feat: YCbCr chroma subsampling for non-JPEG compression#366
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lilith wants to merge 1 commit intoimage-rs:mainfrom
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feat: YCbCr chroma subsampling for non-JPEG compression#366lilith wants to merge 1 commit intoimage-rs:mainfrom
lilith wants to merge 1 commit intoimage-rs:mainfrom
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Add YCbCrUpsamplingReader that converts chroma-subsampled YCbCr block data to full-resolution 3-bytes-per-pixel output. Each block of h×v pixels stores h*v Y samples plus one shared Cb and Cr sample; the reader expands these into per-pixel (Y, Cb, Cr) triples. This enables decoding YCbCr TIFFs with any supported compression method (uncompressed, LZW, Deflate, etc.), not just JPEG. Two more libtiffpic test images now decode with verified hashes: - dscf0013.tif (uncompressed YCbCr 2,1 subsampling) - ycbcr-cat.tif (LZW-compressed YCbCr 2,2 subsampling) The remaining two YCbCr test images (smallliz.tif, zackthecat.tif) use Old JPEG compression (type 6) which is a separate unsupported feature.
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Summary
YCbCrUpsamplingReaderthat converts chroma-subsampled block layout (h×v Y samples + shared Cb/Cr) to full-resolution per-pixel (Y, Cb, Cr) triplesexpand_chunk()for non-JPEG compression methodsChromaSubsamplingerror) since the block layout only applies to chunky/interleaved dataSplit out from #363 per review request.
Test plan
dscf0013.tif(uncompressed, YCbCr 2×1 subsampling): CRC32 hash verified0x6896e83aycbcr-cat.tif(LZW, YCbCr 2×2 subsampling): CRC32 hash verified0x388a9d8fsmallliz.tif/zackthecat.tifremain smoke tests (Old JPEG compression type 6, unsupported regardless)