Transfer decoder to new reader

Make decoder's `Image` an option

src/decoder/cycles.rs

@@ -11,7 +11,7 @@ use std::collections::HashMap;
 ///
 /// 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)]
+#[derive(Clone, Default, Debug)]
 pub struct IfdCycles {
     /// The root of each component union.
     component_union: HashMap<ComponentId, ComponentId>,

src/decoder/entry_reader.rs

@@ -0,0 +1,362 @@
+use std::io::{self, Cursor, Read, Seek};
+
+use crate::{
+    decoder::{ifd::Entry, stream::EndianReader, ValueReader},
+    tags::{ByteOrder, IfdPointer, Type},
+    TiffResult,
+};
+
+pub struct EntryBytesReader<'lt, R> {
+    inner: &'lt mut EndianReader<R>,
+    variant: Variant,
+}
+
+const OFFSET_BUFFER_LEN: usize = 8;
+
+enum Variant {
+    Immediate {
+        /// An in-value reader if the offset field contains the data. Note that we rotate it to the
+        /// back of this buffer, not the front, so that `Cursor` already contains the right end.
+        /// We do not let you seek back beyond our start.
+        data: Cursor<[u8; OFFSET_BUFFER_LEN]>,
+        value_bytes: u8,
+    },
+    InFile {
+        // the length relative to the base so we can seek relative without querying the position.
+        value_bytes: u64,
+        remainder: u64,
+        base: u64,
+    },
+}
+
+impl<'lt, R> EntryBytesReader<'lt, R> {
+    pub(super) fn from_entry(inner: &'lt mut ValueReader<R>, entry: Entry) -> TiffResult<Self>
+    where
+        R: Read + Seek,
+    {
+        let offset: [_; OFFSET_BUFFER_LEN] = *entry.offset_raw();
+        let limit = if inner.bigtiff { 8 } else { 4 };
+        assert!(limit as usize <= OFFSET_BUFFER_LEN);
+
+        let value_bytes = entry.field_type().value_bytes(entry.count())?;
+
+        if value_bytes <= limit {
+            // We move the data to the back of the array whereas it is in the front right now.
+            let mut value = offset;
+            value.rotate_left(value_bytes as usize);
+            let mut cursor = Cursor::new(value);
+            cursor.set_position(OFFSET_BUFFER_LEN as u64 - value_bytes);
+
+            Ok(EntryBytesReader {
+                inner: &mut inner.reader,
+                variant: Variant::Immediate {
+                    data: cursor,
+                    value_bytes: value_bytes as u8,
+                },
+            })
+        } else {
+            // remainder > limit >= 0 so this is an unreachable panic.
+            let pointer = if inner.bigtiff {
+                let mut offset = offset;
+
+                inner
+                    .reader
+                    .byte_order
+                    .convert(Type::IFD8, &mut offset[..], ByteOrder::native());
+
+                IfdPointer(u64::from_ne_bytes(offset))
+            } else {
+                let mut offset = *offset.first_chunk::<4>().unwrap();
+
+                inner
+                    .reader
+                    .byte_order
+                    .convert(Type::IFD, &mut offset[..], ByteOrder::native());
+
+                let offset = u32::from_ne_bytes(offset);
+                IfdPointer(u64::from(offset))
+            };
+
+            inner.reader.goto_offset(pointer.0)?;
+
+            Ok(EntryBytesReader {
+                inner: &mut inner.reader,
+                variant: Variant::InFile {
+                    value_bytes,
+                    remainder: value_bytes,
+                    base: pointer.0,
+                },
+            })
+        }
+    }
+}
+
+impl<R: Read> Read for EntryBytesReader<'_, R> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        match &mut self.variant {
+            Variant::Immediate { data, .. } => data.read(buf),
+            Variant::InFile { remainder, .. } => {
+                let bound = usize::try_from(*remainder).unwrap_or(usize::MAX);
+                let avail = buf.len().min(bound);
+                let read = self.inner.inner().read(&mut buf[..avail])?;
+
+                // On a well-behaved `Read`: read <= avail <= bound <=(mathematical) remainder
+                let consume = read as u64;
+                *remainder -= consume;
+                Ok(read)
+            }
+        }
+    }
+
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        match &mut self.variant {
+            Variant::Immediate { data, .. } => data.read_exact(buf),
+            Variant::InFile { remainder, .. } => {
+                let bound = usize::try_from(*remainder).unwrap_or(usize::MAX);
+
+                // Fail early if we would be sure to over-read.
+                if buf.len() > bound {
+                    return Err(io::Error::new(
+                        io::ErrorKind::UnexpectedEof,
+                        "Tiff value shorter than the requested buffer size",
+                    ));
+                }
+
+                // since buf.len() <= bound <=(mathematical) remainder
+                let consume = buf.len() as u64;
+                self.inner.inner().read_exact(buf)?;
+                *remainder -= consume;
+
+                Ok(())
+            }
+        }
+    }
+}
+
+impl<R: Read + Seek> Seek for EntryBytesReader<'_, R> {
+    fn seek(&mut self, offset: io::SeekFrom) -> io::Result<u64> {
+        let (value_bytes, remainder) = match &self.variant {
+            Variant::Immediate { data, value_bytes } => {
+                let remainder = data.get_ref().len() as u64 - data.position();
+                (*value_bytes as u64, remainder)
+            }
+            &Variant::InFile {
+                value_bytes,
+                remainder,
+                base: _,
+            } => (value_bytes, remainder),
+        };
+
+        // Normalize to an offset from the start
+        let offset_from_start = match offset {
+            io::SeekFrom::Start(n) => Some(n),
+            io::SeekFrom::Current(n) => {
+                let front = value_bytes - remainder;
+                front.checked_add_signed(n)
+            }
+            io::SeekFrom::End(n) => value_bytes.checked_add_signed(n),
+        };
+
+        let Some((offset, remainder)) = offset_from_start.and_then(|n| {
+            let remainder = value_bytes.checked_sub(n)?;
+            Some((n, remainder))
+        }) else {
+            return Err(io::Error::new(
+                io::ErrorKind::InvalidInput,
+                "seek out of bounds of entry value",
+            ));
+        };
+
+        match &mut self.variant {
+            Variant::Immediate { data, .. } => {
+                let start = OFFSET_BUFFER_LEN as u64 - value_bytes;
+                data.set_position(start + offset);
+                Ok(offset)
+            }
+            &mut Variant::InFile { base, .. } => {
+                // We checked this against overflow on construction. Ignore the offset we get.
+                self.inner
+                    .inner()
+                    .seek(io::SeekFrom::Start(base + offset))?;
+
+                self.variant = Variant::InFile {
+                    value_bytes,
+                    remainder,
+                    base,
+                };
+
+                Ok(offset)
+            }
+        }
+    }
+
+    fn stream_position(&mut self) -> io::Result<u64> {
+        Ok(match &self.variant {
+            Variant::Immediate { data, value_bytes } => {
+                // Recall the value is at the end of the buffer.
+                let from_end = OFFSET_BUFFER_LEN as u64 - data.position();
+                u64::from(*value_bytes) - from_end
+            }
+            // Note: we might think about using the inner `stream_position` here. However that
+            // exposes something interesting: If the internal stream is modified concurrently from
+            // another instance (e.g. a shared `File`) then we may lose track of it anyways.
+            // Especially the `Read` implementation is not bounded on `Seek` so it could not
+            // acquire the position. And it should not since that is just a different form of race
+            // where we risk the position being modified by a different thread just as well.
+            //
+            // So in the interest of least surprise we just assume that we can track the position
+            // accurately here and that the inner reader uses for instance `read_at` consistently.
+            // We own the value (or mutable reference) it after all.
+            //
+            // Also this one MUST be compatible with `seek(SeekFrom::Current(0))` as per trait
+            // defintion—so at least keep those consistent if changing anything.
+            &Variant::InFile {
+                remainder,
+                value_bytes,
+                ..
+            } => value_bytes - remainder,
+        })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{decoder::Decoder, encoder, tags};
+    use std::io::{Read as _, Seek as _, SeekFrom};
+
+    #[test]
+    fn can_seek_tag_bytes() -> Result<(), crate::error::TiffError> {
+        let file = std::fs::File::open(concat!(
+            env!("CARGO_MANIFEST_DIR"),
+            "/tests/images/int8_rgb.tif"
+        ))
+        .unwrap();
+
+        let mut decoder = Decoder::open(file).unwrap();
+        decoder.next_directory().unwrap();
+
+        let mut buffer = crate::tags::ValueBuffer::default();
+
+        decoder
+            .current_ifd()
+            .find_tag_buf(tags::Tag::StripByteCounts, &mut buffer)
+            .expect("some strip byte count is present");
+
+        let bytes_of_value = buffer.as_bytes().len() as u64;
+
+        let mut reader = decoder
+            .read_tag(tags::Tag::StripByteCounts)?
+            .expect("some strip byte count is present");
+
+        let mut data = vec![];
+        reader.read_to_end(&mut data)?;
+
+        // `ValueBuffer` also stores its data in the file endianess until converted.
+        assert_eq!(reader.stream_position()?, bytes_of_value);
+        assert_eq!(buffer.as_bytes(), data);
+
+        reader.seek(SeekFrom::Start(0))?;
+        assert_eq!(reader.stream_position()?, 0);
+        reader.read_to_end(&mut data)?;
+        assert_eq!(reader.stream_position()?, bytes_of_value);
+
+        let (first, second) = data.split_at(bytes_of_value as usize);
+        assert_eq!(first, second);
+
+        Ok(())
+    }
+
+    #[test]
+    #[allow(clippy::seek_from_current)] // We want to test that functionality.
+    fn seek_from_end() -> Result<(), crate::error::TiffError> {
+        const ARTIST: &str = "99 little bugs in the code";
+
+        let data = {
+            let mut data = std::io::Cursor::new(vec![]);
+            let mut encoder = encoder::TiffEncoder::new(&mut data)?;
+
+            let mut dir = encoder.image_directory()?;
+            dir.write_tag(tags::Tag::Artist, ARTIST)?;
+            dir.finish()?;
+
+            data.into_inner()
+        };
+
+        let file = std::io::Cursor::new(data);
+        let mut decoder = Decoder::open(file).unwrap();
+        decoder.next_directory().unwrap();
+
+        let mut reader = decoder
+            .read_tag(tags::Tag::Artist)?
+            .expect("the artist is present?");
+
+        let mut value = vec![];
+        reader.read_to_end(&mut value)?;
+
+        let (&nul, value) = value.split_last().unwrap();
+        assert_eq!(value, ARTIST.as_bytes());
+        assert_eq!(nul, b'\0');
+
+        assert_eq!(reader.seek(SeekFrom::End(0))?, ARTIST.len() as u64 + 1);
+        assert!(reader.seek(SeekFrom::End(1)).is_err());
+        assert_eq!(reader.stream_position()?, ARTIST.len() as u64 + 1);
+
+        // Check relative seeks by comparing against the "code"
+        let code_offset = reader.seek(SeekFrom::End(-5))?;
+        let mut is_code = [0u8; 4];
+        reader.read_exact(&mut is_code)?;
+        assert_eq!(is_code, *value.last_chunk::<4>().unwrap());
+
+        assert_eq!(reader.seek(SeekFrom::Start(0))?, 0);
+        assert_eq!(reader.stream_position()?, 0);
+        assert_eq!(reader.seek(SeekFrom::Current(0))?, 0);
+        assert_eq!(reader.stream_position()?, 0);
+        assert!(reader.seek(SeekFrom::Current(-1)).is_err());
+        assert_eq!(reader.stream_position()?, 0);
+
+        reader.seek(SeekFrom::Start(code_offset))?;
+        reader.read_exact(&mut is_code)?;
+        assert_eq!(reader.stream_position()?, ARTIST.len() as u64);
+        assert_eq!(is_code, *value.last_chunk::<4>().unwrap());
+
+        Ok(())
+    }
+
+    /// Check that the reader from an `offset` field also works.
+    #[test]
+    fn read_from_offset() -> Result<(), crate::error::TiffError> {
+        let data = {
+            let mut data = std::io::Cursor::new(vec![]);
+            let mut encoder = encoder::TiffEncoder::new(&mut data)?;
+
+            let mut dir = encoder.image_directory()?;
+            dir.write_tag(
+                tags::Tag::PlanarConfiguration,
+                tags::PlanarConfiguration::Planar,
+            )?;
+            dir.finish()?;
+
+            data.into_inner()
+        };
+
+        let file = std::io::Cursor::new(data);
+        let mut decoder = Decoder::open(file).unwrap();
+        decoder.next_directory().unwrap();
+        let bo = decoder.byte_order();
+
+        let mut reader = decoder
+            .read_tag(tags::Tag::PlanarConfiguration)?
+            .expect("some artist is present");
+
+        let mut buf = [0; 2];
+        reader.read_exact(&mut buf)?;
+
+        bo.convert(tags::Type::SHORT, &mut buf, tags::ByteOrder::BigEndian);
+        assert_eq!(buf, [0, 2]);
+
+        // Check relative seeks, this item is only two bytes long.
+        assert!(reader.seek(SeekFrom::End(-3)).is_err());
+
+        Ok(())
+    }
+}

src/decoder/ifd.rs

@@ -393,7 +393,7 @@ impl Entry {
         self.count
     }
 
-    pub(crate) fn offset(&self) -> &[u8] {
+    pub(crate) fn offset_raw(&self) -> &[u8; 8] {
         &self.offset
     }
 

src/decoder/image.rs

@@ -155,32 +155,6 @@ pub(crate) struct ReadoutLayout {
 }
 
 impl Image {
-    pub(crate) fn no_image() -> Image {
-        Image {
-            width: 0,
-            height: 0,
-            bits_per_sample: 1,
-            samples: 1,
-            extra_samples: vec![],
-            photometric_samples: 1,
-            sample_format: SampleFormat::Uint,
-            photometric_interpretation: PhotometricInterpretation::BlackIsZero,
-            compression_method: CompressionMethod::None,
-            jpeg_tables: None,
-            predictor: Predictor::None,
-            chunk_type: ChunkType::Strip,
-            planar_config: PlanarConfiguration::Chunky,
-            strip_decoder: None,
-            tile_attributes: None,
-            chunk_offsets: Vec::new(),
-            chunk_bytes: Vec::new(),
-            chroma_subsampling: (2, 2),
-            fill_order: 1,
-            color_map: None,
-            decompression_to_host_endian: false,
-        }
-    }
-
     pub fn from_reader<R: Read + Seek>(
         decoder: &mut ValueReader<R>,
         ifd: &Directory,
@@ -528,6 +502,17 @@ impl Image {
         })
     }
 
+    pub(crate) fn check_chunk_type(&self, expected: ChunkType) -> TiffResult<()> {
+        if expected != self.chunk_type {
+            return Err(TiffError::UsageError(UsageError::InvalidChunkType(
+                expected,
+                self.chunk_type,
+            )));
+        }
+
+        Ok(())
+    }
+
     pub(crate) fn colortype(&self) -> TiffResult<ColorType> {
         let is_alpha_extra_samples = matches!(
             self.extra_samples.as_slice(),

src/decoder/mod.rs

@@ -15,6 +15,7 @@ use self::stream::EndianReader;
 
 mod buffer;
 mod cycles;
+mod entry_reader;
 pub mod ifd;
 mod image;
 mod logluv;
@@ -22,6 +23,7 @@ mod stream;
 mod tag_reader;
 
 pub use self::buffer::{DecodingSampleBuffer, DecodingSampleSlice, DecodingSampleType};
+pub use self::entry_reader::EntryBytesReader;
 
 #[deprecated = "Renamed to `DecodingSampleBuffer`"]
 pub type DecodingResult = DecodingSampleBuffer;
@@ -229,12 +231,19 @@ pub struct TiffHeader {
 
 /// The representation of a TIFF decoder
 ///
-/// Currently does not support decoding of interlaced images
+/// There are three groups of operations:
+///
+/// - [Manipulate the directory state](#header-manipulate-directory) to which the decoder currently
+///   refers in the file.
+/// - [Interact with the underlying stream](#header-stream-access) instead of entries in an IFD.
+/// - [Access the encoded image](#header-image).
+///
+/// ## Limitations
+///
+/// Currently does not support decoding of interlaced images. It also does not support images with
+/// downsampling in any direction; except if they are also JPEG or WebP compressed.
 #[derive(Debug)]
-pub struct Decoder<R>
-where
-    R: Read + Seek,
-{
+pub struct Decoder<R> {
     /// There are grouped for borrow checker reasons. This allows us to implement methods that
     /// borrow the stream access and the other fields mutably at the same time.
     value_reader: ValueReader<R>,
@@ -244,7 +253,7 @@ where
     ifd_offsets: Vec<IfdPointer>,
     /// Map from the ifd into the `ifd_offsets` ordered list.
     seen_ifds: cycles::IfdCycles,
-    image: Image,
+    image: Option<Image>,
 }
 
 /// All the information needed to read and interpret byte slices from the underlying file, i.e. to
@@ -617,11 +626,12 @@ impl TiffHeader {
                 dir
             },
             seen_ifds: cycles::IfdCycles::new(),
-            image: Image::no_image(),
+            image: None,
         }
     }
 }
 
+/// <span id=header-manipulate-directory>Methods to manipulate the directory state</span>
 impl<R: Read + Seek> Decoder<R> {
     /// Parse the image header from the underlying file and create a decoder positioned before its
     /// first directory.
@@ -699,22 +709,146 @@ impl<R: Read + Seek> Decoder<R> {
         self
     }
 
-    pub fn dimensions(&mut self) -> TiffResult<(u32, u32)> {
-        Ok((self.image.width, self.image.height))
+    // 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>)> {
+        if self.image.is_some() {
+            return Ok((self.image.as_mut().unwrap(), &mut self.value_reader));
         }
 
-    pub fn colortype(&mut self) -> TiffResult<ColorType> {
-        self.image.colortype()
+        let image = Image::from_reader(&mut self.value_reader, &self.directory)?;
+        Ok((self.image.insert(image), &mut self.value_reader))
     }
 
-    /// Returns the color map (palette) for the current image, if it uses
-    /// `PhotometricInterpretation::RGBPalette`.
+    fn ensure_image(&mut self) -> TiffResult<&mut Image> {
+        Ok(self.ensure_image_and_reader()?.0)
+    }
+
+    /// Change to another reader in a new decoder, taking with it parsed data and limits.
     ///
-    /// The returned slice contains `3 * 2^BitsPerSample` entries as u16 values,
-    /// laid out as: all red values, then all green values, then all blue values.
-    /// Each value is in the range `0..=65535` regardless of the actual bit depth.
-    pub fn color_map(&self) -> Option<&[u16]> {
-        self.image.color_map.as_deref()
+    /// The reader will keep the IFD state but resources and in particular the parsed image are
+    /// transferred to the new decoder. This has two motivations. Firstly, it can not silently
+    /// duplicate any limits and exhaust more memory than expected. Secondly, this is compatible
+    /// with additional resources that can not be cloned at all.
+    ///
+    /// This method assumes that the alternate reader contains the same data as the current reader
+    /// (but not necessarily the same stream).
+    ///
+    /// The limits used by `self` get replaced with default limits. To keep using the reader that
+    /// is the receiver of this method you likely want to reinitialize it with
+    /// [`Self::with_limits`].
+    ///
+    /// # Examples
+    ///
+    /// This method can also be used to 'fork' the reader into multiple independent iterators over
+    /// the file, provided you use access to [`Self::inner`] to clone the underlying stream. Since
+    /// the first call takes the resources, keep that object separate if you want each worker to
+    /// get the same decoder (each with default configured limits).
+    ///
+    /// Here we show the functionality by opening cursors over data in-memory.
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    /// use tiff::{decoder::Decoder, TiffError};
+    ///
+    /// let data: &[u8] = /* */
+    /// # &include_bytes!(concat!(
+    /// #   env!("CARGO_MANIFEST_DIR"), "/tests/images/tiled-gray-i1.tif"
+    /// # ))[..];
+    /// # const N: usize = 1;
+    /// let decoder = Decoder::open(Cursor::new(data))?;
+    ///
+    /// let mut workers = vec![decoder];
+    ///
+    /// for _ in 0..N {
+    ///     let decoder = workers.last_mut().unwrap();
+    ///     // Takes the resources with it, they are always in the last.
+    ///     let dup = decoder.take_with_reader(Cursor::new(data));
+    ///     workers.push(dup);
+    /// }
+    ///
+    /// let with_resources = workers.pop().unwrap();
+    ///
+    /// // Now all `workers` are at the same IFD but in a bare state.
+    ///
+    /// Ok::<_, TiffError>(())
+    /// ```
+    ///
+    /// Please note that you do **not** want to use [`File::try_clone`][`std::fs::File::open`], or
+    /// any other method where multiple readers share a single position variable, in combination
+    /// with concurrent use of decoders. When the independent decoders each seek the file
+    /// descriptor in parallel there is a race as to what position they then read from
+    /// (time-to-check-to-time-to-use). This of course applies to the original file passed to
+    /// [`Decoder::open`] already but may be more tempting here. Instead, use a file reader that
+    /// owns a position and uses [`read_at`] or equivalents.
+    ///
+    /// [`read_at`]: `std::os::unix::fs::FileExt::read_at`
+    ///
+    /// An *incorrect* use would be:
+    ///
+    /// ```no_run
+    /// use std::fs::File;
+    /// use tiff::{decoder::Decoder, TiffError};
+    ///
+    /// let file: File = /* */
+    /// # File::open(concat!(
+    /// #   env!("CARGO_MANIFEST_DIR"), "/tests/images/tiled-gray-i1.tif"
+    /// # ))?;
+    /// let mut decoder = Decoder::open(&file)?;
+    ///
+    /// let concurrent = decoder.inner().try_clone()?;
+    /// // ⚠️Do NOT attempt to use both decoder and racing_decoder in parallel!
+    /// // This will be defined behavior but likely cause both to fail.
+    /// let racing_decoder = decoder.take_with_reader(concurrent);
+    ///
+    /// # Ok::<_, TiffError>(())
+    /// ```
+    ///
+    /// In another use case you have a partially cached file backed by fast storage that you only
+    /// access from slow disk when any operation misses the cache. In that case you may use this
+    /// method to replace the cached reader with a file reader after a miss is detected, without
+    /// losing any of the state collected by the decoder.
+    ///
+    /// ```
+    /// use tiff::{decoder::Decoder, TiffError};
+    ///
+    /// let partial_file = /* */
+    /// # std::io::Cursor::new(include_bytes!(concat!(
+    /// #   env!("CARGO_MANIFEST_DIR"), "/tests/images/tiled-gray-i1.tif"
+    /// # )));
+    /// let mut decoder = Decoder::open(partial_file)?;
+    ///
+    /// # let backing_file = concat!(
+    /// #   env!("CARGO_MANIFEST_DIR"), "/tests/images/tiled-gray-i1.tif"
+    /// # );
+    ///
+    /// // Catch cache errors: we are out of pages we have for this cache..
+    /// if let Err(TiffError::IoError(e)) = decoder.next_image() {
+    ///     if e.kind() == std::io::ErrorKind::OutOfMemory {
+    ///         let file = std::fs::File::open(backing_file)?;
+    ///         let mut slow_decoder = decoder.take_with_reader(file);
+    ///         // Repeat the op, `next_image` is atomic on error.
+    ///         slow_decoder.next_image()?;
+    ///         // …
+    ///     }
+    /// }
+    ///
+    /// # Ok::<_, TiffError>(())
+    /// ```
+    pub fn take_with_reader<S: Read + Seek>(&mut self, reader: S) -> Decoder<S> {
+        Decoder {
+            value_reader: ValueReader {
+                reader: EndianReader::new(reader, self.value_reader.reader.byte_order),
+                bigtiff: self.value_reader.bigtiff,
+                limits: core::mem::take(&mut self.value_reader.limits),
+            },
+            ifd_offsets: self.ifd_offsets.clone(),
+            seen_ifds: self.seen_ifds.clone(),
+            directory: self.directory.clone(),
+            current_ifd_pointer: self.current_ifd_pointer,
+            image: core::mem::take(&mut self.image),
+        }
     }
 
     /// The offset of the directory representing the current image.
@@ -722,13 +856,33 @@ impl<R: Read + Seek> Decoder<R> {
         self.current_ifd_pointer
     }
 
+    /// Get the IFD decoder for our current IFD.
+    ///
+    /// This may be used with any directory, valid image or otherwise. When the decoder has not
+    /// read a directory yet (a state after [`TiffHeader::open`]) the decoder will represent an
+    /// empty directory with only the next pointer set.
+    pub fn current_ifd(&mut self) -> IfdDecoder<'_> {
+        IfdDecoder {
+            inner: tag_reader::TagReader {
+                decoder: &mut self.value_reader,
+                ifd: &self.directory,
+            },
+        }
+    }
+
     /// Loads the IFD at the specified index in the list, if one exists
     ///
     /// Error guarantee: the current directory and [`Self::ifd_pointer`] is not modified when an
     /// error is returned.
+    ///
+    /// The next directory image attributes are *not* parsed on a successful seek, i.e. it need not be
+    /// semantically valid as an image (incoherent sample depths, unsupported planar configuration,
+    /// missing required tags, etc.) but [`Self::read_image`] may not work. Calling methods
+    /// interacting with it as an image after this will first attempt to parse the directory
+    /// instead.
     pub fn seek_to_directory(&mut self, ifd_index: usize) -> TiffResult<()> {
         let (offset, directory) = self.find_nth_ifd(ifd_index)?;
-        self.image = Image::no_image();
+        self.image = None;
         self.current_ifd_pointer = Some(offset);
         self.directory = directory;
         Ok(())
@@ -742,7 +896,7 @@ 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 = Image::from_reader(&mut self.value_reader, &directory)?;
+        self.image = Some(Image::from_reader(&mut self.value_reader, &directory)?);
         self.current_ifd_pointer = Some(offset);
         self.directory = directory;
         Ok(())
@@ -805,15 +959,21 @@ impl<R: Read + Seek> Decoder<R> {
 
     /// Reads the next IFD.
     ///
-    /// If there is no further image in the TIFF file a format error is returned.
-    /// To determine whether there are more images call `TIFFDecoder::more_images` instead.
+    /// If there is no further IFD in the TIFF file a format error is returned.
+    /// To determine whether there are more images call [`Self::more_images`] instead.
+    ///
+    /// The next directory image attributes are *not* parsed on a successful seek, i.e. it need not be
+    /// semantically valid as an image (incoherent sample depths, unsupported planar configuration,
+    /// missing required tags, etc.) but [`Self::read_image`] may not work. Calling methods
+    /// interacting with it as an image after this will first attempt to parse the directory
+    /// instead.
     ///
     /// Error guarantee: the current directory and [`Self::ifd_pointer`] is not modified when an error is
     /// returned.
     pub fn next_directory(&mut self) -> TiffResult<()> {
         let next_ifd = self.directory.next();
         let (offset, directory) = self.read_chained_ifd(self.current_ifd_pointer, next_ifd)?;
-        self.image = Image::no_image();
+        self.image = None;
         self.current_ifd_pointer = Some(offset);
         self.directory = directory;
         Ok(())
@@ -822,19 +982,65 @@ impl<R: Read + Seek> Decoder<R> {
     /// Reads in the next image.
     ///
     /// If there is no further image in the TIFF file a format error is returned.
-    /// To determine whether there are more images call `TIFFDecoder::more_images` instead.
+    /// To determine whether there are more images call [`Self::more_images`] instead.
     ///
     /// Error guarantee: the current directory and [`Self::ifd_pointer`] is not modified when an error is
     /// returned.
     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 = Image::from_reader(&mut self.value_reader, &directory)?;
+        self.image = Some(Image::from_reader(&mut self.value_reader, &directory)?);
         self.current_ifd_pointer = Some(offset);
         self.directory = directory;
         Ok(())
     }
 
+    /// Prepare reading values for tags of a given directory.
+    ///
+    /// # Examples
+    ///
+    /// This method may be used to read the values of tags in directories that have been previously
+    /// read with [`Decoder::read_directory`].
+    ///
+    /// ```no_run
+    /// use tiff::decoder::Decoder;
+    /// use tiff::tags::Tag;
+    ///
+    /// # use std::io::Cursor;
+    /// # let mut data = Cursor::new(vec![0]);
+    /// let mut decoder = Decoder::open(&mut data)?;
+    ///
+    /// decoder.next_directory()?;
+    /// let sub_ifds = decoder.get_tag(Tag::SubIfd)?.into_ifd_vec()?;
+    ///
+    /// for ifd in sub_ifds {
+    ///     let subdir = decoder.read_directory(ifd)?;
+    ///     let subfile = decoder.read_directory_tags(&subdir).find_tag(Tag::SubfileType)?;
+    ///     // omitted: handle the subfiles, e.g. thumbnails
+    /// }
+    ///
+    /// # Ok::<_, tiff::TiffError>(())
+    /// ```
+    pub fn read_directory_tags<'ifd>(&'ifd mut self, ifd: &'ifd Directory) -> IfdDecoder<'ifd> {
+        IfdDecoder {
+            inner: tag_reader::TagReader {
+                decoder: &mut self.value_reader,
+                ifd,
+            },
+        }
+    }
+
+    /// Utility to create a reader like [`Self::read_entry`], from the current IFD.
+    ///
+    /// Returns an `Ok(None)` if the tag is not found in the current directory. See
+    /// [`Self::read_entry´] of which this is a simple wrapper.
+    pub fn read_tag(&mut self, tag: Tag) -> TiffResult<Option<EntryBytesReader<'_, R>>> {
+        match self.current_ifd().find_entry(tag) {
+            None => Ok(None),
+            Some(entry) => self.read_entry(entry).map(Some),
+        }
+    }
+
     /// Returns `true` if there is at least one more image available.
     pub fn more_images(&self) -> bool {
         self.directory.next().is_some()
@@ -849,6 +1055,23 @@ impl<R: Read + Seek> Decoder<R> {
     pub fn byte_order(&self) -> ByteOrder {
         self.value_reader.reader.byte_order
     }
+}
+
+/// <span id=header-stream-access>Methods that interact with the underlying reader.</span>
+impl<R: Read> Decoder<R> {
+    /// Returns a mutable reference to the stream being decoded.
+    pub fn inner(&mut self) -> &mut R {
+        self.value_reader.reader.inner()
+    }
+
+    /// Check if this file is a BigTIFF or a standard TIFF.
+    pub fn variant(&self) -> TiffVariant {
+        if self.value_reader.bigtiff {
+            TiffVariant::BigTiff
+        } else {
+            TiffVariant::Standard
+        }
+    }
 
     #[inline]
     pub fn read_ifd_offset(&mut self) -> Result<u64, io::Error> {
@@ -859,11 +1082,6 @@ impl<R: Read + Seek> Decoder<R> {
         }
     }
 
-    /// Returns a mutable reference to the stream being decoded.
-    pub fn inner(&mut self) -> &mut R {
-        self.value_reader.reader.inner()
-    }
-
     /// Reads a TIFF byte value
     #[inline]
     pub fn read_byte(&mut self) -> Result<u8, io::Error> {
@@ -943,6 +1161,33 @@ impl<R: Read + Seek> Decoder<R> {
         Ok(val)
     }
 
+    /// Create a byte reader for the value of an arbitrary IFD entry.
+    ///
+    /// Small values are stored inline inside the offset field of the entry. The threshold for this
+    /// depends on the [`Self::variant`] of the file. Interpret entries that have not been
+    /// extracted from an IFD of the current decoder with care.
+    ///
+    /// The reader returned form this method reads raw bytes. Values retrieved in this way are not
+    /// yet converted to the host endianess.
+    ///
+    /// Returns an error if the entry count exceeds the possible offset values or if the offset
+    /// of the entry would overflow the offset in the underlying stream. Otherwise returns a reader
+    /// that yields the bytes of the value.
+    ///
+    /// ## Design Notes
+    ///
+    /// This method does not appear on [`IfdDecoder`] as we assume the reader type may be important
+    /// to you. It also allows forward compatibility to additional trait implementations without
+    /// demanding them from all types as `dyn` would. There is also no `M×N` instantiation problem
+    /// with this method: you get one type per reader type and monomorphic methods from the traits
+    /// it implements.
+    pub fn read_entry(&mut self, entry: ifd::Entry) -> TiffResult<EntryBytesReader<'_, R>>
+    where
+        R: Seek,
+    {
+        EntryBytesReader::from_entry(&mut self.value_reader, entry)
+    }
+
     /// Reads a TIFF IFA offset/value field
     #[inline]
     pub fn read_offset_u64(&mut self) -> Result<[u8; 8], io::Error> {
@@ -953,14 +1198,58 @@ impl<R: Read + Seek> Decoder<R> {
 
     /// Moves the cursor to the specified offset
     #[inline]
-    pub fn goto_offset(&mut self, offset: u32) -> io::Result<()> {
+    pub fn goto_offset(&mut self, offset: u32) -> io::Result<()>
+    where
+        R: Seek,
+    {
         self.goto_offset_u64(offset.into())
     }
 
     #[inline]
-    pub fn goto_offset_u64(&mut self, offset: u64) -> io::Result<()> {
+    pub fn goto_offset_u64(&mut self, offset: u64) -> io::Result<()>
+    where
+        R: Seek,
+    {
         self.value_reader.reader.goto_offset(offset)
     }
+}
+
+/// <span id=header-image>Methods that interact with the current directory as an image.</span>
+impl<R: Read + Seek> Decoder<R> {
+    /// Return the current IFD's image size.
+    ///
+    /// Returns an error if the current directory could not be parsed as an image. This can occur
+    /// if [`Self::next_directory`] or [`Self::seek_to_directory`] was used to advance the decoder
+    /// to the current directory which implies the IFD will be validated as an image on-demand by
+    /// this method.
+    ///
+    /// [`Self::image_buffer_layout`] provides further information on reading the image data and
+    /// can be used alternatively to verify further that the image can be read.
+    pub fn dimensions(&mut self) -> TiffResult<(u32, u32)> {
+        let image = self.ensure_image()?;
+        Ok((image.width, image.height))
+    }
+
+    /// Return the color type resulting from reading the image (in the current configuration).
+    ///
+    /// Returns an error if the current directory could not be parsed as an image. This can occur
+    /// if [`Self::next_directory`] or [`Self::seek_to_directory`] was used to advance the decoder
+    /// to the current directory which implies the IFD will be validated as an image on-demand by
+    /// this method.
+    pub fn colortype(&mut self) -> TiffResult<ColorType> {
+        let image = self.ensure_image()?;
+        image.colortype()
+    }
+
+    /// Returns the color map (palette) for the current image, if it uses
+    /// `PhotometricInterpretation::RGBPalette`.
+    ///
+    /// The returned slice contains `3 * 2^BitsPerSample` entries as u16 values,
+    /// laid out as: all red values, then all green values, then all blue values.
+    /// Each value is in the range `0..=65535` regardless of the actual bit depth.
+    pub fn color_map(&self) -> Option<&[u16]> {
+        self.image.as_ref().and_then(|img| img.color_map.as_deref())
+    }
 
     /// Read a tag-entry map from a known offset.
     ///
@@ -985,40 +1274,37 @@ impl<R: Read + Seek> Decoder<R> {
         self.value_reader.read_directory(ptr)
     }
 
-    fn check_chunk_type(&self, expected: ChunkType) -> TiffResult<()> {
-        if expected != self.image.chunk_type {
-            return Err(TiffError::UsageError(UsageError::InvalidChunkType(
-                expected,
-                self.image.chunk_type,
-            )));
-        }
-
-        Ok(())
-    }
-
     /// The chunk type (Strips / Tiles) of the image
+    ///
+    /// # Panics
+    ///
+    /// This method panics if the directory was read with [`Self::next_directory`] or
+    /// [`Self::seek_to_directory`] and [`Self::dimensions`] has not yet succeeded. (Other
+    /// image-related methods with signature (`fn(&mut self) -> TiffResult<_>`) can be substituted
+    /// but use that one by default).
     pub fn get_chunk_type(&self) -> ChunkType {
-        self.image.chunk_type
+        self.image.as_ref().unwrap().chunk_type
     }
 
     /// Number of strips in image
     pub fn strip_count(&mut self) -> TiffResult<u32> {
-        self.check_chunk_type(ChunkType::Strip)?;
-        let rows_per_strip = self.image.strip_decoder.as_ref().unwrap().rows_per_strip;
+        let image = self.ensure_image()?;
+        image.check_chunk_type(ChunkType::Strip)?;
+        let rows_per_strip = image.strip_decoder.as_ref().unwrap().rows_per_strip;
 
         if rows_per_strip == 0 {
             return Ok(0);
         }
 
         // rows_per_strip - 1 can never fail since we know it's at least 1
-        let height = match self.image.height.checked_add(rows_per_strip - 1) {
+        let height = match image.height.checked_add(rows_per_strip - 1) {
             Some(h) => h,
             None => return Err(TiffError::IntSizeError),
         };
 
-        let strips = match self.image.planar_config {
+        let strips = match image.planar_config {
             PlanarConfiguration::Chunky => height / rows_per_strip,
-            PlanarConfiguration::Planar => height / rows_per_strip * self.image.samples as u32,
+            PlanarConfiguration::Planar => height / rows_per_strip * image.samples as u32,
         };
 
         Ok(strips)
@@ -1026,8 +1312,9 @@ impl<R: Read + Seek> Decoder<R> {
 
     /// Number of tiles in image
     pub fn tile_count(&mut self) -> TiffResult<u32> {
-        self.check_chunk_type(ChunkType::Tile)?;
-        Ok(u32::try_from(self.image.chunk_offsets.len())?)
+        let image = self.ensure_image()?;
+        image.check_chunk_type(ChunkType::Tile)?;
+        Ok(u32::try_from(image.chunk_offsets.len())?)
     }
 
     fn read_chunk_to_bytes(
@@ -1036,12 +1323,10 @@ impl<R: Read + Seek> Decoder<R> {
         chunk_index: u32,
         layout: &image::ReadoutLayout,
     ) -> TiffResult<()> {
-        let offset = self.image.chunk_file_range(chunk_index)?.0;
-        self.goto_offset_u64(offset)?;
-
-        self.image
-            .expand_chunk(&mut self.value_reader, buffer, layout, chunk_index)?;
-
+        let (image, value_reader) = 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)?;
         Ok(())
     }
 
@@ -1056,8 +1341,9 @@ impl<R: Read + Seek> Decoder<R> {
         &mut self,
         chunk_index: u32,
     ) -> TiffResult<BufferLayoutPreference> {
-        let data_dims = self.image.chunk_data_dimensions(chunk_index)?;
-        let readout = self.image.readout_for_size(data_dims.0, data_dims.1)?;
+        let image = self.ensure_image()?;
+        let data_dims = image.chunk_data_dimensions(chunk_index)?;
+        let readout = image.readout_for_size(data_dims.0, data_dims.1)?;
 
         let extent = readout.result_extent_for_planes(0..1)?;
         let sample_type = extent.sample_type();
@@ -1072,7 +1358,7 @@ impl<R: Read + Seek> Decoder<R> {
             planes: 1,
             plane_stride,
             complete_len: layout.size(),
-            sample_format: self.image.sample_format,
+            sample_format: image.sample_format,
             sample_type: Some(sample_type),
         })
     }
@@ -1089,17 +1375,15 @@ impl<R: Read + Seek> Decoder<R> {
         &mut self,
         code_unit: TiffCodingUnit,
     ) -> TiffResult<BufferLayoutPreference> {
-        match self.image.planar_config {
+        let image = self.ensure_image()?;
+
+        match image.planar_config {
             PlanarConfiguration::Chunky => return self.image_chunk_buffer_layout(code_unit.0),
             PlanarConfiguration::Planar => {}
         }
 
-        let (width, height) = self.image.chunk_data_dimensions(code_unit.0)?;
-
-        let layout = self
-            .image
-            .readout_for_size(width, height)?
-            .to_plane_layout()?;
+        let (width, height) = image.chunk_data_dimensions(code_unit.0)?;
+        let layout = image.readout_for_size(width, height)?.to_plane_layout()?;
 
         if code_unit.0 >= layout.readout.chunks_per_plane {
             return Err(TiffError::UsageError(UsageError::InvalidCodingUnit(
@@ -1115,9 +1399,10 @@ impl<R: Read + Seek> Decoder<R> {
     ///
     /// Note that for planar images each chunk contains only one sample of the underlying data.
     pub fn read_chunk(&mut self, chunk_index: u32) -> TiffResult<DecodingSampleBuffer> {
-        let (width, height) = self.image.chunk_data_dimensions(chunk_index)?;
+        let image = self.ensure_image()?;
 
-        let readout = self.image.readout_for_size(width, height)?;
+        let (width, height) = image.chunk_data_dimensions(chunk_index)?;
+        let readout = image.readout_for_size(width, height)?;
 
         let mut result = readout
             .result_extent_for_planes(0..1)?
@@ -1136,9 +1421,10 @@ impl<R: Read + Seek> Decoder<R> {
     ///
     /// Note that for planar images each chunk contains only one sample of the underlying data.
     pub fn read_chunk_bytes(&mut self, chunk_index: u32, buffer: &mut [u8]) -> TiffResult<()> {
-        let (width, height) = self.image.chunk_data_dimensions(chunk_index)?;
+        let image = self.ensure_image()?;
 
-        let layout = self.image.readout_for_size(width, height)?;
+        let (width, height) = image.chunk_data_dimensions(chunk_index)?;
+        let layout = image.readout_for_size(width, height)?;
         layout.assert_min_layout(buffer)?;
 
         self.read_chunk_to_bytes(buffer, chunk_index, &layout)?;
@@ -1159,9 +1445,10 @@ impl<R: Read + Seek> Decoder<R> {
         chunk_index: u32,
         output_width: usize,
     ) -> TiffResult<()> {
-        let (width, height) = self.image.chunk_data_dimensions(chunk_index)?;
+        let image = self.ensure_image()?;
 
-        let mut layout = self.image.readout_for_size(width, height)?;
+        let (width, height) = image.chunk_data_dimensions(chunk_index)?;
+        let mut layout = image.readout_for_size(width, height)?;
         layout.set_row_stride(output_width)?;
 
         let extent = layout.result_extent_for_planes(0..1)?;
@@ -1191,8 +1478,10 @@ impl<R: Read + Seek> Decoder<R> {
         slice: TiffCodingUnit,
         buffer: &mut [u8],
     ) -> TiffResult<()> {
-        let (width, height) = self.image.chunk_data_dimensions(slice.0)?;
-        let readout = self.image.readout_for_size(width, height)?;
+        let (image, value_reader) = self.ensure_image_and_reader()?;
+
+        let (width, height) = image.chunk_data_dimensions(slice.0)?;
+        let readout = image.readout_for_size(width, height)?;
 
         let ref layout @ image::PlaneLayout {
             ref plane_offsets,
@@ -1214,14 +1503,9 @@ impl<R: Read + Seek> Decoder<R> {
 
         for (idx, &plane_offset) in plane_offsets[..used_plane_offsets].iter().enumerate() {
             let chunk = slice.0 + idx as u32 * readout.chunks_per_plane;
-            self.goto_offset_u64(self.image.chunk_offsets[chunk as usize])?;
-
-            self.image.expand_chunk(
-                &mut self.value_reader,
-                &mut buffer[plane_offset..],
-                readout,
-                chunk,
-            )?;
+            let offset = image.chunk_offsets[chunk as usize];
+            value_reader.reader.goto_offset(offset)?;
+            image.expand_chunk(value_reader, &mut buffer[plane_offset..], readout, chunk)?;
         }
 
         Ok(())
@@ -1229,14 +1513,32 @@ impl<R: Read + Seek> Decoder<R> {
 
     /// Returns the default chunk size for the current image. Any given chunk in the image is at most as large as
     /// the value returned here. For the size of the data (chunk minus padding), use `chunk_data_dimensions`.
+    ///
+    /// # Panics
+    ///
+    /// This method panics if the directory was read with [`Self::next_directory`] or
+    /// [`Self::seek_to_directory`] and [`Self::dimensions`] has not yet succeeded. (Other
+    /// image-related methods with signature (`fn(&mut self) -> TiffResult<_>`) can be substituted
+    /// but use that one by default).
     pub fn chunk_dimensions(&self) -> (u32, u32) {
-        self.image.chunk_dimensions().unwrap()
+        self.image.as_ref().unwrap().chunk_dimensions().unwrap()
     }
 
     /// Returns the size of the data in the chunk with the specified index. This is the default size of the chunk,
     /// minus any padding.
+    ///
+    /// # Panics
+    ///
+    /// This method panics if the directory was read with [`Self::next_directory`] or
+    /// [`Self::seek_to_directory`] and [`Self::dimensions`] has not yet succeeded. (Other
+    /// image-related methods with signature (`fn(&mut self) -> TiffResult<_>`) can be substituted
+    /// but use that one to be sure).
+    ///
+    /// This method panics if the index is out-of-range.
     pub fn chunk_data_dimensions(&self, chunk_index: u32) -> (u32, u32) {
         self.image
+            .as_ref()
+            .unwrap()
             .chunk_data_dimensions(chunk_index)
             .expect("invalid chunk_index")
     }
@@ -1255,7 +1557,8 @@ impl<R: Read + Seek> Decoder<R> {
     /// indicate the layout needed to read the first data plane. This will be fixed in a future
     /// major version of `tiff`.
     pub fn image_buffer_layout(&mut self) -> TiffResult<BufferLayoutPreference> {
-        let layout = self.image.readout_for_image()?.to_plane_layout()?;
+        let image = self.ensure_image()?;
+        let layout = image.readout_for_image()?.to_plane_layout()?;
         Ok(BufferLayoutPreference::from_planes(&layout))
     }
 
@@ -1279,7 +1582,8 @@ impl<R: Read + Seek> Decoder<R> {
     /// replaced, to ensure that existing code will not run into unexpectedly large allocations
     /// that will error on limits instead.
     pub fn read_image(&mut self) -> TiffResult<DecodingSampleBuffer> {
-        let readout = self.image.readout_for_image()?;
+        let image = self.ensure_image()?;
+        let readout = image.readout_for_image()?;
 
         let mut result = readout
             .result_extent_for_planes(0..1)?
@@ -1341,7 +1645,7 @@ impl<R: Read + Seek> Decoder<R> {
         &mut self,
         result: &mut DecodingSampleBuffer,
     ) -> TiffResult<BufferLayoutPreference> {
-        let readout = self.image.readout_for_image()?;
+        let readout = self.ensure_image()?.readout_for_image()?;
         let planes = readout.to_plane_layout()?;
 
         let num_planes = if planes.total_bytes <= self.value_reader.limits.decoding_buffer_size {
@@ -1372,7 +1676,8 @@ 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 readout = self.image.readout_for_image()?;
+        let (image, value_reader) = self.ensure_image_and_reader()?;
+        let readout = image.readout_for_image()?;
 
         let ref layout @ image::PlaneLayout {
             ref plane_offsets,
@@ -1396,10 +1701,11 @@ impl<R: Read + Seek> Decoder<R> {
 
             for (idx, &plane_offset) in plane_offsets[..used_plane_offsets].iter().enumerate() {
                 let chunk = chunk + idx as u32 * readout.chunks_per_plane;
-                self.goto_offset_u64(self.image.chunk_offsets[chunk as usize])?;
+                let offset = image.chunk_offsets[chunk as usize];
+                value_reader.reader.goto_offset(offset)?;
 
-                self.image.expand_chunk(
-                    &mut self.value_reader,
+                image.expand_chunk(
+                    value_reader,
                     &mut buffer[plane_offset..][buffer_offset..],
                     readout,
                     chunk,
@@ -1409,70 +1715,28 @@ impl<R: Read + Seek> Decoder<R> {
 
         Ok(())
     }
-
-    /// Get the IFD decoder for our current IFD.
-    ///
-    /// This may be used with any directory, valid image or otherwise. When the decoder has not
-    /// read a directory yet (a state after [`TiffHeader::open`]) the decoder will represent an
-    /// empty directory with only the next pointer set.
-    pub fn current_ifd(&mut self) -> IfdDecoder<'_> {
-        IfdDecoder {
-            inner: tag_reader::TagReader {
-                decoder: &mut self.value_reader,
-                ifd: &self.directory,
-            },
-        }
-    }
-
-    /// Prepare reading values for tags of a given directory.
-    ///
-    /// # Examples
-    ///
-    /// This method may be used to read the values of tags in directories that have been previously
-    /// read with [`Decoder::read_directory`].
-    ///
-    /// ```no_run
-    /// use tiff::decoder::Decoder;
-    /// use tiff::tags::Tag;
-    ///
-    /// # use std::io::Cursor;
-    /// # let mut data = Cursor::new(vec![0]);
-    /// let mut decoder = Decoder::open(&mut data)?;
-    ///
-    /// decoder.next_directory()?;
-    /// let sub_ifds = decoder.get_tag(Tag::SubIfd)?.into_ifd_vec()?;
-    ///
-    /// for ifd in sub_ifds {
-    ///     let subdir = decoder.read_directory(ifd)?;
-    ///     let subfile = decoder.read_directory_tags(&subdir).find_tag(Tag::SubfileType)?;
-    ///     // omitted: handle the subfiles, e.g. thumbnails
-    /// }
-    ///
-    /// # Ok::<_, tiff::TiffError>(())
-    /// ```
-    pub fn read_directory_tags<'ifd>(&'ifd mut self, ifd: &'ifd Directory) -> IfdDecoder<'ifd> {
-        IfdDecoder {
-            inner: tag_reader::TagReader {
-                decoder: &mut self.value_reader,
-                ifd,
-            },
-        }
 }
 
+#[doc(hidden)]
+#[allow(deprecated)]
+impl<R: Read + Seek> Decoder<R> {
     /// Tries to retrieve a tag from the current image directory.
     /// Return `Ok(None)` if the tag is not present.
+    #[deprecated = "Replace with `self.current_ifd().find_tag`"]
     pub fn find_tag(&mut self, tag: Tag) -> TiffResult<Option<ifd::Value>> {
         self.current_ifd().find_tag(tag)
     }
 
     /// Tries to retrieve a tag in the current image directory and convert it to the desired
     /// unsigned type.
+    #[deprecated = "Replace with `self.current_ifd().find_tag_unsigned`"]
     pub fn find_tag_unsigned<T: TryFrom<u64>>(&mut self, tag: Tag) -> TiffResult<Option<T>> {
         self.current_ifd().find_tag_unsigned(tag)
     }
 
     /// Tries to retrieve a vector of all a tag's values and convert them to the desired unsigned
     /// type.
+    #[deprecated = "Replace with `self.current_ifd().find_tag_unsigned_vec`"]
     pub fn find_tag_unsigned_vec<T: TryFrom<u64>>(
         &mut self,
         tag: Tag,
@@ -1482,67 +1746,81 @@ impl<R: Read + Seek> Decoder<R> {
 
     /// Tries to retrieve a tag from the current image directory and convert it to the desired
     /// unsigned type. Returns an error if the tag is not present.
+    #[deprecated = "Replace with `self.current_ifd().get_tag_unsigned`"]
     pub fn get_tag_unsigned<T: TryFrom<u64>>(&mut self, tag: Tag) -> TiffResult<T> {
         self.current_ifd().get_tag_unsigned(tag)
     }
 
     /// Tries to retrieve a tag from the current image directory.
     /// Returns an error if the tag is not present
+    #[deprecated = "Replace with `self.current_ifd().get_tag`"]
     pub fn get_tag(&mut self, tag: Tag) -> TiffResult<ifd::Value> {
         self.current_ifd().get_tag(tag)
     }
 
+    #[deprecated = "Replace with `self.current_ifd().get_tag_u32`"]
     pub fn get_tag_u32(&mut self, tag: Tag) -> TiffResult<u32> {
         self.get_tag(tag)?.into_u32()
     }
 
+    #[deprecated = "Replace with `self.current_ifd().get_tag_u64`"]
     pub fn get_tag_u64(&mut self, tag: Tag) -> TiffResult<u64> {
         self.get_tag(tag)?.into_u64()
     }
 
     /// Tries to retrieve a tag and convert it to the desired type.
+    #[deprecated = "Replace with `self.current_ifd().get_tag_f32`"]
     pub fn get_tag_f32(&mut self, tag: Tag) -> TiffResult<f32> {
         self.get_tag(tag)?.into_f32()
     }
 
     /// Tries to retrieve a tag and convert it to the desired type.
+    #[deprecated = "Replace with `self.current_ifd().get_tag_f64`"]
     pub fn get_tag_f64(&mut self, tag: Tag) -> TiffResult<f64> {
         self.get_tag(tag)?.into_f64()
     }
 
     /// Tries to retrieve a tag and convert it to the desired type.
+    #[deprecated = "Replace with `self.current_ifd().get_tag_u32_vec`"]
     pub fn get_tag_u32_vec(&mut self, tag: Tag) -> TiffResult<Vec<u32>> {
         self.get_tag(tag)?.into_u32_vec()
     }
 
+    #[deprecated = "Replace with `self.current_ifd().get_tag_u16_vec`"]
     pub fn get_tag_u16_vec(&mut self, tag: Tag) -> TiffResult<Vec<u16>> {
         self.get_tag(tag)?.into_u16_vec()
     }
 
+    #[deprecated = "Replace with `self.current_ifd().get_tag_u64_vec`"]
     pub fn get_tag_u64_vec(&mut self, tag: Tag) -> TiffResult<Vec<u64>> {
         self.get_tag(tag)?.into_u64_vec()
     }
 
     /// Tries to retrieve a tag and convert it to the desired type.
+    #[deprecated = "Replace with `self.current_ifd().get_tag_f32_vec`"]
     pub fn get_tag_f32_vec(&mut self, tag: Tag) -> TiffResult<Vec<f32>> {
         self.get_tag(tag)?.into_f32_vec()
     }
 
     /// Tries to retrieve a tag and convert it to the desired type.
+    #[deprecated = "Replace with `self.current_ifd().get_tag_f64_vec`"]
     pub fn get_tag_f64_vec(&mut self, tag: Tag) -> TiffResult<Vec<f64>> {
         self.get_tag(tag)?.into_f64_vec()
     }
 
     /// Tries to retrieve a tag and convert it to a 8bit vector.
+    #[deprecated = "Replace with `self.current_ifd().get_tag_u8_vec`"]
     pub fn get_tag_u8_vec(&mut self, tag: Tag) -> TiffResult<Vec<u8>> {
         self.get_tag(tag)?.into_u8_vec()
     }
 
     /// Tries to retrieve a tag and convert it to a ascii vector.
+    #[deprecated = "Replace with `self.current_ifd().get_tag_ascii_string`"]
     pub fn get_tag_ascii_string(&mut self, tag: Tag) -> TiffResult<String> {
         self.get_tag(tag)?.into_string()
     }
 
+    #[deprecated = "Replace with `self.current_ifd().tag_iter`"]
     pub fn tag_iter(&mut self) -> impl Iterator<Item = TiffResult<(Tag, ifd::Value)>> + '_ {
         self.current_ifd().tag_iter()
     }

src/directory.rs

@@ -18,6 +18,7 @@ use crate::{
 /// [`Decoder::read_directory_tags`](crate::decoder::Decoder::read_directory_tags) to read the
 /// values associated with tags from an underlying file.
 #[doc(alias = "IFD")]
+#[derive(Clone)]
 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

src/encoder/mod.rs

@@ -488,7 +488,8 @@ impl<'a, W: 'a + Write + Seek, K: TiffKind> DirectoryEncoder<'a, W, K> {
             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])?;
+            self.writer
+                .write_bytes(&entry.offset_raw()[..offset_bytes])?;
         }
 
         Ok(offset)

src/tags.rs

@@ -567,6 +567,13 @@ impl ValueBuffer {
     }
 }
 
+/// Create an empty value buffer with `Byte` type. This type choice is arbitrary.
+impl Default for ValueBuffer {
+    fn default() -> Self {
+        ValueBuffer::empty(Type::BYTE)
+    }
+}
+
 /// Byte order of the TIFF file.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum ByteOrder {

tests/decode_bigtiff_images.rs

@@ -26,21 +26,20 @@ fn test_big_tiff() {
             decoder.colortype().expect("Cannot get colortype"),
             ColorType::RGB(8)
         );
+
+        let mut ifd = decoder.current_ifd();
         assert_eq!(
-            decoder
-                .get_tag_u64(Tag::StripOffsets)
+            ifd.get_tag_u64(Tag::StripOffsets)
                 .expect("Cannot get StripOffsets"),
             16
         );
         assert_eq!(
-            decoder
-                .get_tag_u64(Tag::RowsPerStrip)
+            ifd.get_tag_u64(Tag::RowsPerStrip)
                 .expect("Cannot get RowsPerStrip"),
             64
         );
         assert_eq!(
-            decoder
-                .get_tag_u64(Tag::StripByteCounts)
+            ifd.get_tag_u64(Tag::StripByteCounts)
                 .expect("Cannot get StripByteCounts"),
             12288
         )

tests/decode_geotiff_images.rs

@@ -30,27 +30,25 @@ fn test_geo_tiff() {
                 num_samples: 5
             }
         );
+
+        let mut ifd = decoder.current_ifd();
         assert_eq!(
-            decoder
-                .get_tag_u64(Tag::StripOffsets)
+            ifd.get_tag_u64(Tag::StripOffsets)
                 .expect("Cannot get StripOffsets"),
             418
         );
         assert_eq!(
-            decoder
-                .get_tag_u64(Tag::RowsPerStrip)
+            ifd.get_tag_u64(Tag::RowsPerStrip)
                 .expect("Cannot get RowsPerStrip"),
             10
         );
         assert_eq!(
-            decoder
-                .get_tag_u64(Tag::StripByteCounts)
+            ifd.get_tag_u64(Tag::StripByteCounts)
                 .expect("Cannot get StripByteCounts"),
             1000
         );
         assert_eq!(
-            decoder
-                .get_tag(Tag::ModelPixelScaleTag)
+            ifd.get_tag(Tag::ModelPixelScaleTag)
                 .expect("Cannot get pixel scale")
                 .into_f64_vec()
                 .expect("Cannot get pixel scale"),

tests/decode_images.rs

@@ -404,7 +404,10 @@ fn test_string_tags() {
         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");
-        let software = decoder.get_tag(tiff::tags::Tag::Software).unwrap();
+        let software = decoder
+            .current_ifd()
+            .get_tag(tiff::tags::Tag::Software)
+            .unwrap();
         match software {
             ifd::Value::Ascii(s) => assert_eq!(
                 &s,
@@ -478,7 +481,8 @@ fn test_inner_access() {
     decoder.next_image().expect("Cannot read image IFD");
     assert_eq!(decoder.colortype().unwrap(), ColorType::RGB(8));
 
-    let c = decoder
+    let mut ifd = decoder.current_ifd();
+    let c = ifd
         .get_tag(tiff::tags::Tag::Compression)
         .unwrap()
         .into_u16()
@@ -487,13 +491,13 @@ fn test_inner_access() {
 
     // 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
+    let first_offset = ifd
         .get_tag_u32_vec(tiff::tags::Tag::TileOffsets)
         .unwrap()
         .into_iter()
         .next()
         .unwrap();
-    let first_byte_count = decoder
+    let first_byte_count = ifd
         .get_tag_u32_vec(tiff::tags::Tag::TileByteCounts)
         .unwrap()
         .into_iter()

tests/encode_images.rs

@@ -40,6 +40,7 @@ fn encode_decode() {
         assert_eq!(decoder.dimensions().unwrap(), (100, 100));
 
         let mut all_tags = decoder
+            .current_ifd()
             .tag_iter()
             .filter_map(Result::ok)
             .collect::<Vec<_>>();
@@ -80,7 +81,7 @@ fn encode_decode() {
         );
 
         assert_eq!(
-            decoder.get_tag(Tag::Artist).unwrap(),
+            decoder.current_ifd().get_tag(Tag::Artist).unwrap(),
             ifd::Value::Ascii("Image-tiff".into())
         );
 
@@ -125,6 +126,7 @@ fn encode_decode_big() {
         assert_eq!(decoder.dimensions().unwrap(), (100, 100));
 
         let mut all_tags = decoder
+            .current_ifd()
             .tag_iter()
             .filter_map(Result::ok)
             .collect::<Vec<_>>();
@@ -165,7 +167,7 @@ fn encode_decode_big() {
         );
 
         assert_eq!(
-            decoder.get_tag(Tag::Artist).unwrap(),
+            decoder.current_ifd().get_tag(Tag::Artist).unwrap(),
             ifd::Value::Ascii("Image-tiff".into())
         );
         let mut buffer = tiff::decoder::DecodingSampleBuffer::U8(vec![]);
@@ -539,37 +541,57 @@ trait AssertDecode {
 
 impl<R: std::io::Read + std::io::Seek> AssertDecode for Decoder<R> {
     fn assert_tag_u32(&mut self, tag: u16) -> u32 {
-        self.get_tag(Tag::Unknown(tag)).unwrap().into_u32().unwrap()
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
+            .unwrap()
+            .into_u32()
+            .unwrap()
     }
     fn assert_tag_u32_vec(&mut self, tag: u16) -> Vec<u32> {
-        self.get_tag(Tag::Unknown(tag))
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
             .unwrap()
             .into_u32_vec()
             .unwrap()
     }
     fn assert_tag_i32(&mut self, tag: u16) -> i32 {
-        self.get_tag(Tag::Unknown(tag)).unwrap().into_i32().unwrap()
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
+            .unwrap()
+            .into_i32()
+            .unwrap()
     }
     fn assert_tag_i32_vec(&mut self, tag: u16) -> Vec<i32> {
-        self.get_tag(Tag::Unknown(tag))
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
             .unwrap()
             .into_i32_vec()
             .unwrap()
     }
     fn assert_tag_u64(&mut self, tag: u16) -> u64 {
-        self.get_tag(Tag::Unknown(tag)).unwrap().into_u64().unwrap()
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
+            .unwrap()
+            .into_u64()
+            .unwrap()
     }
     fn assert_tag_u64_vec(&mut self, tag: u16) -> Vec<u64> {
-        self.get_tag(Tag::Unknown(tag))
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
             .unwrap()
             .into_u64_vec()
             .unwrap()
     }
     fn assert_tag_i64(&mut self, tag: u16) -> i64 {
-        self.get_tag(Tag::Unknown(tag)).unwrap().into_i64().unwrap()
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
+            .unwrap()
+            .into_i64()
+            .unwrap()
     }
     fn assert_tag_i64_vec(&mut self, tag: u16) -> Vec<i64> {
-        self.get_tag(Tag::Unknown(tag))
+        self.current_ifd()
+            .get_tag(Tag::Unknown(tag))
             .unwrap()
             .into_i64_vec()
             .unwrap()
@@ -776,7 +798,13 @@ fn test_rows_per_strip() {
     {
         let mut decoder = Decoder::open(&mut file).unwrap();
         decoder.next_image().unwrap();
-        assert_eq!(decoder.get_tag_u64(Tag::RowsPerStrip).unwrap(), 2);
+        assert_eq!(
+            decoder
+                .current_ifd()
+                .get_tag_u64(Tag::RowsPerStrip)
+                .unwrap(),
+            2
+        );
         assert_eq!(decoder.strip_count().unwrap(), 50);
 
         for i in 0..50 {
@@ -829,6 +857,7 @@ fn test_auxiliary_directory() {
         let _ = decoder.read_image().unwrap();
 
         let exif_location = decoder
+            .current_ifd()
             .get_tag(Tag::ExifDirectory)
             .expect("second directory missing Exif")
             .into_ifd_pointer()