Transparent WebM Turned Opaque in Pipeline With No Error, Developer Finds

A developer found that an 8-second VP9 WebM with an alpha channel, exported from a local browser-based video background removal tool, became fully opaque during conversion without any step reporting an error. Using FFmpeg 7.1, they found that the default VP9 decoder read the alpha plane as entirely opaque, while the libvpx-vp9 decoder showed roughly 79% of frame 60's pixels as transparent. File sizes differed between decoders, but size alone was not treated as proof of correctness. To locate the faulty stage, they built three checks: a container probe that counts alpha data rather than trusting the alpha_mode tag, a decode probe that pins the decoder and reads a fixed frame, and an output probe run after every conversion. The article reports these checks and their results on the test files, but the testing was limited to one clip and one FFmpeg build.