wai-rs: jpegai_pipeline::sigma_index — close the scale path (byte-exact, both planes) #5
2026-07-10
wai-rs: jpegai_pipeline::sigma_index — close the scale path (byte-exact, both planes)
Derives the per-position mANS distribution index from the hyper-scale decoder's output, connecting decode_hyper_scale to the sgm entropy decode: sigma[c,p] = clamp( (i_s[c,p] + gain_per_ch[c] + rvs[c,p] + 64) >> 7, 0, 31 ) The `+64 >> 7` is a round-to-nearest-128 done as an arithmetic right shift (floor) — the same reference-decoder semantics as the hyper_scale power-of-two division. The form was recovered from the reference decoder's own fixtures: sigma is not a function of i_s alone (the ranges overlap), but becomes an exact pure function once the per-channel gain and per-position residual-variance scale are added. Validated BYTE-EXACT against the reference decoder's entropy fixtures on real test256.bin data, both planes: luma (SORP) 40960/40960 and chroma (SORS) 24576/24576 positions match the `indexes` the reference entropy coder actually consumed. Plus a unit test pinning the 128-boundaries, the floor-on-negatives, and the [0,31] clamp. Co-Authored-By: Claude Opus 4.8 <[email protected]>