) -> &Self::OffsetArrayType;
 }
 
+impl<K: TiffKind> DirectoryOffset<K> {
+    /// Construct the directory pointer information from its raw offset.
+    ///
+    /// Fails in these cases:
+    ///
+    /// - if start offset is too large to be represented with the chosen TIFF format, i.e. a value
+    ///   larger than [`u32::MAX`] with a standard tiff which is only valid in a BigTiff. This
+    ///   returns [`TiffError::IntSizeError`].
+    /// - the extent of the directory would overflow the file length. This returns a
+    ///   [`TiffError::UsageError`].
+    ///
+    /// The library does *not* validate the contents or offset in any way, it just performs the
+    /// necessary math assuming the data is an accurate representation of existing content.
+    ///
+    /// # Examples
+    ///
+    /// If you have any custom way of writing a directory to a file that can not go through the
+    /// usual [`TiffEncoder::extra_directory`] then you can reconstruct the offset information as
+    /// long as you provide all the necessary data to the library. You can then use it as a parent
+    /// directory, i.e. have the library overwrite its `next` field.
+    ///
+    /// ```
+    /// use tiff::{
+    ///     encoder::{TiffEncoder, DirectoryOffset},
+    ///     Directory,
+    ///     tags::IfdPointer,
+    /// };
+    ///
+    /// # let mut file = std::io::Cursor::new(Vec::new());
+    /// let mut tiff = TiffEncoder::new(&mut file)?;
+    ///
+    /// // … some custom data writes, assume we know where a directory was written
+    /// # fn reconstruction_of_your_directory() -> Directory { Directory::from_iter([]) }
+    /// let known_offset = IfdPointer(1024);
+    /// let known_contents: Directory = reconstruction_of_your_directory();
+    ///
+    /// let reconstructed = DirectoryOffset::new(known_offset, &known_contents)?;
+    ///
+    /// // Now we can use it as if the directory was written by the `TiffEncoder` itself.
+    /// let mut chain = tiff.extra_directory()?;
+    /// chain.set_parent(&reconstructed);
+    /// chain.finish_with_offsets()?;
+    ///
+    /// # Ok::<_, tiff::TiffError>(())
+    /// ```
+    pub fn new(pointer: IfdPointer, dir: &Directory) -> TiffResult<Self> {
+        let offset = K::convert_offset(pointer.0)?;
+        let encoded = dir.encoded_len::<K>();
+        let offset_field_offset = encoded - mem::size_of_val(&offset) as u64;
+
+        let ifd_chain = pointer
+            .0
+            .checked_add(offset_field_offset)
+            .and_then(NonZeroU64::new)
+            .ok_or(TiffError::UsageError(UsageError::ZeroIfdPointer))?;
+
+        Ok(DirectoryOffset {
+            pointer,
+            offset,
+            ifd_chain,
+            kind: PhantomData,
+        })
+    }
+}
+
 /// Create a standard Tiff file.
 pub struct TiffKindStandard;
 
@@ -737,3 +1118,33 @@ impl TiffKind for TiffKindBig {
         slice
     }
 }
+
+#[test]
+fn directory_offset_new_equivalent_to_writing() {
+    type K = TiffKindStandard;
+
+    let dir = Directory::from_iter(vec![
+        (
+            Tag::ImageWidth,
+            Entry::new(Type::LONG, 1, 100u32.to_ne_bytes()),
+        ),
+        (
+            Tag::ImageLength,
+            Entry::new(Type::LONG, 1, 200u32.to_ne_bytes()),
+        ),
+    ]);
+
+    let data = std::io::Cursor::new(vec![]);
+    let mut file = TiffEncoder::new(data).unwrap();
+
+    let mut as_dir_encoder = file.extra_directory().unwrap();
+    as_dir_encoder.extend_from(&dir);
+    let dir_extent = as_dir_encoder.finish_with_offsets().unwrap();
+
+    // Check we can recreate the extent had we written ourselves.
+    let synth = DirectoryOffset::<K>::new(dir_extent.pointer, &dir).unwrap();
+
+    assert_eq!(synth.pointer, dir_extent.pointer);
+    assert_eq!(synth.offset, dir_extent.offset);
+    assert_eq!(synth.ifd_chain, dir_extent.ifd_chain);
+}

src/encoder/tiff_value.rs

@@ -1,21 +1,34 @@
 use std::{borrow::Cow, io::Write, slice::from_ref};
 
-use crate::{bytecast, tags::Type, TiffError, TiffFormatError, TiffResult};
+use crate::{bytecast, tags, tags::Type, TiffError, TiffFormatError, TiffResult};
 
 use super::writer::TiffWriter;
 
 /// Trait for types that can be encoded in a tiff file
+#[diagnostic::on_unimplemented(
+    message = "the type `{Self}` does not implement `TiffValue`",
+    note = "the trait is implemented for primitive types (`u8`, `i16`, `f32`, etc.)",
+    note = "the trait is implemented for shared references to values",
+    note = "the trait is implemented for slices, pass them by reference (e.g. `&[u8]`)",
+    note = "the trait is implemented for arrays if it is implemented for a slice",
+    note = "values in a `Vec` or `Box` should be dereferenced, e.g. `&vec[..]`"
+)]
 pub trait TiffValue {
     const BYTE_LEN: u8;
     const FIELD_TYPE: Type;
+
+    // FIXME: If we want this to mean 'count' in the sense of `Entry` it should return `u32` or
+    // `u64`, probably the latter with `convert_offset` taking care of the check against `u32::MAX`
+    // for classic tiff.
     fn count(&self) -> usize;
+
     fn bytes(&self) -> usize {
         self.count() * usize::from(Self::BYTE_LEN)
     }
 
     /// Access this value as an contiguous sequence of bytes.
     /// If their is no trivial representation, allocate it on the heap.
-    fn data(&self) -> Cow<[u8]>;
+    fn data(&self) -> Cow<'_, [u8]>;
 
     /// Write this value to a TiffWriter.
     /// While the default implementation will work in all cases, it may require unnecessary allocations.
@@ -34,7 +47,7 @@ impl TiffValue for [u8] {
         self.len()
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(self)
     }
 }
@@ -47,7 +60,7 @@ impl TiffValue for [i8] {
         self.len()
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::i8_as_ne_bytes(self))
     }
 }
@@ -60,7 +73,7 @@ impl TiffValue for [u16] {
         self.len()
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::u16_as_ne_bytes(self))
     }
 }
@@ -73,7 +86,7 @@ impl TiffValue for [i16] {
         self.len()
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::i16_as_ne_bytes(self))
     }
 }
@@ -86,7 +99,7 @@ impl TiffValue for [u32] {
         self.len()
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::u32_as_ne_bytes(self))
     }
 }
@@ -99,7 +112,7 @@ impl TiffValue for [i32] {
         self.len()
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::i32_as_ne_bytes(self))
     }
 }
@@ -112,7 +125,7 @@ impl TiffValue for [u64] {
         self.len()
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::u64_as_ne_bytes(self))
     }
 }
@@ -125,7 +138,7 @@ impl TiffValue for [i64] {
         self.len()
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::i64_as_ne_bytes(self))
     }
 }
@@ -138,7 +151,7 @@ impl TiffValue for [f32] {
         self.len()
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         // We write using native endian so this should be safe
         Cow::Borrowed(bytecast::f32_as_ne_bytes(self))
     }
@@ -152,7 +165,7 @@ impl TiffValue for [f64] {
         self.len()
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         // We write using native endian so this should be safe
         Cow::Borrowed(bytecast::f64_as_ne_bytes(self))
     }
@@ -171,7 +184,7 @@ impl TiffValue for u8 {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(from_ref(self))
     }
 }
@@ -189,7 +202,7 @@ impl TiffValue for i8 {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::i8_as_ne_bytes(from_ref(self)))
     }
 }
@@ -207,7 +220,7 @@ impl TiffValue for u16 {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::u16_as_ne_bytes(from_ref(self)))
     }
 }
@@ -225,7 +238,7 @@ impl TiffValue for i16 {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::i16_as_ne_bytes(from_ref(self)))
     }
 }
@@ -243,7 +256,7 @@ impl TiffValue for u32 {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::u32_as_ne_bytes(from_ref(self)))
     }
 }
@@ -261,7 +274,7 @@ impl TiffValue for i32 {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::i32_as_ne_bytes(from_ref(self)))
     }
 }
@@ -279,7 +292,7 @@ impl TiffValue for u64 {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::u64_as_ne_bytes(from_ref(self)))
     }
 }
@@ -297,7 +310,7 @@ impl TiffValue for i64 {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::i64_as_ne_bytes(from_ref(self)))
     }
 }
@@ -315,7 +328,7 @@ impl TiffValue for f32 {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::f32_as_ne_bytes(from_ref(self)))
     }
 }
@@ -333,7 +346,7 @@ impl TiffValue for f64 {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::f64_as_ne_bytes(from_ref(self)))
     }
 }
@@ -351,7 +364,7 @@ impl TiffValue for Ifd {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::u32_as_ne_bytes(from_ref(&self.0)))
     }
 }
@@ -369,7 +382,7 @@ impl TiffValue for Ifd8 {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Borrowed(bytecast::u64_as_ne_bytes(from_ref(&self.0)))
     }
 }
@@ -388,7 +401,7 @@ impl TiffValue for Rational {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Owned({
             let first_dword = bytecast::u32_as_ne_bytes(from_ref(&self.n));
             let second_dword = bytecast::u32_as_ne_bytes(from_ref(&self.d));
@@ -411,7 +424,7 @@ impl TiffValue for SRational {
         Ok(())
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Owned({
             let first_dword = bytecast::i32_as_ne_bytes(from_ref(&self.n));
             let second_dword = bytecast::i32_as_ne_bytes(from_ref(&self.d));
@@ -438,7 +451,7 @@ impl TiffValue for str {
         }
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         Cow::Owned({
             if self.is_ascii() && !self.bytes().any(|b| b == 0) {
                 let bytes: &[u8] = self.as_bytes();
@@ -450,7 +463,137 @@ impl TiffValue for str {
     }
 }
 
-impl<'a, T: TiffValue + ?Sized> TiffValue for &'a T {
+// FIXME: held up on reading the discriminant for all but the unknown variant.
+//
+// It needs to have `repr(u16)` to have a defined layout and then we may read the tag from its
+// value representation by casting a u16 pointer to a pointer to the enum type.
+//     unsafe { *<*const _>::from(self).cast::<u16>() }
+// But that is quite unsafe and needs careful review to maintain the safety requirements. Maybe we
+// could get `bytemuck` to add a trait for this (it has `TransparentWrapper`) with a derive for
+// enum types that have an explicit repr.
+//
+// This would allow returning borrowed data in more cases. For all types without an unknown, in all
+// cases including as slices, and for all types with and unknown variant when we have a single
+// value (either returning a borrowed discriminant or a borrowed value within `Unknown`).
+impl TiffValue for tags::CompressionMethod {
+    const BYTE_LEN: u8 = <u16 as TiffValue>::BYTE_LEN;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        1
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        let bytes = self.to_u16().to_ne_bytes();
+        Cow::Owned(bytes.to_vec())
+    }
+}
+
+impl TiffValue for tags::PhotometricInterpretation {
+    const BYTE_LEN: u8 = <u16 as TiffValue>::BYTE_LEN;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        1
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        self.to_u16().to_ne_bytes().to_vec().into()
+    }
+}
+
+impl TiffValue for tags::PlanarConfiguration {
+    const BYTE_LEN: u8 = 2;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        1
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        self.to_u16().to_ne_bytes().to_vec().into()
+    }
+}
+
+impl TiffValue for tags::Predictor {
+    const BYTE_LEN: u8 = <u16 as TiffValue>::BYTE_LEN;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        1
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        self.to_u16().to_ne_bytes().to_vec().into()
+    }
+}
+
+impl TiffValue for tags::ResolutionUnit {
+    const BYTE_LEN: u8 = <u16 as TiffValue>::BYTE_LEN;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        1
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        self.to_u16().to_ne_bytes().to_vec().into()
+    }
+}
+
+/// This is implemented for slices as the `SampleFormat` tag takes a count of `N`.
+///
+/// Use `core::slice::from_ref` if you really need to write a single element.
+///
+/// See: <https://web.archive.org/web/20191120220815/https://www.awaresystems.be/imaging/tiff/tifftags/sampleformat.html>
+impl TiffValue for [tags::SampleFormat] {
+    const BYTE_LEN: u8 = <u16 as TiffValue>::BYTE_LEN;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        self.len()
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        let mut buf: Vec<u8> = Vec::with_capacity(self.len() * 2);
+        for x in self {
+            buf.extend_from_slice(&x.to_u16().to_ne_bytes());
+        }
+        Cow::Owned(buf)
+    }
+}
+
+/// This is implemented for slices as the `ExtraSamples` tag takes a count of `N`.
+///
+/// Use `core::slice::from_ref` if you really need to write a single element.
+///
+/// See: <https://web.archive.org/web/20191120220815/https://www.awaresystems.be/imaging/tiff/tifftags/extrasamples.html>
+impl TiffValue for [tags::ExtraSamples] {
+    const BYTE_LEN: u8 = <u16 as TiffValue>::BYTE_LEN;
+    const FIELD_TYPE: Type = Type::SHORT;
+
+    fn count(&self) -> usize {
+        self.len()
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        let mut buf: Vec<u8> = Vec::with_capacity(self.len() * 2);
+        for x in self {
+            buf.extend_from_slice(&x.to_u16().to_ne_bytes());
+        }
+        Cow::Owned(buf)
+    }
+}
+
+// If you pass `&Vec<_>` then you'd get at first sight the  complaint:
+//
+//  `Vec<T>` does not implement `TiffValue`
+//
+// and a list of implementations of `TiffValue` that are quite unrelated. That is not very helpful.
+// We do not *want* `Vec` to implement the trait as an owning type, instead you should pass a
+// reference to a slice. The error message is further customized in the diagnostics of the trait.
+#[diagnostic::do_not_recommend]
+impl<T: TiffValue + ?Sized> TiffValue for &'_ T {
     const BYTE_LEN: u8 = T::BYTE_LEN;
     const FIELD_TYPE: Type = T::FIELD_TYPE;
 
@@ -462,11 +605,32 @@ impl<'a, T: TiffValue + ?Sized> TiffValue for &'a T {
         (*self).write(writer)
     }
 
-    fn data(&self) -> Cow<[u8]> {
+    fn data(&self) -> Cow<'_, [u8]> {
         T::data(self)
     }
 }
 
+/// An array is treated like a slice of the same length.
+impl<T, const N: usize> TiffValue for [T; N]
+where
+    [T]: TiffValue,
+{
+    const BYTE_LEN: u8 = <[T]>::BYTE_LEN;
+    const FIELD_TYPE: Type = <[T]>::FIELD_TYPE;
+
+    fn count(&self) -> usize {
+        self.as_slice().count()
+    }
+
+    fn write<W: Write>(&self, writer: &mut TiffWriter<W>) -> TiffResult<()> {
+        self.as_slice().write(writer)
+    }
+
+    fn data(&self) -> Cow<'_, [u8]> {
+        self.as_slice().data()
+    }
+}
+
 macro_rules! impl_tiff_value_for_contiguous_sequence {
     ($inner_type:ty; $bytes:expr; $field_type:expr) => {
         impl $crate::encoder::TiffValue for [$inner_type] {
@@ -484,7 +648,7 @@ macro_rules! impl_tiff_value_for_contiguous_sequence {
                 Ok(())
             }
 
-            fn data(&self) -> Cow<[u8]> {
+            fn data(&self) -> Cow<'_, [u8]> {
                 let mut buf: Vec<u8> = Vec::with_capacity(Self::BYTE_LEN as usize * self.len());
                 for x in self {
                     buf.extend_from_slice(&x.data());

src/encoder/writer.rs

@@ -69,6 +69,10 @@ impl<W: Write> TiffWriter<W> {
         self.offset
     }
 
+    pub(crate) fn previous_ifd_pointer<K: super::TiffKind>(&self) -> u64 {
+        self.offset() - core::mem::size_of::<K::OffsetType>() as u64
+    }
+
     pub fn last_written(&self) -> u64 {
         self.byte_count
     }
@@ -186,3 +190,64 @@ impl<W: Seek> TiffWriter<W> {
         Ok(())
     }
 }
+
+#[test]
+fn encoder_on_short_writes() {
+    struct ShortWriter<T> {
+        inner: T,
+    }
+
+    impl<T: std::io::Write> std::io::Write for ShortWriter<T> {
+        fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+            let lower_bound = buf.len().min(1);
+            let lower = buf.len().saturating_sub(1).max(lower_bound);
+
+            self.inner.write(&buf[..lower])
+        }
+
+        fn flush(&mut self) -> std::io::Result<()> {
+            self.inner.flush()
+        }
+    }
+
+    impl<T: std::io::Seek> std::io::Seek for ShortWriter<T> {
+        fn seek(&mut self, pos: std::io::SeekFrom) -> std::io::Result<u64> {
+            self.inner.seek(pos)
+        }
+    }
+
+    let mut write = ShortWriter {
+        inner: std::io::Cursor::new(vec![0u8; 0]),
+    };
+
+    let imgdata = vec![42u8; 100 * 100];
+    let mut data = crate::decoder::DecodingResult::U8(vec![]);
+
+    for comp in [
+        super::Compression::Uncompressed,
+        super::Compression::Packbits,
+    ] {
+        write.inner.set_position(0);
+        write.inner.get_mut().clear();
+
+        {
+            let mut tiff_writer = super::TiffEncoder::new(&mut write)
+                .unwrap()
+                .with_compression(comp);
+
+            tiff_writer
+                .write_image::<super::Gray8>(100, 100, &imgdata)
+                .unwrap();
+        }
+
+        write.inner.set_position(0);
+        let mut tiff_reader = crate::decoder::Decoder::new(&mut write.inner).unwrap();
+        let _layout = tiff_reader.read_image_to_buffer(&mut data).unwrap();
+
+        assert!(
+            matches!(&data, crate::decoder::DecodingResult::U8(v) if *v == imgdata),
+            "{data:?} @ {}",
+            data.as_buffer(0).byte_len()
+        );
+    }
+}

src/error.rs

@@ -5,6 +5,7 @@ use std::string;
 use quick_error::quick_error;
 
 use crate::decoder::ChunkType;
+use crate::tags::Type;
 use crate::tags::{
     CompressionMethod, PhotometricInterpretation, PlanarConfiguration, SampleFormat, Tag,
 };
@@ -90,6 +91,9 @@ quick_error! {
         InvalidTypeForTag {
             display("tag has invalid type")
         }
+        InvalidCountForTag(tag: Tag, len: usize) {
+            display("tag `{tag:?}` with incorrect number of elements ({len}) encountered")
+        }
         StripTileTagConflict {
             display("file should contain either (StripByteCounts and StripOffsets) or (TileByteCounts and TileOffsets), other combination was found")
         }
@@ -156,6 +160,9 @@ quick_error! {
         UnsupportedInterpretation(interpretation: PhotometricInterpretation) {
             display("unsupported photometric interpretation \"{interpretation:?}\"")
         }
+        ChromaSubsampling {
+            display("chroma subsampling of YCbCr color is unsupported")
+        }
         MisalignedTileBoundaries {
             display("tile rows are not aligned to byte boundaries")
         }
@@ -175,6 +182,12 @@ quick_error! {
         InvalidChunkIndex(index: u32) {
             display("invalid chunk index ({index}) requested")
         }
+        InvalidPlaneIndex(index: u16) {
+            display("invalid plane index ({index}) requested")
+        }
+        InvalidCodingUnit(index: u32, have: u32) {
+            display("out of bounds coding unit ({index}) requested, only {have} available")
+        }
         PredictorCompressionMismatch {
             display("requested predictor is not compatible with the requested compression")
         }
@@ -184,6 +197,24 @@ quick_error! {
         PredictorUnavailable {
             display("the requested predictor is not available")
         }
+        InsufficientOutputBufferSize { needed: usize, provided: usize } {
+            display("the borrowed output buffer is not large enough for the decoded data, needed {needed} but have {provided}")
+        }
+        InsufficientOutputRowStride { needed: usize, requested: usize } {
+            display("the provided output row stride would alias rows of decoded data, needed {needed} but have {requested}")
+        }
+        ZeroIfdPointer {
+            display("the offset 0 can not point to a valid IFD")
+        }
+        ReconfiguredAfterImageWrite {
+            display("attempted to reconfigure the encoder after image writing has started")
+        }
+        ByteOrderMismatch {
+            display("attempted to use data with a byte order that does not match the required one")
+        }
+        MismatchedEntryLength {ty: Type, found: usize } {
+            display("the length of data is not a multiple of the size of the type {ty:?}, found {found}")
+        }
     }
 }
 

src/lib.rs

@@ -8,10 +8,12 @@
 
 mod bytecast;
 pub mod decoder;
+mod directory;
 pub mod encoder;
 mod error;
 pub mod tags;
 
+pub use self::directory::Directory;
 pub use self::error::{TiffError, TiffFormatError, TiffResult, TiffUnsupportedError, UsageError};
 
 /// An enumeration over supported color types and their bit depths
@@ -42,11 +44,15 @@ pub enum ColorType {
     /// Pixel is YCbCr
     YCbCr(u8),
 
+    /// Pixel is CIE L*a*b* (ICC LAb)
+    Lab(u8),
+
     /// Pixel has multiple bands/channels
     Multiband { bit_depth: u8, num_samples: u16 },
 }
 impl ColorType {
-    fn bit_depth(&self) -> u8 {
+    /// The number of bits used to store each sample.
+    pub fn bit_depth(&self) -> u8 {
         match *self {
             ColorType::Gray(b)
             | ColorType::RGB(b)
@@ -56,7 +62,24 @@ impl ColorType {
             | ColorType::CMYK(b)
             | ColorType::CMYKA(b)
             | ColorType::YCbCr(b)
+            | ColorType::Lab(b)
             | ColorType::Multiband { bit_depth: b, .. } => b,
         }
     }
+
+    /// The number of samples in each representation of a single color.
+    pub fn num_samples(&self) -> u16 {
+        match *self {
+            ColorType::Gray(_) => 1,
+            ColorType::RGB(_) => 3,
+            ColorType::Palette(_) => 1,
+            ColorType::GrayA(_) => 2,
+            ColorType::RGBA(_) => 4,
+            ColorType::CMYK(_) => 4,
+            ColorType::CMYKA(_) => 5,
+            ColorType::YCbCr(_) => 3,
+            ColorType::Lab(_) => 3,
+            ColorType::Multiband { num_samples, .. } => num_samples,
+        }
+    }
 }

src/tags.rs

@@ -1,8 +1,11 @@
+use crate::encoder::TiffValue;
+use core::fmt;
+
 macro_rules! tags {
     {
         // Permit arbitrary meta items, which include documentation.
         $( #[$enum_attr:meta] )*
-        $vis:vis enum $name:ident($ty:tt) $(unknown($unknown_doc:literal))* {
+        $vis:vis enum $name:ident($ty:tt) $(unknown(#[$unknown_meta:meta] $unknown_doc:ident))* {
             // Each of the `Name = Val,` permitting documentation.
             $($(#[$ident_attr:meta])* $tag:ident = $val:expr,)*
         }
@@ -10,17 +13,18 @@ macro_rules! tags {
         $( #[$enum_attr] )*
         #[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
         #[non_exhaustive]
+        #[repr($ty)]
         pub enum $name {
-            $($(#[$ident_attr])* $tag,)*
+            $($(#[$ident_attr])* $tag = $val,)*
             $(
-                #[doc = $unknown_doc]
+                #[$unknown_meta]
                 Unknown($ty),
             )*
         }
 
         impl $name {
             #[inline(always)]
-            fn __from_inner_type(n: $ty) -> Result<Self, $ty> {
+            const fn __from_inner_type(n: $ty) -> Result<Self, $ty> {
                 match n {
                     $( $val => Ok($name::$tag), )*
                     n => Err(n),
@@ -28,10 +32,10 @@ macro_rules! tags {
             }
 
             #[inline(always)]
-            fn __to_inner_type(&self) -> $ty {
+            const fn __to_inner_type(&self) -> $ty {
                 match *self {
                     $( $name::$tag => $val, )*
-                    $( $name::Unknown(n) => { $unknown_doc; n }, )*
+                    $( $name::Unknown($unknown_doc) => { $unknown_doc }, )*
                 }
             }
         }
@@ -39,23 +43,28 @@ macro_rules! tags {
         tags!($name, $ty, $($unknown_doc)*);
     };
     // For u16 tags, provide direct inherent primitive conversion methods.
-    ($name:tt, u16, $($unknown_doc:literal)*) => {
+    ($name:tt, u16, $($unknown_doc:ident)*) => {
         impl $name {
             #[inline(always)]
-            pub fn from_u16(val: u16) -> Option<Self> {
-                Self::__from_inner_type(val).ok()
+            pub const fn from_u16(val: u16) -> Option<Self> {
+                match Self::__from_inner_type(val) {
+                    Ok(v) => Some(v),
+                    Err(_) => None,
+                }
             }
 
             $(
             #[inline(always)]
-            pub fn from_u16_exhaustive(val: u16) -> Self {
-                $unknown_doc;
-                Self::__from_inner_type(val).unwrap_or_else(|_| $name::Unknown(val))
+            pub const fn from_u16_exhaustive($unknown_doc: u16) -> Self {
+                match Self::__from_inner_type($unknown_doc) {
+                    Ok(v) => v,
+                    Err(_) => $name::Unknown($unknown_doc),
+                }
             }
             )*
 
             #[inline(always)]
-            pub fn to_u16(&self) -> u16 {
+            pub const fn to_u16(&self) -> u16 {
                 Self::__to_inner_type(self)
             }
         }
@@ -68,7 +77,10 @@ macro_rules! tags {
 // Note: These tags appear in the order they are mentioned in the TIFF reference
 tags! {
 /// TIFF tags
-pub enum Tag(u16) unknown("A private or extension tag") {
+pub enum Tag(u16) unknown(
+    /// A private or extension tag
+    unknown
+) {
     // Baseline tags:
     Artist = 315,
     // grayscale images PhotometricInterpretation 1 or 3
@@ -78,7 +90,6 @@ pub enum Tag(u16) unknown("A private or extension tag") {
     // palette-color images (PhotometricInterpretation 3)
     ColorMap = 320, // TODO add support
     Compression = 259, // TODO add support for 2 and 32773
-    Copyright = 33_432,
     DateTime = 306,
     ExtraSamples = 338, // TODO add support
     FillOrder = 266, // TODO add support
@@ -114,23 +125,63 @@ pub enum Tag(u16) unknown("A private or extension tag") {
     TileLength = 323,
     TileOffsets = 324,
     TileByteCounts = 325,
+    SubIfd = 330,
     // Data Sample Format
     SampleFormat = 339,
     SMinSampleValue = 340, // TODO add support
     SMaxSampleValue = 341, // TODO add support
     // JPEG
     JPEGTables = 347,
+    // Subsampling
+    #[doc(alias = "YCbCrSubsampling")]
+    ChromaSubsampling = 530, // TODO add support
+    #[doc(alias = "YCbCrPositioning")]
+    ChromaPositioning = 531, // TODO add support
     // GeoTIFF
     ModelPixelScaleTag = 33550, // (SoftDesk)
     ModelTransformationTag = 34264, // (JPL Carto Group)
     ModelTiepointTag = 33922, // (Intergraph)
+    // <https://web.archive.org/web/20131111073619/http://www.exif.org/Exif2-1.PDF>
+    // *Do note its typo in the Decimal id*
+    Copyright = 33_432,
+    // <https://web.archive.org/web/20131111073619/http://www.exif.org/Exif2-1.PDF>
+    ExifDirectory = 0x8769,
+    // <https://web.archive.org/web/20131111073619/http://www.exif.org/Exif2-1.PDF>
+    GpsDirectory = 0x8825,
+    // <https://www.color.org/technotes/ICC-Technote-ProfileEmbedding.pdf>
+    IccProfile = 34675,
     GeoKeyDirectoryTag = 34735, // (SPOT)
     GeoDoubleParamsTag = 34736, // (SPOT)
     GeoAsciiParamsTag = 34737, // (SPOT)
+    ExifVersion = 0x9000,
     GdalNodata = 42113, // Contains areas with missing data
 }
 }
 
+/// Identifies the offset of an IFD.
+///
+/// This is represented as a 64-bit integer but only BigTIFF can utilize the bits. It is encoded
+/// as 32-bit unsigned value ([`Type::LONG`]) in regular TIFF files and as 64-bit unsigned value
+/// ([`Type::IFD8`]) in BigTIFF files.
+#[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
+// We could be using `NonZeroU64` here but I find that this complicates semantics. This type
+// represents the integer value stored as a value in a tag. The semantics of treating `0` as an end
+// marker are imposed by the IFD. (It's unclear if Pointer tags such as Exif would allow `0` but in
+// practice it just returns garbage and the validity does not matter greatly to us).
+pub struct IfdPointer(pub u64);
+
+impl fmt::LowerHex for IfdPointer {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::LowerHex::fmt(&self.0, f)
+    }
+}
+
+impl core::fmt::UpperHex for IfdPointer {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::UpperHex::fmt(&self.0, f)
+    }
+}
+
 tags! {
 /// The type of an IFD entry (a 2 byte field).
 pub enum Type(u16) {
@@ -169,10 +220,52 @@ pub enum Type(u16) {
 }
 }
 
+impl Type {
+    pub(crate) fn byte_len(&self) -> u8 {
+        match *self {
+            Type::BYTE | Type::SBYTE | Type::ASCII | Type::UNDEFINED => 1,
+            Type::SHORT | Type::SSHORT => 2,
+            Type::LONG | Type::SLONG | Type::FLOAT | Type::IFD => 4,
+            Type::LONG8
+            | Type::SLONG8
+            | Type::DOUBLE
+            | Type::RATIONAL
+            | Type::SRATIONAL
+            | Type::IFD8 => 8,
+        }
+    }
+
+    pub(crate) fn value_bytes(&self, count: u64) -> Result<u64, crate::error::TiffError> {
+        let tag_size = u64::from(self.byte_len());
+
+        match count.checked_mul(tag_size) {
+            Some(n) => Ok(n),
+            None => Err(crate::error::TiffError::LimitsExceeded),
+        }
+    }
+
+    pub(crate) fn endian_bytes(self) -> EndianBytes {
+        match self {
+            Type::BYTE | Type::SBYTE | Type::ASCII | Type::UNDEFINED => EndianBytes::One,
+            Type::SHORT | Type::SSHORT => EndianBytes::Two,
+            Type::LONG
+            | Type::SLONG
+            | Type::FLOAT
+            | Type::IFD
+            | Type::RATIONAL
+            | Type::SRATIONAL => EndianBytes::Four,
+            Type::LONG8 | Type::SLONG8 | Type::DOUBLE | Type::IFD8 => EndianBytes::Eight,
+        }
+    }
+}
+
 tags! {
 /// See [TIFF compression tags](https://www.awaresystems.be/imaging/tiff/tifftags/compression.html)
 /// for reference.
-pub enum CompressionMethod(u16) unknown("A custom compression method") {
+pub enum CompressionMethod(u16) unknown(
+    /// A custom compression method
+    unknown
+) {
     None = 1,
     Huffman = 2,
     Fax3 = 3,
@@ -187,6 +280,9 @@ pub enum CompressionMethod(u16) unknown("A custom compression method") {
 
     // Self-assigned by libtiff
     ZSTD = 0xC350,
+
+    // Self-assigned by libtiff
+    WebP = 0xC351,
 }
 }
 
@@ -200,6 +296,8 @@ pub enum PhotometricInterpretation(u16) {
     CMYK = 5,
     YCbCr = 6,
     CIELab = 8,
+    IccLab = 9,
+    ItuLab = 10,
 }
 }
 
@@ -234,10 +332,218 @@ pub enum ResolutionUnit(u16) {
 }
 
 tags! {
-pub enum SampleFormat(u16) unknown("An unknown extension sample format") {
+pub enum SampleFormat(u16) unknown(
+    /// An unknown extension sample format
+    unknown
+) {
     Uint = 1,
     Int = 2,
     IEEEFP = 3,
     Void = 4,
 }
 }
+
+tags! {
+pub enum ExtraSamples(u16) {
+    /// There is no specified association between the sample and the image.
+    Unspecified = 0,
+    /// The sample is associated alpha, i.e. pre-multiplied color.
+    AssociatedAlpha = 1,
+    /// The sample is unassociated alpha such as a mask. There might be more than one such sample.
+    UnassociatedAlpha = 2,
+}
+}
+
+/// A value represented as in-memory bytes with flexible byteorder.
+pub struct ValueBuffer {
+    /// The raw bytes of the value.
+    bytes: Vec<u8>,
+
+    /// The type of the value.
+    ty: Type,
+
+    /// The number of items, as `bytes` may be oversized while holding bytes that are initialized
+    /// but not used by any value.
+    count: u64,
+
+    /// The byte order of the value.
+    byte_order: ByteOrder,
+}
+
+impl ValueBuffer {
+    /// A value with a count of zero.
+    ///
+    /// The byte order is set to the native byte order of the platform.
+    pub fn empty(ty: Type) -> Self {
+        ValueBuffer {
+            bytes: vec![],
+            ty,
+            count: 0,
+            byte_order: ByteOrder::native(),
+        }
+    }
+
+    /// Create a value with native byte order from in-memory data.
+    pub fn from_value<T: TiffValue>(value: &T) -> Self {
+        ValueBuffer {
+            bytes: value.data().into_owned(),
+            ty: <T as TiffValue>::FIELD_TYPE,
+            count: value.count() as u64,
+            byte_order: ByteOrder::native(),
+        }
+    }
+
+    pub fn byte_order(&self) -> ByteOrder {
+        self.byte_order
+    }
+
+    pub fn data_type(&self) -> Type {
+        self.ty
+    }
+
+    /// The count of items in the value.
+    pub fn count(&self) -> u64 {
+        debug_assert!({
+            self.ty
+                .value_bytes(self.count)
+                .is_ok_and(|n| n <= self.bytes.len() as u64)
+        });
+
+        self.count
+    }
+
+    /// View the underlying raw bytes of this value.
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.bytes[..self.assumed_len_from_count()]
+    }
+
+    /// View the underlying mutable raw bytes of this value.
+    pub fn as_bytes_mut(&mut self) -> &mut [u8] {
+        let len = self.assumed_len_from_count();
+        &mut self.bytes[..len]
+    }
+
+    /// Change the byte order of the value representation.
+    pub fn set_byte_order(&mut self, byte_order: ByteOrder) {
+        let len = self.assumed_len_from_count();
+
+        self.byte_order
+            .convert(self.ty, &mut self.bytes[..len], byte_order);
+
+        self.byte_order = byte_order;
+    }
+
+    /// Prepare the internal for a value `to_len` bytes long.
+    ///
+    /// Shrinks the allocation if it is far too large or extends it if it is too small. In either
+    /// case ensures that at least `to_len` bytes are initialized for [`Self::raw_bytes_mut`].
+    pub(crate) fn prepare_length(&mut self, to_len: usize) {
+        if to_len > self.bytes.len() {
+            self.bytes.resize(to_len, 0);
+        }
+
+        if self.bytes.len() < to_len / 2 {
+            self.bytes.truncate(to_len);
+            self.bytes.shrink_to_fit();
+        }
+    }
+
+    /// Internal method to change the type and count while re-interpreting the byte buffer.
+    ///
+    /// Should only be called after writing bytes to the internal buffer prepared with
+    /// `Self::prepare_length`.
+    pub(crate) fn assume_type(&mut self, ty: Type, count: u64, bo: ByteOrder) {
+        debug_assert!({
+            ty.value_bytes(count)
+                .is_ok_and(|n| n <= self.bytes.len() as u64)
+        });
+
+        self.byte_order = bo;
+        self.ty = ty;
+        self.count = count;
+    }
+
+    pub(crate) fn raw_bytes_mut(&mut self) -> &mut [u8] {
+        &mut self.bytes
+    }
+
+    fn assumed_len_from_count(&self) -> usize {
+        usize::from(self.ty.byte_len()) * self.count as usize
+    }
+}
+
+/// Byte order of the TIFF file.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum ByteOrder {
+    /// little endian byte order
+    LittleEndian,
+    /// big endian byte order
+    BigEndian,
+}
+
+impl ByteOrder {
+    /// Get the byte order representing the running target.
+    ///
+    /// The infallibility of this method represents the fact that only little and big endian
+    /// systems are supported by the library. No mixed endian and no other weird stuff. (Note: as
+    /// of Rust 1.90 this is a tautology as Rust itself only has those two kinds).
+    pub const fn native() -> Self {
+        match () {
+            #[cfg(target_endian = "little")]
+            () => ByteOrder::LittleEndian,
+            #[cfg(target_endian = "big")]
+            () => ByteOrder::BigEndian,
+            #[cfg(not(any(target_endian = "big", target_endian = "little")))]
+            () => compile_error!("Unsupported target"),
+        }
+    }
+
+    /// Given a typed buffer, convert its contents to the specified byte order in-place.
+    ///
+    /// The buffer is assumed to represent an array of the given type. If the length of the buffer
+    /// is not divisible into an integer number of values, the behavior for the remaining bytes it
+    /// not specified.
+    pub fn convert(self, ty: Type, buffer: &mut [u8], to: ByteOrder) {
+        self.convert_endian_bytes(ty.endian_bytes(), buffer, to)
+    }
+
+    pub(crate) fn convert_endian_bytes(self, cls: EndianBytes, buffer: &mut [u8], to: ByteOrder) {
+        if self == to {
+            return;
+        }
+
+        // FIXME: at MSRV 1.89 or higher use `slice::as_chunks_mut`.
+        match cls {
+            EndianBytes::One => {
+                // No change needed
+            }
+            EndianBytes::Two => {
+                for chunk in buffer.chunks_exact_mut(2) {
+                    let chunk: &mut [u8; 2] = chunk.try_into().unwrap();
+                    *chunk = u16::from_be_bytes(*chunk).to_le_bytes();
+                }
+            }
+            EndianBytes::Four => {
+                for chunk in buffer.chunks_exact_mut(4) {
+                    let chunk: &mut [u8; 4] = chunk.try_into().unwrap();
+                    *chunk = u32::from_be_bytes(*chunk).to_le_bytes();
+                }
+            }
+            EndianBytes::Eight => {
+                for chunk in buffer.chunks_exact_mut(8) {
+                    let chunk: &mut [u8; 8] = chunk.try_into().unwrap();
+                    *chunk = u64::from_be_bytes(*chunk).to_le_bytes();
+                }
+            }
+        }
+    }
+}
+
+/// The size of individual byte-order corrected elements.
+#[derive(Clone, Copy)]
+pub(crate) enum EndianBytes {
+    One,
+    Two,
+    Four,
+    Eight,
+}

tests/COPYRIGHT

@@ -0,0 +1,8 @@
+subsubifd.tif:
+  url: https://data.kitware.com/api/v1/file/hashsum/sha512/372ca32735c8a8fdbe2286dabead9e779da63f10ba81eda84625e5273f76d74ca1a47a978f67e9c00c12f7f72009c7b2c07a641e643bb0c463812f4ae7f15d6e/download
+  credit: https://github.com/DigitalSlideArchive/tifftools/commit/4d00c70dbce828262f86c1431f7c66b1748965ac
+  license: CC0
+usda/*.tif
+  url: https://registry.opendata.aws/naip/index.html
+  credit: NAIP on AWS was accessed on 2026-01-28 from https://registry.opendata.aws/naip.
+  license: Public Domain

tests/decode_geotiff_images.rs

@@ -7,7 +7,7 @@ use tiff::ColorType;
 use std::fs::File;
 use std::path::PathBuf;
 
-const TEST_IMAGE_DIR: &str = "./tests/images";
+const TEST_IMAGE_DIR: &str = "./tests/geotiff";
 
 #[test]
 fn test_geo_tiff() {
@@ -61,3 +61,26 @@ fn test_geo_tiff() {
         assert_eq!(data.len(), 500);
     }
 }
+
+#[cfg(feature = "webp")]
+#[test]
+fn test_webp_tiff() {
+    let filenames = ["usda_naip_256_webp_z3.tif"];
+    let mut buffer = DecodingResult::U8(vec![]);
+
+    for filename in filenames.iter() {
+        let path = PathBuf::from(TEST_IMAGE_DIR).join(filename);
+        let img_file = File::open(path).expect("Cannot find test image!");
+        let mut decoder = Decoder::new(img_file).expect("Cannot create decoder");
+
+        loop {
+            decoder.read_image_to_buffer(&mut buffer).unwrap();
+
+            if !decoder.more_images() {
+                break;
+            }
+
+            decoder.next_image().unwrap();
+        }
+    }
+}

tests/decode_images.rs

@@ -4,6 +4,7 @@ use tiff::decoder::{ifd, Decoder, DecodingResult};
 use tiff::ColorType;
 
 use std::fs::File;
+use std::io::{Read, Seek};
 use std::path::PathBuf;
 
 const TEST_IMAGE_DIR: &str = "./tests/images/";
@@ -45,12 +46,12 @@ fn test_image_color_type_unsupported(file: &str, expected_type: ColorType) {
     let img_file = File::open(path).expect("Cannot find test image!");
     let mut decoder = Decoder::new(img_file).expect("Cannot create decoder");
     assert_eq!(decoder.colortype().unwrap(), expected_type);
-    assert!(match decoder.read_image() {
+    assert!(matches!(
+        decoder.read_image(),
         Err(tiff::TiffError::UnsupportedError(
             tiff::TiffUnsupportedError::UnsupportedColorType(_),
-        )) => true,
-        _ => false,
-    });
+        ))
+    ));
 }
 
 #[test]
@@ -172,7 +173,7 @@ fn test_string_tags() {
                 &s,
                 "GraphicsMagick 1.2 unreleased Q16 http://www.GraphicsMagick.org/"
             ),
-            _ => assert!(false),
+            other => unreachable!("Expected Ascii tag from file, got {other:?}"),
         };
     }
 }
@@ -231,6 +232,54 @@ fn test_tiled_rect_rgb_u8() {
     test_image_sum_u8("tiled-rect-rgb-u8.tif", ColorType::RGB(8), 62081032);
 }
 
+#[test]
+fn test_inner_access() {
+    let path = PathBuf::from(TEST_IMAGE_DIR).join("tiled-rect-rgb-u8.tif");
+    let img_file = File::open(path).expect("Cannot find test image!");
+    let mut decoder = Decoder::new(img_file).expect("Cannot create decoder");
+    assert_eq!(decoder.colortype().unwrap(), ColorType::RGB(8));
+
+    let c = decoder
+        .get_tag(tiff::tags::Tag::Compression)
+        .unwrap()
+        .into_u16()
+        .unwrap();
+    assert_eq!(c, tiff::tags::CompressionMethod::None.to_u16());
+
+    // Because the image is uncompressed, reading the first tile directly with the inner reader
+    // should yield the same result as reading it with the decoder's read_chunk method.
+    let first_offset = decoder
+        .get_tag_u32_vec(tiff::tags::Tag::TileOffsets)
+        .unwrap()
+        .into_iter()
+        .next()
+        .unwrap();
+    let first_byte_count = decoder
+        .get_tag_u32_vec(tiff::tags::Tag::TileByteCounts)
+        .unwrap()
+        .into_iter()
+        .next()
+        .unwrap();
+    decoder
+        .inner()
+        .seek(std::io::SeekFrom::Start(first_offset as u64))
+        .expect("Cannot seek to tile offset");
+    let mut buf = vec![0; first_byte_count as usize];
+    decoder
+        .inner()
+        .read_exact(&mut buf)
+        .expect("Cannot read tile data");
+    let raw_sum: u64 = buf.into_iter().map(<u64>::from).sum();
+
+    match decoder.read_chunk(0).unwrap() {
+        DecodingResult::U8(chunk) => {
+            let sum: u64 = chunk.into_iter().map(<u64>::from).sum();
+            assert_eq!(sum, raw_sum);
+        }
+        _ => panic!("Wrong bit depth"),
+    }
+}
+
 /* #[test]
 fn test_tiled_jpeg_rgb_u8() {
     test_image_sum_u8("tiled-jpeg-rgb-u8.tif", ColorType::RGB(8), 93031606);
@@ -292,6 +341,11 @@ fn test_tiled_incremental() {
 
 #[test]
 fn test_planar_rgb_u8() {
+    test_image_sum_u8("planar-rgb-u8.tif", ColorType::RGB(8), 15417630);
+}
+
+#[test]
+fn test_read_planar_bands() {
     // gdal_translate tiled-rgb-u8.tif planar-rgb-u8.tif -co INTERLEAVE=BAND -co COMPRESS=LZW -co PROFILE=BASELINE
     let file = "planar-rgb-u8.tif";
     let expected_type = ColorType::RGB(8);
@@ -328,8 +382,6 @@ fn test_planar_rgb_u8() {
         }
         _ => panic!("Wrong bit depth"),
     }
-
-    test_image_sum_u8(file, ColorType::RGB(8), 15417630);
 }
 
 #[test]
@@ -348,7 +400,7 @@ fn test_div_zero() {
 
     match err {
         TiffError::FormatError(TiffFormatError::StripTileTagConflict) => {}
-        unexpected => panic!("Unexpected error {}", unexpected),
+        unexpected => panic!("Unexpected error {unexpected}"),
     }
 }
 
@@ -366,7 +418,7 @@ fn test_too_many_value_bytes() {
 
     match error {
         tiff::TiffError::LimitsExceeded => {}
-        unexpected => panic!("Unexpected error {}", unexpected),
+        unexpected => panic!("Unexpected error {unexpected}"),
     }
 }
 
@@ -497,7 +549,7 @@ fn timeout() {
 
     match error {
         TiffError::FormatError(TiffFormatError::CycleInOffsets) => {}
-        e => panic!("Unexpected error {:?}", e),
+        e => panic!("Unexpected error {e:?}"),
     }
 }
 
@@ -522,3 +574,149 @@ fn test_zstd_compression() {
     // gdal_translate -co COMPRESS=ZSTD -co ZSTD_LEVEL=20 int16.tif int16_zstd.tif
     test_image_sum_i16("int16_zstd.tif", ColorType::Gray(16), 354396);
 }
+
+fn test_image_bytes(
+    file: &str,
+    expected: &[(ColorType, u32)],
+    // Represent all samples as little endian, ensuring check sums on all platforms.
+    normalize_byte_order: fn(&mut [u8]),
+) {
+    let path = PathBuf::from(TEST_IMAGE_DIR).join(file);
+    let img_file = File::open(path).expect("Cannot find test image!");
+    let mut decoder = Decoder::new(img_file).expect("Cannot create decoder");
+
+    let mut buffer = vec![0u8; 0];
+    for (idx, &(expected_type, expected_crc)) in expected.iter().enumerate() {
+        if idx > 0 {
+            decoder
+                .next_image()
+                .expect("mismatched number of directories");
+        }
+
+        assert_eq!(decoder.colortype().unwrap(), expected_type);
+        let layout = decoder
+            .image_buffer_layout()
+            .expect("Cannot describe buffer layout");
+
+        buffer.resize(layout.len, 0u8);
+        decoder.read_image_bytes(&mut buffer).unwrap();
+        normalize_byte_order(&mut buffer);
+
+        // Well almost a crc..
+        assert_eq!(crc32fast::hash(&buffer), expected_crc);
+    }
+}
+
+fn byte_order_u16(bytes: &mut [u8]) {
+    for chunk in bytes.chunks_exact_mut(2) {
+        let n = u16::from_ne_bytes(chunk.try_into().unwrap());
+        chunk.copy_from_slice(&n.to_le_bytes());
+    }
+}
+
+fn byte_order_u32(bytes: &mut [u8]) {
+    for chunk in bytes.chunks_exact_mut(4) {
+        let n = u32::from_ne_bytes(chunk.try_into().unwrap());
+        chunk.copy_from_slice(&n.to_le_bytes());
+    }
+}
+
+fn byte_order_u64(bytes: &mut [u8]) {
+    for chunk in bytes.chunks_exact_mut(8) {
+        let n = u64::from_ne_bytes(chunk.try_into().unwrap());
+        chunk.copy_from_slice(&n.to_le_bytes());
+    }
+}
+
+#[test]
+fn bytes_cmyk_3c_8b() {
+    test_image_bytes(
+        "cmyk-3c-8b.tiff",
+        &[(ColorType::CMYK(8), 2935032230)],
+        |_| (),
+    );
+}
+
+#[test]
+fn bytes_gray_16b() {
+    test_image_bytes(
+        "minisblack-1c-16b.tiff",
+        &[(ColorType::Gray(16), 3524597615)],
+        byte_order_u16,
+    );
+}
+
+#[test]
+fn bytes_gray_32b() {
+    test_image_bytes(
+        "gradient-1c-32b.tiff",
+        &[(ColorType::Gray(32), 2257036530)],
+        byte_order_u32,
+    );
+}
+
+#[test]
+fn bytes_gray_u64() {
+    test_image_bytes(
+        "gradient-1c-64b.tiff",
+        &[(ColorType::Gray(64), 2156547113)],
+        byte_order_u64,
+    );
+}
+
+#[test]
+fn bytes_gray_f32() {
+    test_image_bytes(
+        "gradient-1c-32b-float.tiff",
+        &[(ColorType::Gray(32), 3232356730)],
+        byte_order_u32,
+    );
+}
+
+#[test]
+#[cfg(feature = "fax")]
+fn test_fax4() {
+    test_image_sum_u8("fax4.tiff", ColorType::Gray(1), 7802706);
+}
+
+#[test]
+#[cfg(feature = "fax")]
+fn test_fax4_white_is_min() {
+    test_image_sum_u8("imagemagick_group4.tiff", ColorType::Gray(1), 3742820);
+}
+
+#[test]
+fn write_extra_bits_image() {
+    use tiff::{encoder, tags::ExtraSamples};
+
+    let path = PathBuf::from(TEST_IMAGE_DIR).join("extra_bits_rgb_8b.tiff");
+    let img_file = File::create(path).expect("Cannot find test image!");
+
+    let mut encoder = encoder::TiffEncoder::new(img_file).expect("Cannot create encoder");
+    let mut img = encoder.new_image::<encoder::colortype::RGB8>(8, 8).unwrap();
+
+    img.extra_samples(&[ExtraSamples::Unspecified]).unwrap();
+
+    // all pixels are supposed to be 0, the extra is 1
+    let data: [u8; 256] = core::array::from_fn(|i| (i as u8 % 4).saturating_sub(2));
+
+    img.write_data(&data)
+        .expect("correct amount of data to write");
+}
+
+#[test]
+fn extra_bits_gray() {
+    test_image_sum_u8(
+        "extra_bits_gray_8b.tiff",
+        ColorType::Multiband {
+            bit_depth: 8,
+            num_samples: 2,
+        },
+        64,
+    );
+}
+
+#[test]
+fn extra_bits_rgb() {
+    test_image_sum_u8("extra_bits_rgb_8b.tiff", ColorType::RGB(8), 0);
+}

tests/decode_planes.rs

@@ -0,0 +1,123 @@
+use tiff::decoder::{Decoder, DecodingResult, Limits, TiffCodingUnit};
+
+use std::fs::File;
+use std::path::PathBuf;
+
+const TEST_IMAGE_DIR: &str = "./tests/images/";
+
+#[test]
+fn read_all_planes() {
+    let path = PathBuf::from(TEST_IMAGE_DIR).join("planar-rgb-u8.tif");
+    let file = File::open(path).unwrap();
+    let io = std::io::BufReader::new(file);
+
+    let mut tif = Decoder::new(io).unwrap();
+    let layout = tif.image_buffer_layout().unwrap();
+    assert_eq!(layout.planes, 3);
+    let plane_stride = layout.plane_stride.map_or(0, |x| x.get());
+
+    eprintln!("{:?}", layout);
+    let mut buffer = vec![0u8; layout.complete_len];
+    tif.read_image_bytes(&mut buffer).unwrap();
+
+    // Mainly you can see: these are different and non-zero. Otherwise these magic constants just
+    // depend on the file itself that is being used.
+    const PLANE_EXCPECTED_SUMS: [u32; 3] = [15417630, 13007788, 11103530];
+
+    for (plane, expected) in (0..layout.planes).zip(PLANE_EXCPECTED_SUMS) {
+        let plane_data = &buffer[plane_stride * plane..][..layout.len];
+        let sum: u32 = plane_data.iter().copied().map(u32::from).sum();
+        assert_eq!(sum, expected);
+    }
+}
+
+#[test]
+fn read_first_coding_unit() {
+    let path = PathBuf::from(TEST_IMAGE_DIR).join("planar-rgb-u8.tif");
+    let file = File::open(path).unwrap();
+    let io = std::io::BufReader::new(file);
+
+    let mut tif = Decoder::new(io).unwrap();
+    let layout = tif.image_coding_unit_layout(TiffCodingUnit(0)).unwrap();
+
+    assert_eq!(layout.planes, 3);
+    let plane_stride = layout.plane_stride.map_or(0, |x| x.get());
+    eprintln!("{:?}", layout);
+    let mut buffer = vec![0u8; layout.complete_len];
+
+    struct Expected {
+        plane_sums: [u32; 3],
+        unit: TiffCodingUnit,
+    }
+
+    // Mainly you can see: these are different and non-zero. Otherwise these magic constants just
+    // depend on the file itself that is being used. Also this varies by what unit we are reading.
+    const PLANE_EXCPECTED_SUMS: [Expected; 2] = [
+        Expected {
+            plane_sums: [856853, 767686, 688532],
+            unit: TiffCodingUnit(0),
+        },
+        Expected {
+            plane_sums: [883679, 797329, 711086],
+            unit: TiffCodingUnit(1),
+        },
+    ];
+
+    for expected in PLANE_EXCPECTED_SUMS {
+        tif.read_coding_unit_bytes(expected.unit, &mut buffer)
+            .unwrap();
+
+        for (plane, expected) in (0..layout.planes).zip(expected.plane_sums) {
+            let plane_data = &buffer[plane_stride * plane..][..layout.len];
+            let sum: u32 = plane_data.iter().copied().map(u32::from).sum();
+            assert_eq!(sum, expected);
+        }
+    }
+}
+
+#[test]
+fn read_planes_by_limits() {
+    let path = PathBuf::from(TEST_IMAGE_DIR).join("planar-rgb-u8.tif");
+    let file = File::open(path).unwrap();
+    let io = std::io::BufReader::new(file);
+
+    let mut limits = Limits::default();
+    let mut buffer = DecodingResult::U8(vec![]);
+
+    // Make room for 1 plane.
+    let mut tif = Decoder::new(io).unwrap();
+    tif = tif.with_limits({
+        limits.decoding_buffer_size = 374 * 499;
+        limits.clone()
+    });
+
+    tif.read_image_to_buffer(&mut buffer).unwrap();
+    assert_eq!(buffer.as_buffer(0).byte_len(), 374 * 499);
+
+    // Make room for 2 planes.
+    tif = tif.with_limits({
+        limits.decoding_buffer_size = 374 * 499 * 2;
+        limits.clone()
+    });
+
+    tif.read_image_to_buffer(&mut buffer).unwrap();
+    assert_eq!(buffer.as_buffer(0).byte_len(), 374 * 499);
+
+    // Make room for 3 planes.
+    tif = tif.with_limits({
+        limits.decoding_buffer_size = 374 * 499 * 3;
+        limits.clone()
+    });
+
+    tif.read_image_to_buffer(&mut buffer).unwrap();
+    assert_eq!(buffer.as_buffer(0).byte_len(), 374 * 499 * 3);
+
+    // Make room for 4 planes?
+    tif = tif.with_limits({
+        limits.decoding_buffer_size = 374 * 499 * 4;
+        limits.clone()
+    });
+
+    tif.read_image_to_buffer(&mut buffer).unwrap();
+    assert_eq!(buffer.as_buffer(0).byte_len(), 374 * 499 * 3);
+}

tests/encode_images.rs

@@ -57,7 +57,7 @@ fn encode_decode() {
                 ),
                 (Tag::Compression, ifd::Value::Short(1)),
                 (Tag::PhotometricInterpretation, ifd::Value::Short(2)),
-                (Tag::StripOffsets, ifd::Value::Unsigned(8)),
+                (Tag::StripOffsets, ifd::Value::Unsigned(35)),
                 (Tag::SamplesPerPixel, ifd::Value::Short(3)),
                 (Tag::RowsPerStrip, ifd::Value::Unsigned(3334)),
                 (Tag::StripByteCounts, ifd::Value::Unsigned(30000)),
@@ -81,7 +81,10 @@ fn encode_decode() {
             decoder.get_tag(Tag::Artist).unwrap(),
             ifd::Value::Ascii("Image-tiff".into())
         );
-        if let DecodingResult::U8(img_res) = decoder.read_image().unwrap() {
+
+        let mut buffer = tiff::decoder::DecodingResult::U8(vec![]);
+        let _layout = decoder.read_image_to_buffer(&mut buffer).unwrap();
+        if let tiff::decoder::DecodingResult::U8(img_res) = buffer {
             assert_eq!(image_data, img_res);
         } else {
             panic!("Wrong data type");
@@ -137,7 +140,7 @@ fn encode_decode_big() {
                 ),
                 (Tag::Compression, ifd::Value::Short(1)),
                 (Tag::PhotometricInterpretation, ifd::Value::Short(2)),
-                (Tag::StripOffsets, ifd::Value::UnsignedBig(16)),
+                (Tag::StripOffsets, ifd::Value::UnsignedBig(27)),
                 (Tag::SamplesPerPixel, ifd::Value::Short(3)),
                 (Tag::RowsPerStrip, ifd::Value::Unsigned(3334)),
                 (Tag::StripByteCounts, ifd::Value::UnsignedBig(30000)),
@@ -161,7 +164,9 @@ fn encode_decode_big() {
             decoder.get_tag(Tag::Artist).unwrap(),
             ifd::Value::Ascii("Image-tiff".into())
         );
-        if let DecodingResult::U8(img_res) = decoder.read_image().unwrap() {
+        let mut buffer = tiff::decoder::DecodingResult::U8(vec![]);
+        let _layout = decoder.read_image_to_buffer(&mut buffer).unwrap();
+        if let DecodingResult::U8(img_res) = buffer {
             assert_eq!(image_data, img_res);
         } else {
             panic!("Wrong data type");
@@ -235,6 +240,7 @@ const TEST_IMAGE_DIR: &str = "./tests/images/";
 
 macro_rules! test_roundtrip {
     ($name:ident, $buffer:ident, $buffer_ty:ty) => {
+        #[track_caller]
         fn $name<C: colortype::ColorType<Inner = $buffer_ty>>(
             file: &str,
             expected_type: ColorType,
@@ -244,7 +250,10 @@ macro_rules! test_roundtrip {
             let mut decoder = Decoder::new(img_file).expect("Cannot create decoder");
             assert_eq!(decoder.colortype().unwrap(), expected_type);
 
-            let image_data = match decoder.read_image().unwrap() {
+            let mut buffer = tiff::decoder::DecodingResult::U8(vec![]);
+            decoder.read_image_to_buffer(&mut buffer).unwrap();
+
+            let image_data = match buffer {
                 DecodingResult::$buffer(res) => res,
                 _ => panic!("Wrong data type"),
             };
@@ -258,9 +267,11 @@ macro_rules! test_roundtrip {
             }
             file.seek(SeekFrom::Start(0)).unwrap();
             {
+                let mut buffer = tiff::decoder::DecodingResult::U8(vec![]);
                 let mut decoder = Decoder::new(&mut file).unwrap();
-                if let DecodingResult::$buffer(img_res) = decoder.read_image().unwrap() {
-                    assert_eq!(image_data, img_res);
+                decoder.read_image_to_buffer(&mut buffer).unwrap();
+                if let DecodingResult::$buffer(img_res) = buffer {
+                    assert_eq!(*image_data, img_res);
                 } else {
                     panic!("Wrong data type");
                 }
@@ -621,8 +632,62 @@ fn test_rows_per_strip() {
             let img2 = [i; 2 * 100];
             match decoder.read_chunk(i as u32).unwrap() {
                 DecodingResult::U8(data) => assert_eq!(&img2[..], &data[..]),
-                other => panic!("Incorrect strip type {:?}", other),
+                other => panic!("Incorrect strip type {other:?}"),
             }
         }
     }
 }
+
+#[test]
+fn test_auxiliary_directory() {
+    const ARTIST: &str = "Auxiliary-tiff";
+
+    let mut data = Cursor::new(Vec::new());
+    let mut tiff = TiffEncoder::new(&mut data).unwrap();
+
+    {
+        let mut image_encoder = tiff.new_image::<colortype::Gray8>(1, 1).unwrap();
+        image_encoder.write_strip(&[1]).unwrap();
+    }
+
+    let exif = {
+        let mut extra = tiff.extra_directory().unwrap();
+        extra.write_tag(Tag::Artist, ARTIST).unwrap();
+        extra.finish_with_offsets().unwrap()
+    };
+
+    {
+        let mut image_encoder = tiff.new_image::<colortype::Gray8>(2, 1).unwrap();
+        image_encoder.write_strip(&[1, 2]).unwrap();
+        let encoder = image_encoder.encoder();
+        encoder.write_tag(Tag::ExifDirectory, exif.offset).unwrap();
+    }
+
+    data.set_position(0);
+    let mut decoder = Decoder::new(&mut data).unwrap();
+
+    {
+        assert_eq!(decoder.dimensions().unwrap(), (1, 1));
+        let _ = decoder.read_image().unwrap();
+    }
+
+    {
+        decoder.next_image().unwrap();
+        assert_eq!(decoder.dimensions().unwrap(), (2, 1));
+        let _ = decoder.read_image().unwrap();
+
+        let exif_location = decoder
+            .get_tag(Tag::ExifDirectory)
+            .expect("second directory missing Exif")
+            .into_ifd_pointer()
+            .expect("exif directory is not an IFD pointer");
+
+        let exif = decoder.read_directory(exif_location).unwrap();
+
+        let artist = decoder
+            .read_directory_tags(&exif)
+            .get_tag_ascii_string(Tag::Artist)
+            .expect("EXIF directory missing artist tag");
+        assert_eq!(artist, ARTIST);
+    }
+}

tests/encode_images_with_compression.rs

@@ -116,7 +116,7 @@ fn encode_decode_with_compression(compression: Compression) {
         assert_eq!(
             match decoder.read_image() {
                 Ok(DecodingResult::U16(image_data)) => image_data,
-                unexpected => panic!("Descoding RGB failed: {:?}", unexpected),
+                unexpected => panic!("Descoding RGB failed: {unexpected:?}"),
             },
             image_rgb.reference_data()
         );
@@ -126,7 +126,7 @@ fn encode_decode_with_compression(compression: Compression) {
         assert_eq!(
             match decoder.read_image() {
                 Ok(DecodingResult::U8(image_data)) => image_data,
-                unexpected => panic!("Decoding grayscale failed: {:?}", unexpected),
+                unexpected => panic!("Decoding grayscale failed: {unexpected:?}"),
             },
             image_grayscale.reference_data()
         );

tests/predict.rs

@@ -1,7 +1,7 @@
 extern crate tiff;
 
 use tiff::decoder::{Decoder, DecodingResult};
-use tiff::encoder::{colortype, Compression, Predictor, TiffEncoder};
+use tiff::encoder::{colortype, Predictor, TiffEncoder};
 use tiff::ColorType;
 
 use std::fs::File;

tests/subsubifds.rs

@@ -0,0 +1,41 @@
+use tiff::decoder::Decoder;
+
+// This file has the following IFD structure:
+//
+// ```
+// IFD -> IFD -> IFD
+//        |-> IFD
+//        |-> IFD -> IFD
+//        |          \-> IFD -> IFD
+//        \-> IFD
+// ```
+//
+// where the main chain of IFD has 3 images, the second of which has several sub-IFDs and some have
+// more nested IFDs.
+const TEST_IMAGE_SUBIFD: &str = "./tests/subsubifds.tif";
+use std::fs::File;
+
+#[test]
+fn decode_seek_chain() {
+    let file = File::open(TEST_IMAGE_SUBIFD).expect("Cannot open test image");
+    let mut decoder = Decoder::new(file).expect("Invalid format to create decoder");
+
+    // Remember the offsets in the first iteration.
+    let offset0 = decoder.ifd_pointer().expect("First IFD pointer not found");
+    decoder.next_image().unwrap();
+    let offset1 = decoder.ifd_pointer().expect("Second IFD pointer not found");
+    decoder.next_image().unwrap();
+    let offset2 = decoder.ifd_pointer().expect("Third IFD pointer not found");
+    assert!(!decoder.more_images());
+
+    // Ensure we can seek, to do this again.
+    decoder
+        .seek_to_image(0)
+        .expect("Failed to seek to first image");
+    assert_eq!(Some(offset0), decoder.ifd_pointer());
+    decoder.next_image().unwrap();
+    assert_eq!(Some(offset1), decoder.ifd_pointer());
+    decoder.next_image().unwrap();
+    assert_eq!(Some(offset2), decoder.ifd_pointer());
+    assert!(!decoder.more_images());
+}