support palette expansion, CIE L*a*b*, transparency mask, and lzw end marker tolerance

Add expand_palette_to_rgb() for converting palette indices to RGB using the color map (1/2/4/8/16-bit indices, row-aligned sub-byte).

Accept CIE L*a*b* (photometric=8) as Lab(8) for 8-bit 3-channel images. Re-bias a/b from signed to unsigned during expand_chunk so callers handle CIELab and ICCLab the same way.

Treat TransparencyMask as grayscale instead of erroring.

Handle missing LZW end-of-information code gracefully when output was already produced.

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,
@@ -659,6 +659,97 @@ fn invert_colors(
     Ok(())
 }
 
+/// Convert CIE L*a*b* (photometric=8) byte encoding to ICC L*a*b* (photometric=9).
+///
+/// CIELab stores a/b as signed int8 (-128..127). ICCLab stores them as uint8 with
+/// a +128 bias (0..255, where 128 = neutral). The L channel is identical in both.
+/// This re-biases a/b so that downstream code can treat CIELab the same as ICCLab.
+fn cielab_to_icclab(buf: &mut [u8]) {
+    for pixel in buf.chunks_exact_mut(3) {
+        // L stays as-is; a and b get +128 (wrapping handles the two's complement)
+        pixel[1] = pixel[1].wrapping_add(128);
+        pixel[2] = pixel[2].wrapping_add(128);
+    }
+}
+
+/// Expand palette indices in `buf` to RGB using the color map.
+///
+/// `buf` contains packed palette indices at `bits_per_sample` depth. The color map
+/// has layout `[R0..Rn, G0..Gn, B0..Bn]` with 16-bit entries. Each index is looked
+/// up and replaced with an 8-bit RGB triplet (high byte of the 16-bit color map value).
+///
+/// Returns a new `Vec<u8>` with 3 bytes (R,G,B) per pixel. Sub-byte indices (1, 2, 4-bit)
+/// are unpacked from the row-aligned packed representation.
+fn expand_palette(
+    buf: &[u8],
+    color_map: &[u16],
+    width: u32,
+    height: u32,
+    bits_per_sample: u8,
+) -> Vec<u8> {
+    let num_colors = 1usize << bits_per_sample;
+    let pixels = (width as usize) * (height as usize);
+    let mut rgb = Vec::with_capacity(pixels * 3);
+
+    if bits_per_sample == 8 {
+        for &idx in buf.iter().take(pixels) {
+            let i = idx as usize;
+            if i < num_colors {
+                rgb.push((color_map[i] >> 8) as u8);
+                rgb.push((color_map[num_colors + i] >> 8) as u8);
+                rgb.push((color_map[2 * num_colors + i] >> 8) as u8);
+            } else {
+                rgb.extend_from_slice(&[0, 0, 0]);
+            }
+        }
+    } else if bits_per_sample == 16 {
+        for chunk in buf.chunks(2).take(pixels) {
+            if chunk.len() < 2 {
+                break;
+            }
+            let i = u16::from_ne_bytes([chunk[0], chunk[1]]) as usize;
+            if i < num_colors {
+                rgb.push((color_map[i] >> 8) as u8);
+                rgb.push((color_map[num_colors + i] >> 8) as u8);
+                rgb.push((color_map[2 * num_colors + i] >> 8) as u8);
+            } else {
+                rgb.extend_from_slice(&[0, 0, 0]);
+            }
+        }
+    } else {
+        // Sub-byte (1, 2, 4-bit): rows are byte-aligned with padding bits at end
+        let samples_per_byte = 8 / bits_per_sample as usize;
+        let bytes_per_row = ((width as usize) * bits_per_sample as usize).div_ceil(8);
+
+        for row in 0..height as usize {
+            let row_start = row * bytes_per_row;
+            let row_end = (row_start + bytes_per_row).min(buf.len());
+            let mut col = 0u32;
+
+            for &byte in &buf[row_start..row_end] {
+                for bit_idx in (0..samples_per_byte).rev() {
+                    if col >= width {
+                        break;
+                    }
+                    let shift = bit_idx as u8 * bits_per_sample;
+                    let mask = ((1u16 << bits_per_sample) - 1) as u8;
+                    let i = ((byte >> shift) & mask) as usize;
+                    if i < num_colors {
+                        rgb.push((color_map[i] >> 8) as u8);
+                        rgb.push((color_map[num_colors + i] >> 8) as u8);
+                        rgb.push((color_map[2 * num_colors + i] >> 8) as u8);
+                    } else {
+                        rgb.extend_from_slice(&[0, 0, 0]);
+                    }
+                    col += 1;
+                }
+            }
+        }
+    }
+
+    rgb
+}
+
 /// 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();
@@ -1385,6 +1476,28 @@ impl<R: Read + Seek> Decoder<R> {
         self.image.as_ref().and_then(|img| img.color_map.as_deref())
     }
 
+    /// Expand a palette-indexed image buffer to 8-bit RGB.
+    ///
+    /// Takes the raw decoded buffer (palette indices at `bits_per_sample` depth) and
+    /// returns a new buffer with 3 bytes per pixel (R, G, B), using the image's color map.
+    /// Sub-byte indices (1, 2, 4-bit) are unpacked from the row-aligned representation.
+    ///
+    /// Returns `None` if the current image is not a palette image or has no color map.
+    pub fn expand_palette_to_rgb(&self, indices: &[u8]) -> Option<Vec<u8>> {
+        let image = self.image.as_ref()?;
+        let color_map = image.color_map.as_ref()?;
+        if image.photometric_interpretation != PhotometricInterpretation::RGBPalette {
+            return None;
+        }
+        Some(expand_palette(
+            indices,
+            color_map,
+            image.width,
+            image.height,
+            image.bits_per_sample,
+        ))
+    }
+
     /// Read a tag-entry map from a known offset.
     ///
     /// A TIFF [`Directory`], aka. image file directory aka. IFD, refers to a map from