nx_av1_adst.nx

nx_av1_adst.nx -- AV1/AV2 inverse ADST (asymmetric discrete sine transform). AV1 picks between DCT and ADST per block and per axis, because they have opposite shapes at the block edge: the DCT's first basis is FLAT, the ADST's is a RISING RAMP. Intra-predicted residuals are typically small next to the predicted edge and grow with distance from it, which is exactly the ADST's basis-0 shape -- that is the whole reason it exists in the codec. SUBSTITUTING ONE FOR THE OTHER DECODES. That is the danger: a block coded with ADST and reconstructed with DCT produces a plausible image with wrong gradients near block edges, which reads as banding or a soft grid rather than as corruption. Nothing errors. THE SINPI CONSTANTS ARE NOT THE COSINE TABLE. ADST uses sin(k*pi/9) at 12-bit -- 1321, 2482, 3344, 3803 -- which appear nowhere in the DCT's quarter-period cosine table. Reaching for cos128() here silently builds a transform with the wrong basis entirely. THE OUTPUT OF A BASIS-0 IMPULSE IS THE CONSTANT SET ITSELF. Feeding [4096,0,0,0] returns exactly [1321, 2482, 3344, 3803] -- the four constants, in order. That makes the transform exactly checkable without reference vectors, and it is what the gate asserts. genealogy_id: av1_spec_7_13_2_6_inverse_adst lineage_id: nx_av1_adst_v1 license_tier: ORIGINAL