Bringing WebM Alpha support to GStreamer
summary
Retro alpha uses two VP8/VP9 tracks packed into WebM with a per-frame block addition flagged by an alpha mode tag so the second stream carries luma-as-alpha. GStreamer maps that to a boolean codec-alpha caps field and stores the extra buffer in gst_video_codec_alpha_meta, adds a zero‑copy alpha-combine element and vp9-alpha decode bin (plus V4L2 stateless wrappers and a nv12+alpha pixel format) so software and hardware decoding work; streams must match resolution and timing and hardware must decode two concurrent streams.
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chapters (17)
- 0:00Introduction
- 1:00Welcome
- 1:23Who is Nicolas
- 3:27What are tenures
- 5:22WebM Alpha
- 6:54WebM Container
- 10:01Prior considerations
- 14:22Block Addition
- 16:02CodecAlphaDemax
- 16:46AlphaCombine
- 18:52NV12 format
- 20:26Decoders
- 21:57Demo
- 26:31Webkit
- 30:54Launching a demo
- 32:21Hardware acceleration considerations
- 37:04Questions
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