[Libav-user] Difficulty decoding libvpx VP8 with Alpha as RGBA

Craig Turner turner.craig at gmail.com
Mon Feb 1 01:08:58 EET 2021

You are absolutely right - the format is AV_PIX_FMT_YUV420P, so it seems
the alpha channel is not being decoded. Is there something specific I need
to do to ensure it does indeed get decoded?

The codec is indeed the "libvpx VP8" and the (presumably well-known) file I
am using is dancer1.webm, which has alpha_mode = 1:
    Input #0, matroska,webm, from '.\dancer1.webm':
        encoder         : libwebm-
      Duration: 00:01:02.83, start: 0.000000, bitrate: 520 kb/s
        Stream #0:0(eng): Video: vp8, yuv420p(progressive), 640x360, SAR
1:1 DAR 16:9, 30 fps, 30 tbr, 1k tbn, 1k tbc (default)
      alpha_mode      : 1


On Sun, Jan 31, 2021 at 11:12 PM Carl Eugen Hoyos <ceffmpeg at gmail.com>

> Am So., 31. Jan. 2021 um 22:43 Uhr schrieb Craig Turner
> <turner.craig at gmail.com>:
> > I'm trying to decode and convert a VP8 video with an alpha channel to a
> 32-bit
> > RGBA for rendering in my application. The decoding with libvpx works
> fine, but
> > I can't get the alpha channel through into the RGBA. Am I doing something
> > wrong with libswscale?
> Did you check the value of _codecCtx->pix_fmt ?
> If it is not yuva420p, then the alpha channel was not decoded.
> Carl Eugen
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