vpcc: decode chroma_subsampling as 3 bits, not 1#168
Conversation
chromaSubsampling in the VPCodecConfigurationRecord is a 3-bit field (0/1 = 4:2:0, 2 = 4:2:2, 3 = 4:4:4) but decode masked it with 0x01, so 4:2:2 parsed as 0 and 4:4:4 as 1. The encode side already writes all 3 bits, so a decode/encode round-trip silently rewrote a 4:2:2/4:4:4 vpcC as 4:2:0. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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2 participants
Problem
In the
vpcC(VPCodecConfigurationRecord) decoder, the byte holdingbitDepth (4) | chromaSubsampling (3) | videoFullRangeFlag (1)is decoded with(b >> 1) & 0x01forchromaSubsampling— masking only 1 of the 3 bits. Per the VP9 Codec ISO Media File Format Binding,chromaSubsamplingis 3 bits with defined values 0–3 (4:2:0 vertical / 4:2:0 colocated / 4:2:2 / 4:4:4).The corruption is value-only and asymmetric: the encoder writes all 3 bits correctly, so a decode→encode round trip through this crate silently rewrites 4:2:2 (2) and 4:4:4 (3) streams as 4:2:0 variants (0/1).
Change
One-line decode fix:
& 0x01→& 0x07.Tests
Round-trip tests for
chroma_subsampling2 and 3 (both fail against the old mask), plus a decode of raw record bytes asserting the value survives.cargo test --all-features, clippy-D warnings, andfmt --checkare green.🤖 Generated with Claude Code