forked from unom/punktfunk
enricobuehler
5d0b269e58
Both vendored vectors carry three-byte Annex-B start codes throughout. The real host emits four-byte ones on 100% of access units in both codecs — 1514/1514 H.264 and 1133/1133 HEVC, measured off the M0 NVENC corpus through the capture hook's own .idx offsets. So every parity verdict this program has recorded was taken on a prefix form that never ships, and the one form that does ship was exercised by nothing. That gap is not hypothetical. Submitting four-byte start codes to vkCmdDecodeVideoKHR unchanged is exactly what made HEVC unplayable on every driver tested: drivers are validated on the three-byte form, and a fixed +3 + 2 skip into a four-byte-prefixed slice reads a nonsense pps_id — the 115 and 119 both NVIDIAs printed. H.264 was never safe here by structure, only by its vendored encoder's convention, which is why the cure lives in the shared ring layer and why this coverage is generic over both codecs. Each codec's parity body now takes its access units as a parameter and runs twice: once over the vector as it sits, once over the same vector rewritten to four-byte prefixes. Prefix width carries no information, so both runs must reproduce the same goldens — sharing one body is what makes that an equality rather than two assertions that can drift. The rewrite copies nalu.data[nalu.offset..], the same nal_size bytes the parser hands the planner, so trailing_zero_8bits are dropped exactly where the production parser drops them: the only difference between the two streams is the width of every prefix. Two CPU guards keep the new legs from passing vacuously, which is the failure mode they are most exposed to — a rewrite that quietly returned its input would make them trivially green and nothing on the fleet would notice. They assert the original really does carry three-byte prefixes, that the rewritten stream carries none, that the NAL count is preserved exactly, and that the planner still yields 250 pictures. Hardware: all four legs 250/250 bit-identical to libavcodec on two independent driver stacks — AMD VanGogh on RADV/Mesa 26.0-devel (the Steam Deck) and NVIDIA 610.43.03 on Linux. NVIDIA is the family that rejected the four-byte form outright, so it is the meaningful witness for this regression.