Replace Python-generated test TIFFs with libtiff-generated ones to
eliminate circular validation. The C program (gen_seq_tiff.c) uses
libtiff's TIFFWriteScanline with MSB-first packed bit streams.
Cross-validated: tifffile independently confirms all 27 images match
the original Python-generated versions byte-for-byte. tiffinfo validates
all IFD structures. All 27 image-tiff decode tests pass unchanged.
Previously, sub-byte bit depths (1-7 bits per sample) were only
accepted for Gray and Multiband color types. This extends the
validation to accept all color types (RGB, RGBA, CMYK, CMYKA, YCbCr)
at sub-byte depths.
The data format is unchanged: sub-byte samples remain packed MSB-first
in the output buffer, exactly as for grayscale. Predictors are still
rejected for sub-byte data.
Tested via libtiffpic flower-rgb-contig-02/04 and flower-rgb-planar-02/04.
TIFF files can store samples at non-standard bit depths (e.g. 10, 12,
14, 24 bits) packed tightly in a continuous bit stream. Previously
these were rejected as unsupported color types.
This adds:
- MSB-first bit extraction from packed byte streams (unpack_bits,
extract_bits_msb) to expand N-bit samples to standard-width output
(9-16 bit → u16, 17-32 bit → u32)
- Output buffer sizing using the promoted sample width instead of the
packed bit depth (output_bits_per_sample field in ReadoutLayout)
- A new code path in expand_chunk that reads packed rows, unpacks to
standard-width samples, then applies predictor and color inversion
- Horizontal predictor support for extended depths
Tested with 12-bit grayscale and RGB images (existing test suite),
plus all libtiffpic depth test images (10/12/14/24-bit in gray, RGB
contig, and RGB planar configurations).
Read the ColorMap tag (320) for RGBPalette images and return raw
palette indices as the decoded data with ColorType::Palette(n).
Changes:
- Read and validate ColorMap tag during IFD parsing (3 * 2^bps entries)
- Return ColorType::Palette from colortype() for RGBPalette images
- Accept Palette in expand_chunk validation (standard + sub-byte depths)
- Expose color_map() method on Decoder for palette lookup
- Add tests for 1-bit, 4-bit, 8-bit palette images with checksums
- Add test verifying ColorMap accessibility and structure
The decoder returns raw palette indices; consumers apply the lookup
using the color map returned by Decoder::color_map().
This prepares allowing to iterate over a TIFF file with invalid images
while `Image` maintains invariants. For instance we precalculate the
amount of buffer space and verify that the count of strips makes sense
in the constructor—so we do not have to redo that for every other entry
point.
Lilith River