TIFFs stored as YCbCr (PhotometricInterpretation 6) without JPEG
compression failed with "does not support the color type Unknown(24)":
the image crate reads YCbCr only inside JPEG compression. ffmpeg writes
these whenever the source is YUV, so an ordinary frame grab
(`ffmpeg -i clip.mp4 -frames:v 1 grab.tiff`) couldn't be imported.
`still::ycbcr_tiff` decodes them from TIFF 6.0 (§21 YCbCr images, §20
ReferenceBlackWhite, §9 PackBits), reading the directory with
filmcraft-dng's TIFF parser: 8-bit chunky data units with subsampling 1, 2
or 4 each way (the image padded to whole data units), converted with the
file's YCbCrCoefficients and ReferenceBlackWhite (or the §21 defaults).
Every size, offset and count is checked, the picture is capped at 2^28
pixels, and separate planes, tiles and other bit depths are an
"unsupported" error. JPEG-compressed and RGB TIFFs still go to the image
crate.
Tests: hand-built files for data-unit placement (4:2:0), video-range
reference levels with PackBits (4:4:4), sizes that aren't whole data
units, and truncated/bit-flipped/hostile files under catch_unwind; an
ffmpeg oracle compares ffmpeg-written 4:2:0, 4:2:2 and 4:4:4 TIFFs (95×63)
with ffmpeg's own RGB decode (every channel within 6).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
lloyd