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'

I am using ffmpeg version 2.8.4-1+b1 Copyright (c) 2000-2015 the FFmpeg developers.

Solution 1

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

Solution 2

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.

Comments

[sebastian]

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

[Gyan]

Switch to a recent ffmpeg build and try.

[sebastian]

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]

In your answer (updated now) you wrote avconv instead of ffmpeg. So I used avconv, too. Working with ffmpeg, thanks!

[Jason Pratt]

I needed to use '-metadata:s:v:0 alpha_mode="1"' as mentioned here to get alpha to work for VP8.

[Gyan]

Works here, without. Which version of ffmpeg and libvpx?

[Kagami Sascha Rosylight]

For anyone frustrated on Ubuntu LTS: VP9+alpha doesn't work on ffmpeg 2.8.x, you should get ffmpeg 3.x.

[Alex]

I added -b:v 800k for an increased video bit rate to get a better image quality. The default wasn't good enough.

[matteo]

Works for me, but produces an output with garbage quality. How do you set a decent quality or configure the bitrate?

[Gyan]

Set the bitrate e.g. -b:v 2M for 2 mbps.

[JavaRunner]

Doesn't work for me. ffmpeg 4.3.2, video doesn't transparent.