feat: support 16-bit CIELab decoding

src/decoder/image.rs

@@ -597,18 +597,23 @@ impl Image {
                     ),
                 )),
             },
-            // CIE L*a*b* (TIFF6 Section 23, pp 110-115) encodes a/b as signed int8
+            // CIE L*a*b* (TIFF6 Section 23, pp 110-115) encodes a/b as signed integers
-            // (-128..=127) while ICC L*a*b* (photometric=9) uses unsigned samples with
+            // (int8 at 8-bit, int16 at 16-bit) with 0 neutral, while ICC L*a*b*
-            // a +128 bias. For 8-bit we re-bias a/b in post-processing (see
+            // (photometric=9) uses unsigned samples with the neutral at the middle of
-            // `cielab_to_icclab`) so callers see the same encoding as ICCLab.
+            // 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
-            // Only 8-bit is accepted here. 16-bit CIELab is a valid format that libtiff
+            // from the photometric interpretation (requiring `SampleFormat::Uint`, like
-            // does decode (`putcontig8bitCIELab16`, selected by `initCIELabConversion`
+            // the ICCLab arm) and re-bias them to the unsigned ICCLab encoding in
-            // in tif_getimage.c line 2216 @ 04884181e8903915038be553bfab404302d98ea0),
+            // post-processing (see `cielab_to_icclab`). libtiff does the same, decoding
-            // but it uses L=uint16 with a/b=int16 — a mixed layout that `SampleFormat`,
+            // both depths (`putcontig8bitCIELab8` / `putcontig8bitCIELab16`, selected by
-            // which applies uniformly to every sample, cannot express. Left as follow-up.
+            // `initCIELabConversion` in tif_getimage.c line 2216
+            // @ 04884181e8903915038be553bfab404302d98ea0).
             PhotometricInterpretation::CIELab => match self.photometric_samples {
-                3 if matches!(self.bits_per_sample, 8) => Ok(ColorType::Lab(self.bits_per_sample)),
+                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,

src/decoder/mod.rs

@@ -713,17 +713,33 @@ fn invert_colors(
 
 /// Re-bias CIE L*a*b* a/b channels from signed to unsigned encoding.
 ///
-/// CIELab (photometric=8) stores a/b as int8 where 0 is neutral.
+/// CIELab (photometric=8) stores a/b as signed integers where 0 is neutral;
-/// ICCLab (photometric=9) stores them as uint8 where 128 is neutral.
+/// ICCLab (photometric=9) stores them as unsigned with the neutral shifted to
-/// Adding 128 (wrapping) converts between the two representations.
+/// 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]) {
+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.
 ///
@@ -739,7 +755,7 @@ fn post_process_row(
 ) -> TiffResult<()> {
     match photometric_interpretation {
         PhotometricInterpretation::WhiteIsZero => invert_colors(buf, color_type, sample_format)?,
-        PhotometricInterpretation::CIELab => cielab_to_icclab(buf),
+        PhotometricInterpretation::CIELab => cielab_to_icclab(buf, color_type.bit_depth()),
         _ => {}
     }
     Ok(())

tests/decode_images.rs

@@ -1,7 +1,7 @@
 extern crate tiff;
 
 use tiff::decoder::{ifd, Decoder, DecodingSampleBuffer};
-use tiff::tags::{ByteOrder, PhotometricInterpretation, Type};
+use tiff::tags::{ByteOrder, PhotometricInterpretation, SampleFormat, Type};
 use tiff::ColorType;
 
 use std::fs::File;
@@ -1062,31 +1062,46 @@ fn test_gray_u12_hpredict() {
 
 // ---- TransparencyMask ----
 
-/// Build a minimal uncompressed TIFF in memory with the given photometric interpretation.
+/// Build a minimal uncompressed 2x2 TIFF in memory, in the requested byte order,
-/// Returns a 2x2 image with the provided pixel data as a single strip.
+/// 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 = 8;
+    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;
 
-    let mut buf = Vec::new();
-    buf.extend_from_slice(b"II");
-    buf.extend_from_slice(&42u16.to_le_bytes());
-    buf.extend_from_slice(&ifd_offset.to_le_bytes());
-    buf.extend_from_slice(&num_tags.to_le_bytes());
-
     // IFD entries (must be ascending tag order). Tag IDs stay raw for the
     // ascending-order property; the field type uses the `Type` enum.
-    for &(tag, typ, value) in &[
+    let mut tags: Vec<(u16, Type, u32)> = vec![
-        (256u16, Type::SHORT, width as u32),             // ImageWidth
+        (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
@@ -1094,14 +1109,33 @@ fn build_raw_tiff(
         (273, Type::LONG, strip_offset),                 // StripOffsets
         (277, Type::SHORT, samples_per_pixel as u32),    // SamplesPerPixel
         (279, Type::LONG, pixel_data.len() as u32),      // StripByteCounts
-    ] {
+    ];
-        buf.extend_from_slice(&tag.to_le_bytes());
+    if let Some(sf) = sample_format {
-        buf.extend_from_slice(&typ.to_u16().to_le_bytes());
+        tags.push((339, Type::SHORT, sf.to_u16() as u32)); // SampleFormat (tag 339 > 279)
-        buf.extend_from_slice(&1u32.to_le_bytes());
-        buf.extend_from_slice(&value.to_le_bytes());
     }
 
-    buf.extend_from_slice(&0u32.to_le_bytes()); // next IFD = 0
+    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
 }
@@ -1110,9 +1144,11 @@ fn build_raw_tiff(
 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();
@@ -1148,7 +1184,14 @@ fn test_cielab_decode() {
         0, 128, 128, // pixel 3: L=0, a=-128 (as i8), b=-128 (as i8)
     ];
 
-    let data = build_raw_tiff(PhotometricInterpretation::CIELab, 8, 3, &pixels);
+    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();
 
@@ -1182,3 +1225,119 @@ fn test_cielab_decode() {
         _ => 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"
+    );
+}