I'm trying to convert a mov file (with alpha channel) to webm, vp8 with alpha, on windows.

ffmpeg -i C0016.mov -s 960x540 -vcodec vp8 -pix_fmt yuva420p -metadata:s:v:0 alpha_mode="1" c0016_alpha.webm​

gives me:

[NULL @ 00000204af221e80] Unable to find a suitable output format for 'c0016_alpha.webm​' c0016_alpha.webm​: Invalid argument

How can I fix it?

  • Share full log.
    – Gyan
    Commented Oct 20, 2020 at 4:23
  • May i know the reason of using the option -pix_fmt yuva420p ?
    – xer-rex
    Commented Oct 20, 2020 at 13:50

1 Answer 1


I took the sample from apple https://developer.apple.com/videos/play/wwdc2019/506/

My command line using on colab was

!ffmpeg -y -i https://devstreaming-cdn.apple.com/videos/wwdc/2019/506lqy7sprpfyo800/506/506_hd_hevc_video_with_alpha.mp4 -s 960x540 -vcodec vp8 -metadata:s:v:0 alpha_mode="1" c0016_alpha.webm

Hope it matches your scenario.

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