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wai-rs: derive the dequant gain — the last fixture-sourced decoder input #29

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dcharlot merged 17 commits from jpegai-dequant-derive into main 2026-07-26 05:56:22 -04:00
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dcharlot commented 2026-07-19 00:31:30 -04:00
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The per-position dequant gain was the one input still read from a dumped fixture. It is now built from normative assets alone, closing the derivation chain.

Verified on real test256.bin: the RVS stage is bit-exact against the ISO reference on both planes (luma 40960/40960, chroma 24576/24576), and the derived tables match the reference buildTables() output for all three cwg variants. The same scale list reproduces RVS_ADDITIVE exactly, so those previously-tabulated constants are now cross-checked against their derivation.

Three hazards pinned down by assertions, each of which silently corrupts the result:

  • scale rounding is half-to-even (125*1.06 is exactly 132.5; half-away-from-zero picks the wrong multiplier)
  • the reference has integer >>16 and float /2^16 paths selected by a device comparison, disagreeing on negative input
  • fusing the two stages into one gain map costs a last-bit f32 difference

70 existing tests still pass.

Co-Authored-By: Claude Opus 4.8 [email protected]

The per-position dequant gain was the one input still read from a dumped fixture. It is now built from normative assets alone, closing the derivation chain. **Verified on real `test256.bin`:** the RVS stage is bit-exact against the ISO reference on both planes (luma 40960/40960, chroma 24576/24576), and the derived tables match the reference `buildTables()` output for all three cwg variants. The same scale list reproduces `RVS_ADDITIVE` exactly, so those previously-tabulated constants are now cross-checked against their derivation. Three hazards pinned down by assertions, each of which silently corrupts the result: - scale rounding is **half-to-even** (`125*1.06` is exactly `132.5`; half-away-from-zero picks the wrong multiplier) - the reference has integer `>>16` and float `/2^16` paths selected by a *device comparison*, disagreeing on negative input - fusing the two stages into one gain map costs a last-bit f32 difference 70 existing tests still pass. Co-Authored-By: Claude Opus 4.8 <[email protected]>
dcharlot force-pushed jpegai-dequant-derive from 9593fa71bd
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