nx_vp8.nx

nx_vp8.nx -- VP8 lossy: the boolean entropy decoder and keyframe header. The last missing piece of WebP. VP8's entropy coder is a BINARY arithmetic coder -- one bit per call against an 8-bit probability -- which is a different animal from VP8L's canonical Huffman and from AV1's multi-symbol CDF coder. All three now exist in this tree and none substitutes for another. PROBABILITY 128 IS A RAW BIT. With prob 128 the split lands exactly at the midpoint, so the coder degenerates to reading the bitstream MSB-first. That is not a curiosity -- it is the property that makes the renormalisation loop testable without an encoder: decode a run of prob-128 literals and they must reproduce the raw bits exactly. Any error in the shift, the carry or the byte refill breaks it immediately. THE RANGE INVARIANT. After every decode the range must satisfy 128 <= range <= 255. A renormalisation loop that under- or over-shifts still returns plausible bits for a while and then diverges; asserting the invariant after every call catches it at the first bit rather than the thousandth. THE START CODE IS NOT OPTIONAL. A keyframe carries 9d 01 2a after the 3-byte tag. Skipping the check means a corrupt or non-VP8 payload is decoded as if it were a frame, with dimensions read out of arbitrary bytes. genealogy_id: vp8_rfc6386 lineage_id: nx_vp8_v1 license_tier: ORIGINAL