Paint RTs were ARGB32 (8 bpp). Repeated source-over at low alpha rounds
back to the previous value once the per-stroke decrement dst*alpha drops
below half an LSB, so it freezes at a grey floor (~0.5/alpha in 255ths) -
e.g. black at low opacity never reaches 0. Switch the accumulation buffers
to ARGBHalf (16-bit linear). The sRGB round-trip stays at the byte boundary
(seed decodes on upload, ReadRtPixels re-encodes on readback), so canonical
bytes and CPU/GPU parity are unchanged - only the accumulation gained bits.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
adminandClaude Opus 4.8
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