Lilith River 35c14f1e3c decoder: reconstruct extended-depth samples modulo declared bit depth ...

Promoted extended-depth grayscale (9-15 bits -> u16, 17-31 bits -> u32)
can carry bits above the declared depth after the horizontal predictor,
which reconstructs prev+delta in the promoted storage width rather than
modulo 2^BitsPerSample. This underflowed `max - v` in invert_colors (the
image-rs/image-tiff#396 panic in debug, wrong pixel in release) and left
junk in the high bits of the non-inverted output (#396 review, @197g:
"clamp the channel values in the non-inverted color case").

An n-bit predictor delta is stored in exactly n bits, i.e. modulo 2^n,
so reconstruction is defined modulo 2^n as well. Since 2^n divides the
u16/u32 storage modulus, masking each promoted sample with (1<<n)-1 once
after the predictor yields exactly the mod-2^n value -- including
legitimate wrap-arounds -- and also bounds any out-of-range sample from
malformed input. invert_colors' `max - v` is then always in range,
superseding the targeted saturating_sub from #396.

mask_samples_to_bit_depth runs in every integer decode branch of
expand_chunk; outside the extended-depth unpacking branch it is a
statically provable no-op (promoted layouts, declared < storage, occur
only there -- sub-byte depths keep byte-granular storage and reject the
horizontal predictor, standard depths have declared == storage, and the
float predictor requires IEEEFP samples that are never promoted), so the
branches cannot diverge.

Adds exact-pixel regression tests on hand-built TIFFs: 12/20-bit
wrap-around (plain and WhiteIsZero) and a 9-31-bit encode/decode
identity sweep, alongside the two fuzzer-minimized panic seeds. A
saturating clamp corrupts the wrap-around cases -- 12-bit [4095, 0, 1]
is stored as deltas [4095, 1, 1] and masks back to [4095, 0, 1], but
saturating produced [4095, 4095, 4095] -- so the tests lock the modular
semantics.

2026-07-18 16:04:42 -06:00

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