#11045: vp9 in mp4 does not maintain transparency (yuva420p)
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Reporter: Jozef Chutka | Type: defect
Status: new | Priority: normal
Component: undetermined | Version: 7.0
Keywords: | Blocked By:
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
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Using ffmpeg 7.0, I am able to create .mkv, .webm output with alpha
channel, however not when creating .mp4
creating transparent video vp9 in mp4 does not seem to maintain alpha
channel:
{{{
ffmpeg -i input.png -vcodec libvpx-vp9 -auto-alt-ref 0 -y out.mkv
ffmpeg -i input.png -vcodec libvpx-vp9 -auto-alt-ref 0 -y out.webm
ffmpeg -i input.png -vcodec libvpx-vp9 -auto-alt-ref 0 -y out.mp4
}}}
The output log looks similar for all commands
{{{
Output #0, mp4, to 'out.mp4':
Metadata:
encoder : Lavf61.1.100
Stream #0:0: Video: vp9 (vp09 / 0x39307076),
yuva420p(unknown/bt709/iec61966-2-1, progressive), 256x256 [SAR 1:1 DAR
1:1], q=2-31, 25 fps, 12800 tbn
Metadata:
encoder : Lavc61.3.100 libvpx-vp9
Side data:
cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: N/A
}}}
Notice, ffmpeg claims yuva420p is used, however, mp4 does not really store
yuva420p:
{{{
ffmpeg -vcodec libvpx-vp9 -i out.mkv
...
Stream #0:0: Video: vp9 (libvpx-vp9) (Profile 0), yuva420p(tv,
unknown/bt709/iec61966-2-1, progressive), 256x256, SAR 1:1 DAR 1:1, 25
fps, 25 tbr, 1k tbn
ffmpeg -vcodec libvpx-vp9 -i out.webm
...
Stream #0:0: Video: vp9 (libvpx-vp9) (Profile 0), yuva420p(tv,
unknown/bt709/iec61966-2-1, progressive), 256x256, SAR 1:1 DAR 1:1, 25
fps, 25 tbr, 1k tbn
ffmpeg -vcodec libvpx-vp9 -i out.mp4
...
Stream #0:0[0x1](und): Video: vp9 (libvpx-vp9) (Profile 0) (vp09 /
0x39307076), yuv420p(tv, unknown/bt709/iec61966-2-1, progressive),
256x256, 211 kb/s, SAR 1:1 DAR 1:1, 25 fps, 25 tbr, 12800 tbn (default)
}}}
When tested in a chrome browser which supports all these formats, only
.mkv and .webm is rendered with transparency, while .mp4 being opaque.
So the question is, does mp4 support vp9 with yuva420p or not? If not,
should ffmpeg throw an error during encoding?
--
Ticket URL: <https://trac.ffmpeg.org/ticket/11045>
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