The dword-swap that fixes DXT1 is a property of the BC1 COLOUR block, not of
BCn in general. RE on a DXT5A (BC4) mask showed the alpha block is byte-indexed
and wants the endian swap ALONE — dword-swapping it makes it worse. The blanket
swap from the previous commit was therefore breaking the (common) DXT5A/DXN
alpha & normal masks.
Now targeted by format:
- DXT1 : swap dwords of each 8-byte colour block (verified).
- DXT2_3 / DXT4_5 : swap only the colour half (bytes 8..16) of each 16-byte
block; alpha half endian-only. TENTATIVE — these formats
are near-absent in the game assets, so unvalidated against
a clear image.
- DXT5A / DXN : no dword swap (endian only).
DXT1 export re-verified clean (weapon skins); 16 formats tests pass.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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