[FFmpeg-trac] #5734(undetermined:new): Coherent support to VP8 with alpha channel

FFmpeg trac at avcodec.org
Mon Jul 25 15:04:30 EEST 2016

#5734: Coherent support to VP8 with alpha channel
             Reporter:  cdlvcdlv     |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
 Summary of the bug:

 You can retrieve alpha channel from any video format, except VP8/webm.

 % ffmpeg -i
 -c:v libvpx -pix_fmt yuva420p -metadata:s:v:0 alpha_mode="1"
 % ffmpeg -i
 -c:v qtrle -pix_fmt yuva420p -metadata:s:v:0 alpha_mode="1"

 ffmpeg version - all

 The problem is that ffmpeg and ffplay ignore the encoded alpha channel
 (that I think is encoded in the container webm, i.e. matroska).

 You can use any video with alpha (lagarith, qtrle, huffyuv...) and
 transcode from one to another, given that some of them can only be decoded
 but not encoded: you can transcode from lagarith to qtrle but no the other
 way round. In any case, the alpha channel is never lost unless you specify
 it to be discarded.

 But if you use VP8/webm, you will not be able to make further transcodings
 and retain the transparency. If you deleted the original, your alpha
 channel is there, but in a black hole. The only way I know to show it is
 there is using Chrome (I think Opera too, but I have not tested it). I
 suppose that how they do it is in the source code of Chromium.

 I have read it is difficult (see link below and its response). But, is it
 true what he says: "In a simple test, I passed AV_PIX_FMT_YUVA420P to
 avpicture_alloc() and was able to get the linesize[] and data[] filled
 correctly for a YUVA picture"?


 To date, there's no tool that can retrieve alpha channel in a webm video.


Ticket URL: <https://trac.ffmpeg.org/ticket/5734>
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