nx_webp_alpha.nx

nx_webp_alpha.nx -- WebP VP8X extended container + ALPH alpha channel. Completes the extended half of WebP. Transparent WebP is everywhere on the real web, and it does NOT live in the VP8/VP8L bitstream: a VP8X file carries the colour planes in one chunk and an entirely separate ALPH chunk holding the alpha plane, filtered independently. Filters are implemented in BOTH directions so the pair proves by exact round-trip rather than against reference images. THE FIRST ROW AND COLUMN PREDICT DIFFERENTLY PER FILTER. Horizontal predicts the first column from ABOVE; vertical predicts the first row from the LEFT. Using the nominal direction at the edges reads outside the plane, and clamping that to zero instead produces an alpha channel that is correct in the interior and wrong along two edges -- which looks like a subtle halo rather than a decode failure. THE GRADIENT PREDICTOR CLAMPS BEFORE ADDING. clamp(left + top - topleft) is clamped to 0..255 and THEN the residual is added modulo 256. Clamping after the addition, or not clamping at all, diverges only where the gradient overshoots -- i.e. at sharp alpha edges, exactly where it is visible. genealogy_id: webp_container_spec_vp8x_alph lineage_id: nx_webp_alpha_v1 license_tier: ORIGINAL