Convert PNGs to webm video with transparency

Question

I would like to use avconv to convert a series of PNG images to a WebM video, preserving transparency.

I understand that the pixel format used in the output video must support transparency. So I tried:

$ avconv -framerate 25 -f image2 -i frames/%03d.png -pix_fmt yuva420p output.webm

Unfortunately, avconv complains:

Incompatible pixel format 'yuva420p' for codec 'libvpx-vp9', auto-selecting format 'yuv420p'

[avconv != ffmpeg](http://stackoverflow.com/q/9477115/5726027) — Gyan, Jan 24 '16 at 11:07

Gyan · Accepted Answer · 2016-09-20T11:11:07.920

22

With VP8:

ffmpeg -framerate 25 -f image2 -i frames/%03d.png -c:v libvpx -pix_fmt yuva420p output.webm

Edit: Now, with VP9

ffmpeg -framerate 25 -f image2 -i frames/%03d.png -c:v libvpx-vp9 -pix_fmt yuva420p output.webm

edited Sep 20 '16 at 11:11

answered Jan 24 '16 at 11:04

Gyan

Thanks for the insight. Can't get it to work though, if I run your command, avconv complains that `Specified pix_fmt is not supported`. I am using `avconv version 9.18-6:9.18-0ubuntu0.14.04.1, Copyright (c) 2000-2014` – sebastian Jan 25 '16 at 12:15
Switch to a recent ffmpeg build and try. – Gyan Jan 25 '16 at 12:17
Still no luck. I am using `avconv version 11.3-6:11.3-1~trusty` now and still get `Specified pix_fmt is not supported`. – sebastian Jan 25 '16 at 12:32
In your answer (updated now) you wrote `avconv` instead of `ffmpeg`. So I used `avconv`, too. Working with ffmpeg, thanks! – sebastian Jan 25 '16 at 14:13
I needed to use '-metadata:s:v:0 alpha_mode="1"' as mentioned [here](https://trac.ffmpeg.org/wiki/Encode/VP8) to get alpha to work for VP8. – Jason Pratt May 17 '17 at 13:16
Works here, without. Which version of ffmpeg and libvpx? – Gyan May 17 '17 at 13:52
3

For anyone frustrated on Ubuntu LTS: VP9+alpha doesn't work on ffmpeg 2.8.x, you should get ffmpeg 3.x. – Kagami Sascha Rosylight Sep 01 '17 at 05:14
Works for me, but produces an output with garbage quality. How do you set a decent quality or configure the bitrate? – matteo Jul 11 '18 at 09:20
Set the bitrate e.g. `-b:v 2M` for 2 mbps. – Gyan Jul 11 '18 at 10:07

cdlvcdlv · Answer 2 · 2016-08-23T23:50:42.440

14

Since 2016-07-13, it's possible to encode VP9/webm videos with alpha channel (VP9a).

You only need a copy of ffmpeg compiled after that date. By the way, all you need to write is:

ffmpeg -i frames/%03d.png output.webm

FFmpeg understands png format and will set a default framerate of 25 fps and a yuva420p pixel format to the output.

edited Aug 23 '16 at 23:50

answered Aug 23 '16 at 23:32

cdlvcdlv

3

I added `-b:v 800k` for an increased video bit rate to get a better image quality. The default wasn't good enough. – Alex Jan 28 '18 at 21:55
Doesn't work for me. ffmpeg 4.3.2, video doesn't transparent. – JavaRunner Mar 27 '21 at 03:44

score 0 · Answer 3 · answered Jul 02 '18 at 21:39

You can also convert a video wich allready contains an alpha channel to a webm video with transparency with this command:

ffmpeg -i myVideoWithAlphaChannel.mov -c:v libvpx -vf format=rgba myVideoWithAlphaChannel.mov.webm

The format of myVideoWithAlphaChannel.mov (generated with Blender3D) is:

With as Ouputs:

3 Answers3