nx_av1_txfm.nx

nx_av1_txfm.nx -- AV1/AV2 transform primitives. Layer six, the last one before actual residual reconstruction. Everything here is integer and bit-exact by mandate: an inverse transform that is off by one anywhere drifts the reference frames apart from the encoder's, and the picture degrades over a GOP rather than failing. Round2 AND Round2Signed BOTH EXIST AND DIFFER. Round2 adds half and arithmetic-shifts, so it rounds negatives toward positive infinity: Round2(-3, 1) is -1. Round2Signed rounds AWAY from zero by construction: Round2Signed(-3, 1) is -2. The spec uses each in specific places and they are not interchangeable -- substituting one for the other produces a transform that is correct on non-negative input and half-wrong on real signal, which is exactly the kind of defect that survives a smoke test. THE COSINE TABLE IS 12-BIT AND QUARTER-PERIOD. Cos128_Lookup holds only angles 0..64; the other three quadrants are reflections with sign flips. Getting a quadrant sign wrong inverts half the basis functions, which decodes to a recognisable but wrong image -- it does not look like noise, so it is easy to ship. THE WHT IS EXACTLY INVERTIBLE. The lossless path's Walsh-Hadamard pair is integer-exact in both directions, which makes it the one transform provable by pure round-trip rather than against reference vectors. genealogy_id: av1_spec_7_13_inverse_transform lineage_id: nx_av1_txfm_v1 license_tier: ORIGINAL