log16_to_y() uses f32::exp() which is not required to be correctly
rounded by IEEE 754. Apple libm on arm64 differs by 1 ULP from glibc
on x86_64, producing different CRC32 checksums for the decoded output.
Same issue as off_luv32.tif which was already demoted in commit 617dc6e.
A deterministic LUT-based approach could fix both in a follow-up.
Fix two CI failures on i686 and powerpc:
1. chunk_col_stride overflow: RowsPerStrip = u32::MAX causes
data_row_bytes * chunk_height to exceed 32-bit usize. Clamp
the chunk height to the image height before computing stride
values, since a chunk never contributes more rows than the
image contains. Applied to both readout_for_size() and
set_row_stride().
2. off_luv32.tif hash mismatch: the LogLuv decoder produces
platform-dependent output (different hashes on x86_64, i686,
and powerpc), so demote to smoke test.
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.
Add Group3Reader for decoding CCITT Group 3 (T.4) 1D compressed TIFF
images. The fax crate's Group3Decoder has a bug where new() verifies
but doesn't consume the initial EOL marker, causing its lookup tables
to destructively consume bits from the EOL code. We work around this
by using the crate's lower-level white::decode / black::decode
functions and handling EOL markers ourselves.
Also adds FillOrder tag (266) support: FillOrder=2 (LSB-to-MSB,
common in fax images) is handled by reversing bits in each byte
before decoding. This applies to both Group 3 and Group 4 readers.
Both libtiffpic fax test images (fax2d.tif, g3test.tif) now decode
correctly with verified CRC32 hashes.
Upgrade the libtiffpic test harness from decode-only smoke tests to
full output verification using CRC32 reference hashes. 50 of 61 images
now have hash-verified decoded output; the remaining 11 use unsupported
formats (YCbCr subsampling, Fax3, etc.) and remain as decode-only tests.
Uses read_image_to_buffer (the non-deprecated API) with ByteOrder::convert
for cross-platform LE normalization.
Remove the 13 individual depth tests from decode_images.rs — they tested
the same libtiffpic flower images that are now hash-verified here.
Two tests are disabled, both related to tiles, since they do not follow
the fully released spec themselves but incidentally decode with libtiff
anyways. The two images (`cramps-tile.tif`, `quad-tile.tif`) are tiled
layout which libtiff detects by the presence of TileWidth/TileHeight
tags. We would detect that by chunks encoded via TileOffsets and
TileByteLengths tags. However the images encoded their chunk data into
the StripOffsets tag; additionally they contain a RowsPerStrip value
anyways.
Tests disabled as this is to be resolved in a separate issue (#338) with
a decision on treatment of this situation (strictness setting or copying
libtiff, also error late for mismatched lengths).
A. Molzer