Retain LZW decoder across tiles

Fix: clip border tile rows when discarding extra samples

CHANGES.md

@@ -1,5 +1,12 @@
 # Version 0.12.0 (unreleased)
 
+Fixes:
+- Decoding a tiled image whose width is not a multiple of the tile width no
+  longer corrupts the columns below the right border tiles when extra samples
+  are discarded (e.g. RGB with an unspecified extra sample). The horizontal
+  tile padding was written past the end of the output row, spilling into the
+  leftmost columns of the following row.
+
 Additions:
 - Added the `TiffHeader` type. It can be used to create a `Decoder` in a state
   before the first IFD and allows opening files at an arbitrary first IFD

Cargo.toml

@@ -25,6 +25,7 @@ fax34 = { package = "fax", version = "0.2.6", optional = true }
 flate2 = { version = "1.0.20", optional = true }
 weezl = { version = "0.2.1", optional = true }
 zstd = { version = "0.13", optional = true }
+zrip-decode = { version = "0.8.2", features = ["paranoid"], optional = true }
 zune-jpeg = { version = "0.5.1", optional = true }
 image-webp = {version = "0.2.4", optional = true }
 
@@ -45,6 +46,9 @@ jpeg = ["dep:zune-jpeg"]
 lzw = ["dep:weezl"]
 webp = ["dep:image-webp"]
 zstd = ["dep:zstd"]
+# Fully safe Rust but somewhat slower Zstd decoding.
+# When both zstd and zstd-safe-rust are enabled, zstd takes precedence.
+zstd-safe-rust = ["dep:zrip-decode"]
 
 [[bench]]
 name = "lzw"

examples/decode.rs

@@ -2,13 +2,15 @@ use tiff::decoder::{BufferLayoutPreference, Decoder, DecodingSampleBuffer, Decod
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
     let Some(image) = std::env::args_os().nth(1) else {
-        eprintln!("Usage: decode FILE");
+        eprintln!("Usage: decode FILE [--lenient]");
         return Ok(());
     };
 
+    let lenient = std::env::args().nth(2).as_deref() == Some("--lenient");
+
     let file = std::fs::File::open(image)?;
     let io = std::io::BufReader::new(file);
-    let mut reader = Decoder::open(io)?;
+    let mut reader = Decoder::open(io)?.with_lenient(lenient);
 
     let mut data = DecodingSampleBuffer::I8(vec![]);
 

src/decoder/ifd.rs

@@ -363,7 +363,7 @@ impl ::std::fmt::Debug for Entry {
     fn fmt(&self, fmt: &mut ::std::fmt::Formatter) -> Result<(), ::std::fmt::Error> {
         fmt.write_str(&format!(
             "Entry {{ type_: {:?}, count: {:?}, offset: {:?} }}",
-            self.type_, self.count, &self.offset
+            self.type_, self.count, self.offset
         ))
     }
 }

src/decoder/image.rs

@@ -1,8 +1,9 @@
 use super::ifd::Value;
-use super::stream::PackBitsReader;
+use super::stream::{PackBitsReader, ReadSamples};
 use super::tag_reader::TagReader;
 use super::ChunkType;
 use super::{predict_f16, predict_f32, predict_f64, ValueReader};
+use crate::decoder::stream::CompressionEngines;
 use crate::tags::{
     ByteOrder, CompressionMethod, ExtraSamples, PhotometricInterpretation, PlanarConfiguration,
     Predictor, SampleFormat, Tag,
@@ -11,7 +12,7 @@ use crate::{
     ColorType, Directory, TiffError, TiffFormatError, TiffResult, TiffUnsupportedError, UsageError,
 };
 
-use std::io::{self, Cursor, Read, Seek};
+use std::io::{Cursor, Read, Seek};
 use std::sync::Arc;
 
 #[derive(Debug)]
@@ -97,6 +98,7 @@ pub(crate) struct Image {
     /// run on samples directly and thus skip the outer byte order entirely, except potentially
     /// they have interior byte order treatment.
     pub decompression_to_host_endian: bool,
+    pub lenient: bool,
 }
 
 /// Describes how to read a tile-aligned portion of the image.
@@ -158,6 +160,7 @@ impl Image {
     pub fn from_reader<R: Read + Seek>(
         decoder: &mut ValueReader<R>,
         ifd: &Directory,
+        lenient: bool,
     ) -> TiffResult<Image> {
         let mut tag_reader = TagReader { decoder, ifd };
 
@@ -500,6 +503,7 @@ impl Image {
                 compression_method,
                 CompressionMethod::SgiLog | CompressionMethod::SgiLog24
             ),
+            lenient,
         })
     }
 
@@ -515,10 +519,13 @@ impl Image {
     }
 
     pub(crate) fn colortype(&self) -> TiffResult<ColorType> {
-        let is_alpha_extra_samples = matches!(
-            self.extra_samples.as_slice(),
-            [ExtraSamples::AssociatedAlpha, ..] | [ExtraSamples::UnassociatedAlpha, ..]
-        );
+        let is_alpha_extra_samples = match self.extra_samples.as_slice().first() {
+            Some(ExtraSamples::AssociatedAlpha) | Some(ExtraSamples::UnassociatedAlpha) => true,
+            // In lenient mode, keep unspecified alpha interpretation logic compatible with libtiff
+            // See Issue #341
+            Some(ExtraSamples::Unspecified) if self.lenient && self.samples > 3 => true,
+            _ => false,
+        };
 
         match self.photometric_interpretation {
             PhotometricInterpretation::RGB => match self.photometric_samples {
@@ -591,20 +598,30 @@ impl Image {
                     ),
                 )),
             },
-            // 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(
+            // CIE L*a*b* (TIFF6 Section 23, pp 110-115) encodes a/b as signed integers
+            // (int8 at 8-bit, int16 at 16-bit) with 0 neutral, while ICC L*a*b*
+            // (photometric=9) uses unsigned samples with the neutral at the middle of
+            // the range. `SampleFormat` applies uniformly to all samples, so it can't
+            // describe the L=unsigned + a/b=signed mix; instead we take a/b as signed
+            // from the photometric interpretation (requiring `SampleFormat::Uint`, like
+            // the ICCLab arm) and re-bias them to the unsigned ICCLab encoding in
+            // post-processing (see `cielab_to_icclab`). libtiff does the same, decoding
+            // both depths (`putcontig8bitCIELab8` / `putcontig8bitCIELab16`, selected by
+            // `initCIELabConversion` in tif_getimage.c line 2216
+            // @ 04884181e8903915038be553bfab404302d98ea0).
+            PhotometricInterpretation::CIELab => match self.photometric_samples {
+                3 if matches!(self.bits_per_sample, 8 | 16)
+                    && matches!(self.sample_format, SampleFormat::Uint) =>
+                {
+                    Ok(ColorType::Lab(self.bits_per_sample))
+                }
+                _ => Err(TiffError::UnsupportedError(
                     TiffUnsupportedError::InterpretationWithBits(
                         PhotometricInterpretation::CIELab,
                         vec![self.bits_per_sample; self.samples as usize],
                     ),
                 )),
+            },
             PhotometricInterpretation::CIELogL => match self.samples {
                 1 if matches!(self.bits_per_sample, 32)
                     && matches!(self.sample_format, SampleFormat::IEEEFP) =>
@@ -660,7 +677,11 @@ impl Image {
                 ),
             )),
             PhotometricInterpretation::RGBPalette => Ok(ColorType::Palette(self.bits_per_sample)),
-            PhotometricInterpretation::TransparencyMask => Err(TiffError::UnsupportedError(
+            // TransparencyMask is a single-channel mask, treat as grayscale.
+            PhotometricInterpretation::TransparencyMask => {
+                Ok(ColorType::Gray(self.bits_per_sample))
+            }
+            PhotometricInterpretation::Unknown(_) => Err(TiffError::UnsupportedError(
                 TiffUnsupportedError::InterpretationWithBits(
                     self.photometric_interpretation,
                     vec![self.bits_per_sample; self.samples as usize],
@@ -678,7 +699,8 @@ impl Image {
         dimensions: (u32, u32),
         samples: u16,
         #[cfg_attr(not(feature = "fax"), allow(unused_variables))] fill_order: u16,
-    ) -> TiffResult<Box<dyn Read + 'r>> {
+        engines: &mut CompressionEngines,
+    ) -> TiffResult<Box<dyn ReadSamples + 'r>> {
         Ok(match compression_method {
             CompressionMethod::None => Box::new(reader),
             CompressionMethod::SgiLog => match samples {
@@ -709,9 +731,12 @@ impl Image {
             CompressionMethod::LZW => Box::new(super::stream::LZWReader::new(
                 reader,
                 usize::try_from(compressed_length)?,
+                engines,
             )),
             #[cfg(feature = "zstd")]
             CompressionMethod::ZSTD => Box::new(zstd::Decoder::new(reader)?),
+            #[cfg(all(not(feature = "zstd"), feature = "zstd-safe-rust"))]
+            CompressionMethod::ZSTD => Box::new(zrip_decode::streaming::FrameDecoder::new(reader)),
             CompressionMethod::PackBits => Box::new(PackBitsReader::new(reader, compressed_length)),
             #[cfg(feature = "deflate")]
             CompressionMethod::Deflate | CompressionMethod::OldDeflate => {
@@ -1011,6 +1036,7 @@ impl Image {
         layout: &ReadoutLayout,
         chunk_index: u32,
         scratch: &mut Vec<u8>,
+        engines: &mut CompressionEngines,
     ) -> TiffResult<()> {
         let ValueReader {
             reader,
@@ -1036,6 +1062,7 @@ impl Image {
             | ColorType::YCbCr(n)
             | ColorType::Gray(n)
             | ColorType::Palette(n)
+            | ColorType::Lab(n)
             | ColorType::Multiband {
                 bit_depth: n,
                 num_samples: _,
@@ -1047,6 +1074,7 @@ impl Image {
             | ColorType::YCbCr(n)
             | ColorType::Gray(n)
             | ColorType::Palette(n)
+            | ColorType::Lab(n)
             | ColorType::Multiband {
                 bit_depth: n,
                 num_samples: _,
@@ -1155,6 +1183,7 @@ impl Image {
             chunk_dims,
             self.samples,
             self.fill_order,
+            engines,
         )?;
 
         // Extended bit depths (9-15, 17-31): packed N-bit samples must be unpacked to
@@ -1190,15 +1219,26 @@ impl Image {
                 super::fix_endianness_and_predict(
                     out_row,
                     color_type.bit_depth(),
-                    samples,
+                    data_samples,
                     ByteOrder::native(),
                     predictor,
                     scratch,
                 );
 
-                if photometric_interpretation == PhotometricInterpretation::WhiteIsZero {
-                    super::invert_colors(out_row, color_type, self.sample_format)?;
-                }
+                // An n-bit predictor delta is stored modulo 2^n, but the
+                // reconstruction above accumulated it in the promoted u16/u32
+                // storage width. Reduce back modulo the declared depth so both
+                // the inverted and non-inverted paths see exact, in-range
+                // values (2^n divides the storage modulus, so one mask after
+                // the row is exact).
+                super::mask_samples_to_bit_depth(out_row, tiff_bps, out_bps, self.sample_format);
+
+                super::post_process_row(
+                    out_row,
+                    photometric_interpretation,
+                    color_type,
+                    self.sample_format,
+                )?;
             }
         } else if is_output_chunk_rows && is_all_bits {
             // Here we can read directly into the output buffer itself.
@@ -1216,9 +1256,22 @@ impl Image {
                 );
             }
 
-            if photometric_interpretation == PhotometricInterpretation::WhiteIsZero {
-                super::invert_colors(tile, color_type, self.sample_format)?;
-            }
+            // No-op in this branch: promoted layouts (declared depth < storage
+            // width) are handled exclusively by the unpacking branch above.
+            // Called in every integer branch so they cannot diverge.
+            super::mask_samples_to_bit_depth(
+                tile,
+                layout.tiff_bits_per_sample,
+                layout.output_bits_per_sample,
+                self.sample_format,
+            );
+
+            super::post_process_row(
+                tile,
+                photometric_interpretation,
+                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.
@@ -1242,9 +1295,14 @@ impl Image {
                     }
                     _ => unreachable!(),
                 }
-                if photometric_interpretation == PhotometricInterpretation::WhiteIsZero {
-                    super::invert_colors(row, color_type, self.sample_format)?;
-                }
+                // No masking here: the floating point predictor requires
+                // IEEEFP samples, which are never depth-promoted.
+                super::post_process_row(
+                    row,
+                    photometric_interpretation,
+                    color_type,
+                    self.sample_format,
+                )?;
             }
         } else if is_all_bits {
             // We read row-by-row but each row fits in its output buffer.
@@ -1255,11 +1313,12 @@ impl Image {
                 // 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)?;
-                    io::copy(&mut reader.by_ref().take(len), &mut io::sink())?;
+                    // Skip horizontal padding
+                    let skip = u64::try_from(chunk_row_bytes - data_row_bytes)?;
+                    reader.read_padded_samples(&mut row[..used], skip)?;
+                } else {
+                    reader.read_exact(&mut row[..used])?;
                 }
 
                 super::fix_endianness_and_predict(
@@ -1271,9 +1330,22 @@ impl Image {
                     scratch,
                 );
 
-                if photometric_interpretation == PhotometricInterpretation::WhiteIsZero {
-                    super::invert_colors(row, color_type, self.sample_format)?;
-                }
+                // No-op in this branch: promoted layouts only occur in the
+                // unpacking branch. Called so the integer branches cannot
+                // diverge.
+                super::mask_samples_to_bit_depth(
+                    row,
+                    layout.tiff_bits_per_sample,
+                    layout.output_bits_per_sample,
+                    self.sample_format,
+                );
+
+                super::post_process_row(
+                    row,
+                    photometric_interpretation,
+                    color_type,
+                    self.sample_format,
+                )?;
             }
         } else {
             // The encoded data potentially takes up more space than the output data so we must be
@@ -1300,6 +1372,10 @@ impl Image {
             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) {
+                // Clip to the chunk's data width: border chunks are stored at the full chunk
+                // width, and writing their horizontal padding would spill into the following
+                // output row (and predict/invert would run past the chunk's own columns).
+                let row = &mut row[..data_row_bytes];
                 reader.read_exact(&mut encoded)?;
 
                 Self::compact_photometric_bytes(&mut encoded, row, &photo_range);
@@ -1313,12 +1389,26 @@ impl Image {
                     scratch,
                 );
 
-                if photometric_interpretation == PhotometricInterpretation::WhiteIsZero {
-                    super::invert_colors(row, color_type, self.sample_format)?;
-                }
+                // No-op in this branch: promoted layouts only occur in the
+                // unpacking branch. Called so the integer branches cannot
+                // diverge.
+                super::mask_samples_to_bit_depth(
+                    row,
+                    layout.tiff_bits_per_sample,
+                    layout.output_bits_per_sample,
+                    self.sample_format,
+                );
+
+                super::post_process_row(
+                    row,
+                    photometric_interpretation,
+                    color_type,
+                    self.sample_format,
+                )?;
             }
         }
 
+        reader.cache_or_destroy(engines);
         Ok(())
     }
 

src/decoder/logluv.rs

@@ -83,6 +83,14 @@ impl<R: Read> Read for LogLuv24<R> {
     }
 }
 
+impl<R: Read> super::stream::ReadSamples for LogLuv24<R> {
+    fn read_padded_samples(&mut self, buf: &mut [u8], discard: u64) -> std::io::Result<()> {
+        self.read_exact(buf)?;
+        std::io::copy(&mut self.by_ref().take(discard), &mut std::io::sink())?;
+        Ok(())
+    }
+}
+
 fn packbits_decode<R: Read, const N: usize>(
     reader: &mut R,
     buf: &mut [[u8; N]],
@@ -212,6 +220,14 @@ impl<R: Read> Read for LogLuv32<R> {
     }
 }
 
+impl<R: Read> super::stream::ReadSamples for LogLuv32<R> {
+    fn read_padded_samples(&mut self, buf: &mut [u8], discard: u64) -> std::io::Result<()> {
+        self.read_exact(buf)?;
+        std::io::copy(&mut self.by_ref().take(discard), &mut std::io::sink())?;
+        Ok(())
+    }
+}
+
 /// Pack-Bits like encoding for a single 16-bit log-L sample.
 pub struct LogLuv16<R> {
     reader: Take<R>,
@@ -278,6 +294,14 @@ impl<R: Read> Read for LogLuv16<R> {
     }
 }
 
+impl<R: Read> super::stream::ReadSamples for LogLuv16<R> {
+    fn read_padded_samples(&mut self, buf: &mut [u8], discard: u64) -> std::io::Result<()> {
+        self.read_exact(buf)?;
+        std::io::copy(&mut self.by_ref().take(discard), &mut std::io::sink())?;
+        Ok(())
+    }
+}
+
 // Table information from: `libtiff`, 1997 by Greg Ward Larson, SGI
 #[doc(alias = "UV_SQSIZ")]
 const UV_QUANTIZATION_WIDTH: f32 = 0.003500;

src/decoder/mod.rs

@@ -3,8 +3,8 @@ use std::io::{self, Read, Seek};
 use std::num::NonZeroUsize;
 
 use crate::tags::{
-    ByteOrder, IfdPointer, PlanarConfiguration, Predictor, SampleFormat, Tag, TiffVariant, Type,
-    ValueBuffer,
+    ByteOrder, IfdPointer, PhotometricInterpretation, PlanarConfiguration, Predictor, SampleFormat,
+    Tag, TiffVariant, Type, ValueBuffer,
 };
 use crate::{
     ColorType, Directory, TiffError, TiffFormatError, TiffResult, TiffUnsupportedError, UsageError,
@@ -257,6 +257,9 @@ pub struct Decoder<R> {
     /// Reusable scratch buffer for the floating-point predictor. Stored here so that the
     /// allocation persists across strips/tiles and images, avoiding per-row heap allocation.
     scratch: Vec<u8>,
+    /// Leniency flag that allows libtiff-like TIFF color interpretation behavior
+    lenient: bool,
+    compression: stream::CompressionEngines,
 }
 
 /// All the information needed to read and interpret byte slices from the underlying file, i.e. to
@@ -566,6 +569,56 @@ fn fix_endianness_and_predict(
     }
 }
 
+/// Reduce promoted extended-depth samples (9–15 bits → `u16`, 17–31 bits →
+/// `u32`) modulo their declared bit depth, i.e. mask with `(1 << n) - 1`.
+///
+/// An n-bit horizontal-predicted sample stores its delta in exactly n bits,
+/// i.e. modulo `2^n`, so reconstruction is also defined modulo `2^n`. The
+/// predictor accumulates `prev + delta` in the promoted storage width
+/// (`u16`/`u32`, see `rev_hpredict_nsamp`); since `2^n` divides the storage
+/// modulus, masking once after the predictor yields exactly the mod-`2^n`
+/// value for every sample — including legitimate wrap-arounds (a 9-bit
+/// `511, 0` pair is stored as delta `1`, and `511 + 1 = 512` masks back to
+/// `0`). Masking also bounds any out-of-range sample in malformed input, so
+/// all downstream consumers (`invert_colors`' `max - v` and the non-inverted
+/// output path) see in-range values. For samples already in range it is a
+/// no-op.
+///
+/// This runs in every integer decode branch of `expand_chunk` so the
+/// branches cannot diverge. Outside the unpacking branch it reduces to the
+/// two guards below and touches nothing: promoted layouts (`declared <
+/// storage`) exist only there — standard depths have `declared == storage`,
+/// and sub-byte depths keep byte-granular storage.
+#[inline]
+fn mask_samples_to_bit_depth(
+    buf: &mut [u8],
+    declared_bits: u8,
+    storage_bits: u8,
+    sample_format: SampleFormat,
+) {
+    // Only unsigned samples are promoted with zero-extension; signed
+    // extended depths would need sign extension instead and are not
+    // supported by the unpacking path.
+    if sample_format != SampleFormat::Uint {
+        return;
+    }
+    match storage_bits {
+        16 if declared_bits < 16 => {
+            let mask = (1u16 << declared_bits) - 1;
+            for chunk in buf.as_chunks_mut::<2>().0 {
+                *chunk = (u16::from_ne_bytes(*chunk) & mask).to_ne_bytes();
+            }
+        }
+        32 if declared_bits < 32 => {
+            let mask = (1u32 << declared_bits) - 1;
+            for chunk in buf.as_chunks_mut::<4>().0 {
+                *chunk = (u32::from_ne_bytes(*chunk) & mask).to_ne_bytes();
+            }
+        }
+        _ => {}
+    }
+}
+
 fn invert_colors(
     buf: &mut [u8],
     color_type: ColorType,
@@ -659,6 +712,56 @@ fn invert_colors(
     Ok(())
 }
 
+/// Re-bias CIE L*a*b* a/b channels from signed to unsigned encoding.
+///
+/// CIELab (photometric=8) stores a/b as signed integers where 0 is neutral;
+/// ICCLab (photometric=9) stores them as unsigned with the neutral shifted to
+/// the middle of the range. The two differ by exactly half the sample range, so
+/// the conversion is a wrapping add of 2^(bits-1) — i.e. flipping the sign bit —
+/// applied per a/b sample. `bit_depth` is 8 or 16 (guaranteed by `colortype`);
+/// samples are in native byte order here (post `fix_endianness_and_predict`).
+/// See libtiff's `TIFFCIELabToXYZ` for reference (tif_color.c line 45
+/// @ 04884181e8903915038be553bfab404302d98ea0).
+fn cielab_to_icclab(buf: &mut [u8], bit_depth: u8) {
+    match bit_depth {
+        16 => {
+            for pixel in buf.as_chunks_mut::<6>().0 {
+                // pixel is [L, a, b] as three native-endian u16 samples.
+                let (samples, _) = pixel.as_chunks_mut::<2>();
+                for s in &mut samples[1..=2] {
+                    *s = u16::from_ne_bytes(*s).wrapping_add(0x8000).to_ne_bytes();
+                }
+            }
+        }
+        _ => {
+            for [_, a, b] in buf.as_chunks_mut::<3>().0 {
+                *a = a.wrapping_add(128);
+                *b = b.wrapping_add(128);
+            }
+        }
+    }
+}
+
+/// Apply photometric-interpretation-specific post-processing to a decoded row.
+///
+/// Some interpretations need a fixup once the raw samples are in the output
+/// buffer: `WhiteIsZero` inverts intensities, and `CIELab` re-biases the signed
+/// a/b channels into the unsigned ICCLab encoding. These are mutually exclusive,
+/// so at most one applies to any given row.
+fn post_process_row(
+    buf: &mut [u8],
+    photometric_interpretation: PhotometricInterpretation,
+    color_type: ColorType,
+    sample_format: SampleFormat,
+) -> TiffResult<()> {
+    match photometric_interpretation {
+        PhotometricInterpretation::WhiteIsZero => invert_colors(buf, color_type, sample_format)?,
+        PhotometricInterpretation::CIELab => cielab_to_icclab(buf, color_type.bit_depth()),
+        _ => {}
+    }
+    Ok(())
+}
+
 /// Fix endianness. If `byte_order` matches the host, then conversion is a no-op.
 fn fix_endianness(buf: &mut [u8], byte_order: ByteOrder, bit_depth: u8) {
     let host = ByteOrder::native();
@@ -750,6 +853,8 @@ impl TiffHeader {
             seen_ifds: cycles::IfdCycles::new(),
             image: None,
             scratch: Vec::new(),
+            lenient: false,
+            compression: stream::CompressionEngines::default(),
         }
     }
 }
@@ -832,25 +937,41 @@ impl<R: Read + Seek> Decoder<R> {
         self
     }
 
+    /// Use a lenient decoder.
+    ///
+    /// Configuring this make the parser respect libtiff's behavior to maintain compatibility with
+    /// some older softwares (e.g. some old Adobe suite products).
+    pub fn with_lenient(mut self, lenient: bool) -> Decoder<R> {
+        self.lenient = lenient;
+        self
+    }
+
     // FIXME: this tuple would be a good candidate for a proxy, like IfdDecoder. That has been
     // requested in some form or another in a number of issues.
     #[expect(clippy::unnecessary_unwrap)] // Lifetimes would overlap due to return.
     fn ensure_image_and_reader(
         &mut self,
-    ) -> TiffResult<(&mut Image, &mut ValueReader<R>, &mut Vec<u8>)> {
+    ) -> TiffResult<(
+        &mut Image,
+        &mut ValueReader<R>,
+        &mut Vec<u8>,
+        &mut stream::CompressionEngines,
+    )> {
         if self.image.is_some() {
             return Ok((
                 self.image.as_mut().unwrap(),
                 &mut self.value_reader,
                 &mut self.scratch,
+                &mut self.compression,
             ));
         }
 
-        let image = Image::from_reader(&mut self.value_reader, &self.directory)?;
+        let image = Image::from_reader(&mut self.value_reader, &self.directory, self.lenient)?;
         Ok((
             self.image.insert(image),
             &mut self.value_reader,
             &mut self.scratch,
+            &mut self.compression,
         ))
     }
 
@@ -982,6 +1103,8 @@ impl<R: Read + Seek> Decoder<R> {
             current_ifd_pointer: self.current_ifd_pointer,
             image: core::mem::take(&mut self.image),
             scratch: core::mem::take(&mut self.scratch),
+            lenient: self.lenient,
+            compression: core::mem::take(&mut self.compression),
         }
     }
 
@@ -1030,7 +1153,11 @@ impl<R: Read + Seek> Decoder<R> {
     /// error is returned.
     pub fn seek_to_image(&mut self, ifd_index: usize) -> TiffResult<()> {
         let (offset, directory) = self.find_nth_ifd(ifd_index)?;
-        self.image = Some(Image::from_reader(&mut self.value_reader, &directory)?);
+        self.image = Some(Image::from_reader(
+            &mut self.value_reader,
+            &directory,
+            self.lenient,
+        )?);
         self.current_ifd_pointer = Some(offset);
         self.directory = directory;
         Ok(())
@@ -1123,7 +1250,11 @@ impl<R: Read + Seek> Decoder<R> {
     pub fn next_image(&mut self) -> TiffResult<()> {
         let next_ifd = self.directory.next();
         let (offset, directory) = self.read_chained_ifd(self.current_ifd_pointer, next_ifd)?;
-        self.image = Some(Image::from_reader(&mut self.value_reader, &directory)?);
+        self.image = Some(Image::from_reader(
+            &mut self.value_reader,
+            &directory,
+            self.lenient,
+        )?);
         self.current_ifd_pointer = Some(offset);
         self.directory = directory;
         Ok(())
@@ -1457,10 +1588,10 @@ impl<R: Read + Seek> Decoder<R> {
         chunk_index: u32,
         layout: &image::ReadoutLayout,
     ) -> TiffResult<()> {
-        let (image, value_reader, scratch) = self.ensure_image_and_reader()?;
+        let (image, value_reader, scratch, engines) = self.ensure_image_and_reader()?;
         let offset = image.chunk_file_range(chunk_index)?.0;
         value_reader.reader.goto_offset(offset)?;
-        image.expand_chunk(value_reader, buffer, layout, chunk_index, scratch)?;
+        image.expand_chunk(value_reader, buffer, layout, chunk_index, scratch, engines)?;
         Ok(())
     }
 
@@ -1612,7 +1743,7 @@ impl<R: Read + Seek> Decoder<R> {
         slice: TiffCodingUnit,
         buffer: &mut [u8],
     ) -> TiffResult<()> {
-        let (image, value_reader, scratch) = self.ensure_image_and_reader()?;
+        let (image, value_reader, scratch, engines) = self.ensure_image_and_reader()?;
 
         let (width, height) = image.chunk_data_dimensions(slice.0)?;
         let readout = image.readout_for_size(width, height)?;
@@ -1645,6 +1776,7 @@ impl<R: Read + Seek> Decoder<R> {
                 readout,
                 chunk,
                 scratch,
+                engines,
             )?;
         }
 
@@ -1816,7 +1948,7 @@ impl<R: Read + Seek> Decoder<R> {
     /// Returns an error if the buffer fits less than one plane. In particular, for non-planar
     /// images returns an error if the buffer does not fit the required size.
     pub fn read_image_bytes(&mut self, buffer: &mut [u8]) -> TiffResult<()> {
-        let (image, value_reader, scratch) = self.ensure_image_and_reader()?;
+        let (image, value_reader, scratch, engines) = self.ensure_image_and_reader()?;
         let readout = image.readout_for_image()?;
 
         let ref layout @ image::PlaneLayout {
@@ -1850,6 +1982,7 @@ impl<R: Read + Seek> Decoder<R> {
                     readout,
                     chunk,
                     scratch,
+                    engines,
                 )?;
             }
         }

src/decoder/stream.rs

@@ -122,9 +122,86 @@ impl<R: Read> EndianReader<R> {
 // # READERS
 //
 
+#[derive(Default)]
+pub(crate) struct CompressionEngines {
+    #[cfg(feature = "lzw")]
+    lzw: Option<weezl::decode::Decoder>,
+}
+
+impl core::fmt::Debug for CompressionEngines {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        let mut dbg = f.debug_struct("CompressionEngines");
+        #[cfg(feature = "lzw")]
+        dbg.field("lzw", &self.lzw.is_some().then_some("<weezl>"));
+        dbg.finish()
+    }
+}
+
+/// A variant of `io::Read` that takes one block to the end.
+///
+/// This is derived from `Read` so that any special optimization done by `std` (or any nightly crate
+/// for that matter) surely remains usable.
+///
+/// We want our decompression engines to be much more specialized than the standard library trait.
+/// Firstly there is more information available to communicate (the tiff Type, known decompressed
+/// size, …) and secondly they can potentially *reset*. This could be solved with an extension trait
+/// but since we want to avoid `read` in favor of `read_exact` (or an equivalent if we want to pass
+/// in a strided block buffer) a separate trait that's auto derived is favored.
+///
+/// FIXME: Also consider that we will want to read subsets of samples. Via IO this is done by
+/// discarding data *after* it is put into the output buffer. Yet, for some decompression that write
+/// alone is considerable work where it could simply skip bits more efficiently. So the reader would
+/// likely want to know about a sample mask.
+pub trait ReadSamples: Read {
+    /// Read a row of samples and throw away some additional padding samples at the end.
+    ///
+    /// This is automatically implemented for `Self: Read` as a copy into `io::Sink`.
+    fn read_padded_samples(&mut self, buf: &mut [u8], discard: u64) -> io::Result<()>;
+
+    /// Put this back into our cache of compression engines.
+    fn cache_or_destroy(self: Box<Self>, _: &mut CompressionEngines) {}
+}
+
+/// Note: the `Seek` bound ensures this does not overlap actual decompression.. This is an odd
+/// design problem born out of wanting both the underlying `Read` for uncompressed streams due to
+/// performance as well as the override to `cache_or_destroy` for all compressed streams.
+impl<R: Read + Seek> ReadSamples for R {
+    fn read_padded_samples(&mut self, buf: &mut [u8], discard: u64) -> io::Result<()> {
+        self.read_exact(buf)?;
+        if discard > 0 {
+            std::io::copy(&mut self.by_ref().take(discard), &mut std::io::sink())?;
+        }
+        Ok(())
+    }
+}
+
 /// Type alias for the deflate Reader
 #[cfg(feature = "deflate")]
-pub type DeflateReader<R> = flate2::read::ZlibDecoder<R>;
+pub struct DeflateReader<R> {
+    inner: flate2::read::ZlibDecoder<R>,
+}
+
+impl<R: Read> DeflateReader<R> {
+    pub fn new(inner: R) -> Self {
+        DeflateReader {
+            inner: flate2::read::ZlibDecoder::new(inner),
+        }
+    }
+}
+
+impl<R: Read> Read for DeflateReader<R> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        self.inner.read(buf)
+    }
+}
+
+impl<R: Read> ReadSamples for DeflateReader<R> {
+    fn read_padded_samples(&mut self, buf: &mut [u8], discard: u64) -> io::Result<()> {
+        self.read_exact(buf)?;
+        std::io::copy(&mut self.by_ref().take(discard), &mut std::io::sink())?;
+        Ok(())
+    }
+}
 
 //
 // ## LZW Reader
@@ -135,31 +212,103 @@ pub type DeflateReader<R> = flate2::read::ZlibDecoder<R>;
 pub struct LZWReader<R: Read> {
     reader: BufReader<Take<R>>,
     decoder: weezl::decode::Decoder,
+    is_old_style: bool,
 }
 
 #[cfg(feature = "lzw")]
 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(
+    pub fn new(
+        reader: R,
+        compressed_length: usize,
+        engine: &mut CompressionEngines,
+    ) -> LZWReader<R> {
+        let mut buffered = BufReader::with_capacity(
             (32 * 1024).min(compressed_length),
             reader.take(u64::try_from(compressed_length).unwrap()),
-            ),
-            decoder: configuration.build(),
+        );
+
+        // `Compression = 5` covers both the current LZW variant and the pre-TIFF-6.0
+        // "old-style" one, with no tag to distinguish them. New-style is MSB-first with
+        // the "early change" code-size switch, and its stream begins with the Clear code
+        // (256) -> first byte 0x80. Old-style is LSB-first *without* the early change
+        // (GIF-style increment timing), and begins with 0x00 followed by a byte with bit
+        // 0 set. This is the heuristic libtiff uses to select its compatibility decoder;
+        // on a short/failed peek we default to the modern variant.
+        let old_style = matches!(
+            buffered.fill_buf(),
+            Ok(head) if head.len() >= 2 && head[0] == 0x00 && head[1] & 0x01 != 0
+        );
+
+        let decoder = if old_style {
+            weezl::decode::Configuration::new(weezl::BitOrder::Lsb, 8)
+                .with_yield_on_full_buffer(true)
+                .build()
+        } else {
+            // All standard LZW decoders have the same configuration. We can thus reuse any that we
+            // have previously constructed, skipping internal buffer allocation. We only need to
+            // reset them to a new stream.
+            if let Some(mut lzw) = engine.lzw.take() {
+                lzw.reset();
+                lzw
+            } else {
+                weezl::decode::Configuration::with_tiff_size_switch(weezl::BitOrder::Msb, 8)
+                    .with_yield_on_full_buffer(true)
+                    .build()
             }
+        };
+
+        Self {
+            reader: buffered,
+            decoder: decoder,
+            is_old_style: old_style,
+        }
+    }
+
+    fn discard_n(&mut self, mut n: u64) -> io::Result<()> {
+        let mut buf = [0u8; 2048];
+
+        while n > 0 {
+            let limit = n.min(2048) as usize;
+            let result = self
+                .decoder
+                .decode_bytes(self.reader.fill_buf()?, &mut buf[..limit]);
+
+            self.reader.consume(result.consumed_in);
+            n -= result.consumed_out as u64;
+
+            match result.status {
+                Ok(weezl::LzwStatus::Ok) => {
+                    continue;
+                }
+                Ok(weezl::LzwStatus::NoProgress) => {
+                    return Err(io::Error::new(
+                        io::ErrorKind::UnexpectedEof,
+                        "no lzw end code found",
+                    ));
+                }
+                Ok(weezl::LzwStatus::Done) => {
+                    if n != 0 {
+                        // stream is too short.
+                        return Err(std::io::ErrorKind::UnexpectedEof)?;
+                    }
+                }
+                Err(err) => return Err(io::Error::new(io::ErrorKind::InvalidData, err)),
+            }
+        }
+
+        Ok(())
     }
 }
 
 #[cfg(feature = "lzw")]
 impl<R: Read> Read for LZWReader<R> {
-    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+    fn read(&mut self, mut buf: &mut [u8]) -> io::Result<usize> {
         loop {
             let result = self.decoder.decode_bytes(self.reader.fill_buf()?, buf);
+
             self.reader.consume(result.consumed_in);
+            let _ = buf.split_off_mut(..result.consumed_out);
 
             match result.status {
                 Ok(weezl::LzwStatus::Ok) => {
@@ -184,6 +333,24 @@ impl<R: Read> Read for LZWReader<R> {
     }
 }
 
+impl<R: Read> ReadSamples for LZWReader<R> {
+    fn read_padded_samples(&mut self, buf: &mut [u8], discard: u64) -> io::Result<()> {
+        self.read_exact(buf)?;
+
+        if discard > 0 {
+            self.discard_n(discard)?;
+        }
+
+        Ok(())
+    }
+
+    fn cache_or_destroy(self: Box<Self>, engines: &mut CompressionEngines) {
+        if !self.is_old_style && engines.lzw.is_none() {
+            engines.lzw = Some(self.decoder);
+        }
+    }
+}
+
 //
 // ## PackBits Reader
 //
@@ -256,6 +423,14 @@ impl<R: Read> Read for PackBitsReader<R> {
     }
 }
 
+impl<R: Read> ReadSamples for PackBitsReader<R> {
+    fn read_padded_samples(&mut self, buf: &mut [u8], discard: u64) -> io::Result<()> {
+        self.read_exact(buf)?;
+        std::io::copy(&mut self.by_ref().take(discard), &mut std::io::sink())?;
+        Ok(())
+    }
+}
+
 /// Iterator adapter that reverses bits in each byte for FillOrder=2 (LSB-to-MSB).
 #[cfg(feature = "fax")]
 type FaxBytes<R> =
@@ -372,6 +547,14 @@ impl<R: Read> Read for Group4Reader<R> {
     }
 }
 
+impl<R: Read> ReadSamples for Group4Reader<R> {
+    fn read_padded_samples(&mut self, buf: &mut [u8], discard: u64) -> io::Result<()> {
+        self.read_exact(buf)?;
+        std::io::copy(&mut self.by_ref().take(discard), &mut std::io::sink())?;
+        Ok(())
+    }
+}
+
 /// Owning byte reader for Group 3 decoding: the compressed data is consumed
 /// into an iterator so the reader doesn't borrow the buffer.
 #[cfg(feature = "fax")]
@@ -510,6 +693,14 @@ impl Read for Group3Reader {
     }
 }
 
+impl ReadSamples for Group3Reader {
+    fn read_padded_samples(&mut self, buf: &mut [u8], discard: u64) -> io::Result<()> {
+        self.read_exact(buf)?;
+        std::io::copy(&mut self.by_ref().take(discard), &mut std::io::sink())?;
+        Ok(())
+    }
+}
+
 #[cfg(feature = "webp")]
 pub struct WebPReader {
     inner: Cursor<Vec<u8>>,
@@ -565,6 +756,15 @@ impl Read for WebPReader {
     }
 }
 
+#[cfg(feature = "webp")]
+impl ReadSamples for WebPReader {
+    fn read_padded_samples(&mut self, buf: &mut [u8], discard: u64) -> io::Result<()> {
+        self.read_exact(buf)?;
+        std::io::copy(&mut self.by_ref().take(discard), &mut std::io::sink())?;
+        Ok(())
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;

src/tags.rs

@@ -295,7 +295,10 @@ pub enum CompressionMethod(u16) unknown(
 }
 
 tags! {
-pub enum PhotometricInterpretation(u16) {
+pub enum PhotometricInterpretation(u16) unknown(
+    /// A private or extension tag
+    unknown
+) {
     WhiteIsZero = 0,
     BlackIsZero = 1,
     RGB = 2,

tests/decode_images.rs

@@ -1,11 +1,11 @@
 extern crate tiff;
 
 use tiff::decoder::{ifd, Decoder, DecodingSampleBuffer};
-use tiff::tags::{ByteOrder, Type};
+use tiff::tags::{ByteOrder, PhotometricInterpretation, SampleFormat, Type};
 use tiff::ColorType;
 
 use std::fs::File;
-use std::io::{Read, Seek};
+use std::io::{Cursor, Read, Seek};
 use std::path::PathBuf;
 
 const TEST_IMAGE_DIR: &str = "./tests/images/";
@@ -16,7 +16,9 @@ macro_rules! test_image_sum {
         fn $name(file: &str, expected_type: ColorType, expected_sum: $sum_ty) {
             let path = PathBuf::from(TEST_IMAGE_DIR).join(file);
             let img_file = File::open(path).expect("Cannot find test image!");
-            let mut decoder = Decoder::open(img_file).expect("Cannot create decoder");
+            let mut decoder = Decoder::open(img_file)
+                .expect("Cannot create decoder")
+                .with_lenient(true);
 
             decoder.next_image().expect("Cannot read image IFD");
             assert_eq!(decoder.colortype().unwrap(), expected_type);
@@ -473,6 +475,37 @@ fn test_tiled_rect_rgb_u8() {
     test_image_sum_u8("tiled-rect-rgb-u8.tif", ColorType::RGB(8), 62081032);
 }
 
+#[test]
+fn test_tiled_extra_samples_border_u8() {
+    // Regression test for https://github.com/image-rs/image-tiff/issues/400
+    //
+    // A tiled image whose dimensions are not multiples of the tile size, with
+    // an extra sample that is discarded on decode. The horizontal padding of
+    // the right border tiles used to be written past the end of the output
+    // row, spilling into the leftmost columns of the following row. The
+    // fixture's tile padding is filled with 0xEE so any spill is visible.
+    let path = PathBuf::from(TEST_IMAGE_DIR).join("tiled-extra-samples-border-u8.tif");
+    let img_file = File::open(path).expect("Cannot find test image!");
+    let mut decoder = Decoder::open(img_file).expect("Cannot create decoder");
+    decoder.next_image().expect("Cannot read image IFD");
+    assert_eq!(decoder.dimensions().unwrap(), (40, 40));
+    assert_eq!(decoder.colortype().unwrap(), ColorType::RGB(8));
+
+    let DecodingSampleBuffer::U8(data) = decoder.read_image().unwrap() else {
+        panic!("Wrong bit depth")
+    };
+
+    let mut expected = Vec::with_capacity(40 * 40 * 3);
+    for y in 0..40u32 {
+        for x in 0..40u32 {
+            expected.push(((x * 3) % 251) as u8);
+            expected.push(((y * 7) % 251) as u8);
+            expected.push(((x + y) % 251) as u8);
+        }
+    }
+    assert_eq!(data, expected);
+}
+
 #[test]
 fn test_inner_access() {
     let path = PathBuf::from(TEST_IMAGE_DIR).join("tiled-rect-rgb-u8.tif");
@@ -825,7 +858,7 @@ fn test_predictor_3_gray_f32() {
 }
 
 #[test]
-#[cfg(feature = "zstd")]
+#[cfg(any(feature = "zstd", feature = "zstd-safe-rust"))]
 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);
@@ -975,7 +1008,7 @@ fn extra_bits_gray() {
 
 #[test]
 fn extra_bits_rgb() {
-    test_image_sum_u8("extra_bits_rgb_8b.tiff", ColorType::RGB(8), 0);
+    test_image_sum_u8("extra_bits_rgb_8b.tiff", ColorType::RGBA(8), 64);
 }
 
 #[test]
@@ -1057,3 +1090,295 @@ fn test_gray_u12_hpredict() {
     // Delta-encoded 12-bit values: [100, 200, 300, 400, 1000, 2000, 3000, 4000]
     test_image_sum_u16("hpredict-1c-12b.tiff", ColorType::Gray(12), 11000);
 }
+
+// ---- Old-style LZW ----
+
+#[test]
+fn test_old_style_lzw() {
+    // Pre-TIFF-6.0 "old-style" LZW: LSB-first bit order with no early-change size
+    // switch. Auto-detected from the stream (first byte 0x00, second byte odd).
+    // Same image as libtiffpic's quad-lzw.tif; the decoded pixels must match.
+    test_image_sum_u8("quad-lzw-compat.tiff", ColorType::RGB(8), 21048924);
+}
+
+// ---- TransparencyMask ----
+
+/// Build a minimal uncompressed 2x2 TIFF in memory, in the requested byte order,
+/// with the given photometric interpretation and the pixel data as a single strip.
+/// `sample_format` is emitted as a SampleFormat tag when `Some`; when `None` the
+/// tag is omitted (the decoder then defaults to unsigned). Pixel bytes must already
+/// be in `byte_order`.
+fn build_raw_tiff(
+    byte_order: ByteOrder,
+    photometric: PhotometricInterpretation,
+    bits_per_sample: u16,
+    samples_per_pixel: u16,
+    sample_format: Option<SampleFormat>,
+    pixel_data: &[u8],
+) -> Vec<u8> {
+    let le = matches!(byte_order, ByteOrder::LittleEndian);
+    let put16 = |buf: &mut Vec<u8>, v: u16| {
+        if le {
+            buf.extend_from_slice(&v.to_le_bytes());
+        } else {
+            buf.extend_from_slice(&v.to_be_bytes());
+        }
+    };
+    let put32 = |buf: &mut Vec<u8>, v: u32| {
+        if le {
+            buf.extend_from_slice(&v.to_le_bytes());
+        } else {
+            buf.extend_from_slice(&v.to_be_bytes());
+        }
+    };
+
+    let width: u16 = 2;
+    let height: u16 = 2;
+    let num_tags: u16 = if sample_format.is_some() { 9 } else { 8 };
+    let ifd_offset: u32 = 8;
+    let ifd_size = 2 + (num_tags as u32) * 12 + 4;
+    let strip_offset = 8 + ifd_size;
+
+    // IFD entries (must be ascending tag order). Tag IDs stay raw for the
+    // ascending-order property; the field type uses the `Type` enum.
+    let mut tags: Vec<(u16, Type, u32)> = vec![
+        (256, Type::SHORT, width as u32),                // ImageWidth
+        (257, Type::SHORT, height as u32),               // ImageLength
+        (258, Type::SHORT, bits_per_sample as u32),      // BitsPerSample
+        (259, Type::SHORT, 1),                           // Compression = None
+        (262, Type::SHORT, photometric.to_u16() as u32), // PhotometricInterpretation
+        (273, Type::LONG, strip_offset),                 // StripOffsets
+        (277, Type::SHORT, samples_per_pixel as u32),    // SamplesPerPixel
+        (279, Type::LONG, pixel_data.len() as u32),      // StripByteCounts
+    ];
+    if let Some(sf) = sample_format {
+        tags.push((339, Type::SHORT, sf.to_u16() as u32)); // SampleFormat (tag 339 > 279)
+    }
+
+    let mut buf = Vec::new();
+    buf.extend_from_slice(if le { b"II" } else { b"MM" });
+    put16(&mut buf, 42);
+    put32(&mut buf, ifd_offset);
+    put16(&mut buf, num_tags);
+
+    for (tag, typ, value) in tags {
+        put16(&mut buf, tag);
+        put16(&mut buf, typ.to_u16());
+        put32(&mut buf, 1); // count
+                            // A SHORT value is left-justified in the 4-byte value field (value in the
+                            // first two bytes, then padding); a LONG fills all four.
+        match typ {
+            Type::SHORT => {
+                put16(&mut buf, value as u16);
+                put16(&mut buf, 0);
+            }
+            _ => put32(&mut buf, value),
+        }
+    }
+
+    put32(&mut buf, 0); // next IFD = 0
+    buf.extend_from_slice(pixel_data);
+    buf
+}
+
+#[test]
+fn test_transparency_mask() {
+    // 2x2 mask: values 0, 255, 128, 64
+    let data = build_raw_tiff(
+        ByteOrder::LittleEndian,
+        PhotometricInterpretation::TransparencyMask,
+        8,
+        1,
+        None,
+        &[0, 255, 128, 64],
+    );
+    let mut decoder = Decoder::open(Cursor::new(&data)).unwrap();
+    decoder.next_image().unwrap();
+
+    assert_eq!(decoder.colortype().unwrap(), ColorType::Gray(8));
+
+    let result = decoder.read_image().unwrap();
+    match result {
+        DecodingSampleBuffer::U8(pixels) => {
+            // TransparencyMask is decoded as grayscale
+            assert_eq!(pixels.len(), 4);
+            assert_eq!(pixels[0], 0);
+            assert_eq!(pixels[1], 255);
+            assert_eq!(pixels[2], 128);
+            assert_eq!(pixels[3], 64);
+        }
+        _ => panic!("Expected U8 buffer"),
+    }
+}
+
+// ---- CIE L*a*b* ----
+
+#[test]
+fn test_cielab_decode() {
+    // 2x2 CIELab image: 3 bytes per pixel (L, a, b)
+    // L=128, a=0 (signed neutral), b=0 (signed neutral)
+    // After re-biasing: a=128 (unsigned neutral), b=128 (unsigned neutral)
+    let pixels: Vec<u8> = vec![
+        128, 0, 0, // pixel 0: L=128, a=0, b=0 (neutral gray)
+        255, 50, 50, // pixel 1: L=255, a=+50, b=+50
+        100, 206, 206, // pixel 2: L=100, a=-50 (as i8), b=-50 (as i8)
+        0, 128, 128, // pixel 3: L=0, a=-128 (as i8), b=-128 (as i8)
+    ];
+
+    let data = build_raw_tiff(
+        ByteOrder::LittleEndian,
+        PhotometricInterpretation::CIELab,
+        8,
+        3,
+        None,
+        &pixels,
+    );
+    let mut decoder = Decoder::open(Cursor::new(&data)).unwrap();
+    decoder.next_image().unwrap();
+
+    assert_eq!(decoder.colortype().unwrap(), ColorType::Lab(8));
+
+    let result = decoder.read_image().unwrap();
+    match result {
+        DecodingSampleBuffer::U8(decoded) => {
+            assert_eq!(decoded.len(), 12); // 4 pixels * 3 channels
+
+            // Pixel 0: L=128, a=0+128=128, b=0+128=128
+            assert_eq!(decoded[0], 128);
+            assert_eq!(decoded[1], 128);
+            assert_eq!(decoded[2], 128);
+
+            // Pixel 1: L=255, a=50+128=178, b=50+128=178
+            assert_eq!(decoded[3], 255);
+            assert_eq!(decoded[4], 178);
+            assert_eq!(decoded[5], 178);
+
+            // Pixel 2: L=100, a=206+128=78 (wrapping), b=206+128=78
+            assert_eq!(decoded[6], 100);
+            assert_eq!(decoded[7], 78);
+            assert_eq!(decoded[8], 78);
+
+            // Pixel 3: L=0, a=128+128=0 (wrapping), b=128+128=0
+            assert_eq!(decoded[9], 0);
+            assert_eq!(decoded[10], 0);
+            assert_eq!(decoded[11], 0);
+        }
+        _ => panic!("Expected U8 buffer"),
+    }
+}
+
+#[test]
+fn test_cielab_decode_16() {
+    // 2x2 16-bit CIELab image: 3 u16 samples per pixel (L, a, b), a/b signed.
+    // Re-biasing maps signed a/b to unsigned by adding 0x8000 (wrapping):
+    //   0 -> 32768, +1000 -> 33768, -1000 -> 31768, -32768 -> 0, +32767 -> 65535.
+    let samples: [u16; 12] = [
+        32768, 0, 0, // pixel 0: L, a=0, b=0 (neutral)
+        65535, 1000, 1000, // pixel 1: a=+1000, b=+1000
+        10000, 64536, 64536, // pixel 2: a=-1000, b=-1000 (as u16)
+        0, 32768, 32767, // pixel 3: a=-32768, b=+32767 (as u16)
+    ];
+    let mut pixels = Vec::new();
+    for s in samples {
+        pixels.extend_from_slice(&s.to_le_bytes());
+    }
+
+    let data = build_raw_tiff(
+        ByteOrder::LittleEndian,
+        PhotometricInterpretation::CIELab,
+        16,
+        3,
+        None,
+        &pixels,
+    );
+    let mut decoder = Decoder::open(Cursor::new(&data)).unwrap();
+    decoder.next_image().unwrap();
+
+    assert_eq!(decoder.colortype().unwrap(), ColorType::Lab(16));
+
+    let result = decoder.read_image().unwrap();
+    match result {
+        DecodingSampleBuffer::U16(decoded) => {
+            assert_eq!(decoded.len(), 12); // 4 pixels * 3 channels
+
+            // L unchanged; a/b re-biased by +0x8000 (wrapping).
+            let expected: [u16; 12] = [
+                32768, 32768, 32768, // pixel 0
+                65535, 33768, 33768, // pixel 1
+                10000, 31768, 31768, // pixel 2
+                0, 0, 65535, // pixel 3
+            ];
+            assert_eq!(decoded, expected);
+        }
+        _ => panic!("Expected U16 buffer"),
+    }
+}
+
+#[test]
+fn test_cielab_decode_16_big_endian() {
+    // Same image and expected output as `test_cielab_decode_16`, but stored as a
+    // big-endian ("MM") TIFF. 16-bit is the case where byte order matters: the
+    // decoder must swap to native before the a/b re-bias, so the result is
+    // identical to the little-endian case.
+    let samples: [u16; 12] = [
+        32768, 0, 0, // pixel 0
+        65535, 1000, 1000, // pixel 1
+        10000, 64536, 64536, // pixel 2
+        0, 32768, 32767, // pixel 3
+    ];
+    let mut pixels = Vec::new();
+    for s in samples {
+        pixels.extend_from_slice(&s.to_be_bytes());
+    }
+
+    let data = build_raw_tiff(
+        ByteOrder::BigEndian,
+        PhotometricInterpretation::CIELab,
+        16,
+        3,
+        None,
+        &pixels,
+    );
+    let mut decoder = Decoder::open(Cursor::new(&data)).unwrap();
+    decoder.next_image().unwrap();
+
+    assert_eq!(decoder.colortype().unwrap(), ColorType::Lab(16));
+
+    let result = decoder.read_image().unwrap();
+    match result {
+        DecodingSampleBuffer::U16(decoded) => {
+            let expected: [u16; 12] = [
+                32768, 32768, 32768, // pixel 0
+                65535, 33768, 33768, // pixel 1
+                10000, 31768, 31768, // pixel 2
+                0, 0, 65535, // pixel 3
+            ];
+            assert_eq!(decoded, expected);
+        }
+        _ => panic!("Expected U16 buffer"),
+    }
+}
+
+#[test]
+fn test_cielab_rejects_non_uint_sample_format() {
+    // CIELab treats a/b as signed via the photometric interpretation and requires
+    // SampleFormat::Uint (matching the ICCLab arm). A file declaring Int must be
+    // rejected rather than silently producing a signed output buffer.
+    let pixels = vec![0u8; 12]; // 2x2, 3 samples, 8-bit
+    let data = build_raw_tiff(
+        ByteOrder::LittleEndian,
+        PhotometricInterpretation::CIELab,
+        8,
+        3,
+        Some(SampleFormat::Int),
+        &pixels,
+    );
+
+    let mut decoder = Decoder::open(Cursor::new(&data)).unwrap();
+    // Rejection may surface at either next_image() or colortype().
+    let rejected = decoder.next_image().is_err() || decoder.colortype().is_err();
+    assert!(
+        rejected,
+        "CIELab with SampleFormat=Int must be rejected, not decoded"
+    );
+}

tests/decode_libtiffpic.rs

@@ -190,7 +190,7 @@ const FILES: &[(&str, Option<u32>)] = &[
     ("tests/libtiffpic/oxford.tif", Some(0x355d2e3e)),
     ("tests/libtiffpic/pc260001.tif", Some(0xdb56b753)),
     ("tests/libtiffpic/quad-jpeg.tif", Some(0xcba22475)),
-    ("tests/libtiffpic/quad-lzw.tif", Some(0)), // invalid LZW stream
+    ("tests/libtiffpic/quad-lzw.tif", Some(0x6055f9ee)), // old-style (LSB) LZW
     ("tests/libtiffpic/quad-tile.tif", Some(0x6055f9ee)),
     ("tests/libtiffpic/README", None),
     ("tests/libtiffpic/smallliz.tif", Some(0)), // unsupported YCbCr chroma subsampling

tests/predict.rs

@@ -233,3 +233,167 @@ fn test_rgb_u64_predict_roundtrip() {
 fn test_ycbcr_u8_predict_roundtrip() {
     test_u8_predict_roundtrip::<colortype::YCbCr8>("tiled-jpeg-ycbcr.tif", ColorType::YCbCr(8));
 }
+
+// ---------------------------------------------------------------------------
+// Extended bit depths (9-15 promoted to u16, 17-31 promoted to u32).
+//
+// A horizontal-predicted n-bit sample stores its delta in exactly n bits,
+// i.e. modulo 2^n, so reconstruction must also be modulo 2^n. The decoder
+// accumulates in the promoted storage width and reduces back to n bits;
+// these tests pin that reduction down with exact pixel values, including
+// deltas that wrap modulo 2^n. The encoder does not support extended
+// depths, so the TIFFs are hand-built.
+// ---------------------------------------------------------------------------
+
+/// Hand-build a minimal little-endian, single-strip, uncompressed grayscale
+/// TIFF with `Predictor::Horizontal` and the given (possibly non-standard)
+/// bit depth. `pixels` are the intended sample values; they are converted to
+/// horizontal differences modulo `2^bits` and bit-packed MSB-first with each
+/// row padded to a byte boundary — the only representation an n-bit field
+/// can hold.
+fn build_gray_hpredict_tiff(
+    width: u32,
+    height: u32,
+    bits: u8,
+    photometric: u16,
+    pixels: &[u32],
+) -> Vec<u8> {
+    let max = ((1u64 << bits) - 1) as u32;
+    let row_samples = width as usize;
+    assert_eq!(pixels.len(), row_samples * height as usize);
+    assert!(pixels.iter().all(|&v| v <= max));
+
+    // Per-row horizontal differencing modulo 2^bits.
+    let mut deltas = Vec::with_capacity(pixels.len());
+    for row in pixels.chunks(row_samples) {
+        for (i, &v) in row.iter().enumerate() {
+            deltas.push(if i == 0 {
+                v
+            } else {
+                v.wrapping_sub(row[i - 1]) & max
+            });
+        }
+    }
+
+    // Bit-pack MSB-first, each row padded to a byte boundary.
+    let row_bytes = (row_samples * bits as usize).div_ceil(8);
+    let mut data = vec![0u8; row_bytes * height as usize];
+    for (r, row) in deltas.chunks(row_samples).enumerate() {
+        for (i, &d) in row.iter().enumerate() {
+            let start = r * row_bytes * 8 + i * bits as usize;
+            for (off, b) in (start..).zip((0..bits).rev()) {
+                if (d >> b) & 1 == 1 {
+                    data[off / 8] |= 0x80 >> (off % 8);
+                }
+            }
+        }
+    }
+
+    // Classic little-endian TIFF: header, strip data, then a single IFD.
+    let mut out = Vec::new();
+    out.extend_from_slice(b"II");
+    out.extend_from_slice(&42u16.to_le_bytes());
+    let data_offset = 8u32;
+    let ifd_offset = data_offset + data.len() as u32;
+    out.extend_from_slice(&ifd_offset.to_le_bytes());
+    out.extend_from_slice(&data);
+
+    const SHORT: u16 = 3;
+    const LONG: u16 = 4;
+    let entries: &[(u16, u16, u32)] = &[
+        (256, LONG, width),                   // ImageWidth
+        (257, LONG, height),                  // ImageLength
+        (258, SHORT, u32::from(bits)),        // BitsPerSample
+        (259, SHORT, 1),                      // Compression: none
+        (262, SHORT, u32::from(photometric)), // PhotometricInterpretation
+        (273, LONG, data_offset),             // StripOffsets
+        (277, SHORT, 1),                      // SamplesPerPixel
+        (278, LONG, height),                  // RowsPerStrip
+        (279, LONG, data.len() as u32),       // StripByteCounts
+        (317, SHORT, 2),                      // Predictor: horizontal
+        (339, SHORT, 1),                      // SampleFormat: unsigned integer
+    ];
+    out.extend_from_slice(&(entries.len() as u16).to_le_bytes());
+    for &(tag, ty, value) in entries {
+        out.extend_from_slice(&tag.to_le_bytes());
+        out.extend_from_slice(&ty.to_le_bytes());
+        out.extend_from_slice(&1u32.to_le_bytes());
+        // A count-1 SHORT payload occupies the low bytes of the value field.
+        out.extend_from_slice(&value.to_le_bytes());
+    }
+    out.extend_from_slice(&0u32.to_le_bytes()); // no next IFD
+    out
+}
+
+fn decode_gray_u16(tiff: Vec<u8>, bits: u8) -> Vec<u16> {
+    let mut decoder = Decoder::open(Cursor::new(tiff)).expect("Cannot create decoder");
+    decoder.next_image().unwrap();
+    assert_eq!(decoder.colortype().unwrap(), ColorType::Gray(bits));
+    match decoder.read_image().unwrap() {
+        DecodingSampleBuffer::U16(res) => res,
+        _ => panic!("Wrong data type"),
+    }
+}
+
+fn decode_gray_u32(tiff: Vec<u8>, bits: u8) -> Vec<u32> {
+    let mut decoder = Decoder::open(Cursor::new(tiff)).expect("Cannot create decoder");
+    decoder.next_image().unwrap();
+    assert_eq!(decoder.colortype().unwrap(), ColorType::Gray(bits));
+    match decoder.read_image().unwrap() {
+        DecodingSampleBuffer::U32(res) => res,
+        _ => panic!("Wrong data type"),
+    }
+}
+
+#[test]
+fn test_gray12_hpredict_wraparound_exact() {
+    // Pixel row [4095, 0, 1] encodes as deltas [4095, 1, 1] (mod 2^12).
+    // u16 accumulation yields [4095, 4096, 4097]; reduction modulo 2^12
+    // must restore [4095, 0, 1]. Saturating instead would corrupt the two
+    // wrapped samples to 4095.
+    let tiff = build_gray_hpredict_tiff(3, 1, 12, 1, &[4095, 0, 1]);
+    assert_eq!(decode_gray_u16(tiff, 12), vec![4095, 0, 1]);
+}
+
+#[test]
+fn test_gray12_hpredict_wraparound_white_is_zero_exact() {
+    // Same stored samples with PhotometricInterpretation = WhiteIsZero:
+    // after the mod-2^12 reduction the decoder inverts as (2^12 - 1) - v.
+    let tiff = build_gray_hpredict_tiff(3, 1, 12, 0, &[4095, 0, 1]);
+    assert_eq!(decode_gray_u16(tiff, 12), vec![0, 4095, 4094]);
+}
+
+#[test]
+fn test_gray20_hpredict_wraparound_exact() {
+    // The same wrap-around shape on the u32 storage path (17-31 bits).
+    let tiff = build_gray_hpredict_tiff(3, 1, 20, 1, &[0xF_FFFF, 0, 1]);
+    assert_eq!(decode_gray_u32(tiff, 20), vec![0xF_FFFF, 0, 1]);
+}
+
+#[test]
+fn test_extended_depth_hpredict_random_roundtrip() {
+    // Deterministic pseudo-random pixels across a spread of extended depths;
+    // encode-then-decode must be the identity for every sample, including
+    // every mod-2^n wrap-around the differencing produces. Three rows verify
+    // that prediction restarts per row.
+    let mut state = 0x243F_6A88_85A3_08D3u64;
+    let mut next = move || {
+        state = state
+            .wrapping_mul(6364136223846793005)
+            .wrapping_add(1442695040888963407);
+        (state >> 33) as u32
+    };
+    for &bits in &[9u8, 11, 12, 15] {
+        let max = (1u32 << bits) - 1;
+        let pixels: Vec<u32> = (0..16 * 3).map(|_| next() & max).collect();
+        let tiff = build_gray_hpredict_tiff(16, 3, bits, 1, &pixels);
+        let expected: Vec<u16> = pixels.iter().map(|&v| v as u16).collect();
+        assert_eq!(decode_gray_u16(tiff, bits), expected, "bits={bits}");
+    }
+    for &bits in &[17u8, 20, 25, 31] {
+        let max = ((1u64 << bits) - 1) as u32;
+        let pixels: Vec<u32> = (0..16 * 3).map(|_| next() & max).collect();
+        let tiff = build_gray_hpredict_tiff(16, 3, bits, 1, &pixels);
+        assert_eq!(decode_gray_u32(tiff, bits), pixels, "bits={bits}");
+    }
+}