nx_av1_obu.nx

nx_av1_obu.nx -- AV1/AV2 OBU layer: reader and writer. The layer directly above nx_av1_ec.nx. Every AV1 and AV2 bitstream is a sequence of Open Bitstream Units, each a 1-2 byte header plus an optional LEB128 size. AVIF's still-picture path uses the same units. Written both directions so it proves by round-trip. LEB128 IS UNSIGNED AND LITTLE-ENDIAN BY GROUP. Seven payload bits per byte, low group first, high bit as the continuation flag. AV1 caps it at 8 bytes. The natural mistake is to stop at the first byte with the high bit clear WITHOUT bounding the count -- a hostile stream then walks the parser off the buffer one byte at a time. THE TWO RESERVED BITS ARE NOT DECORATION. obu_forbidden_bit must be 0 and obu_reserved_1bit must be 0. A parser that ignores them will happily consume arbitrary bytes as OBU headers and resynchronise onto garbage, which is exactly how a fuzzer turns a malformed file into a long walk through memory. Both are REFUSED here. UVLC IS NOT LEB128. Frame headers use an Exp-Golomb-style variable code read MSB-first off the bitstream: leading zeros, then that many value bits, biased by (1 << zeros) - 1. Confusing the two silently misreads every frame header field after the first. genealogy_id: av1_spec_5_3_obu lineage_id: nx_av1_obu_v1 license_tier: ORIGINAL