VP8 WebM showed black video: there was no VP8 decoder. New L0 crate
filmcraft-vp8, implemented from RFC 6386 sections 1-19 (tables extracted
from the RFC text, nothing from the attached reference decoder): every
coding tool, versions 0-3, hidden altref frames, size changes. Bit-exact
with libvpx on 62 libvpx VP8 test vectors, real-world FATE samples and
generated fixtures; never panics on corrupted input.
Wired into codecs for Matroska / WebM (V_VP8, including the alpha layer
of transparent WebM) and MP4 (vp08). VP8 colour is BT.601 (RFC 6386 9.2).
A hidden altref frame is a WebM block of its own with the time of the
frame that follows it: the VP8 time lookup now moves on to that shown
frame instead of repeating the previous one.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
A WebM VP9 track with AlphaMode 1 stores its alpha as a second, independently coded VP9
stream in each block's BlockAdditional (BlockAddID 1). The Matroska reader detected
AlphaMode (has_alpha was true) but never read the additions, so transparent areas
rendered black.
- matroska: BlockGroups' BlockAdditions are parsed; the BlockAdditional with ID 1 of an
unlaced block is kept as Sample::addition, read with MkvFile::read_sample_addition.
- codecs: a WebM VP9 track with AlphaMode and additions gets an AlphaDecoder
(webm_alpha.rs): it decodes each sample's alpha frame with a second software VP9
decoder and attaches its luma plane (full range, converted to the picture's depth)
as the frame's alpha. After a seek the alpha decoder resynchronises from the nearest
alpha key frame. Colour pictures are held until their alpha picture is out, since the
two decoders may return pictures after different calls. A missing or undecodable alpha
frame leaves that picture opaque; it never fails the colour decode.
Tests: vp9_webm_alpha_layer_matches_libvpx (frames out of order and forward, colour and
alpha exact against ffmpeg's libvpx decode; fails before: no alpha plane; passed 200 of
200 runs with four in parallel), set_alpha unit tests.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qq8BK1LmohzE1ga4NgAdWM
