[FFmpeg-user] encoding video from png diffs

[Moritz Barsnick]

On Thu, Sep 01, 2016 at 22:29:01 -0700, Joshua Grauman wrote:
> > from my program that I'm screencasting, which I haven't found screencast 
> > software that can do. Unfortunately, I'm not sure that I'm going to be able 
> > to get vcdiff to run fast enough to create the diffs at 30 frames/sec :(.
> I have another different, but related question. What format would you 
> suggest for storing my screencast in? The resulting video is going to be 
> 1080p30 and it needs to have an alpha channel (the input png's will use 
> the alpha channel). The screencast is going to be overlaid at a later time 
> (with ffmpeg) onto another 1080p30 avchd video from my canon camera.

All along, I was going to suggest: Why not use an existing codec? These
can have various advantages, such as they have already been analyzed
and optimized for performance, they can go into containers, they may
have safety measures such as I-frames (picture recovery mid-stream),
and so on. There are dozens of lossless codecs, and as many comparisons
out there. Some of them do support alpha, probably with ffmpeg as well.
You can check with:
$ ffmpeg -h encoder=ffvhuff
for the "Supported pixel formats:" output.

You will probably even find articles about saving screencasts
losslessly, and which codec to choose. But you can also experiment for
yourself.

> > Plus, this allows me to save the alpha channel from my program that
> > I'm screencasting, which I haven't found screencast software that
> > can do.

I don't understand your specific requirements, but ffmpeg can also
screencast:
https://trac.ffmpeg.org/wiki/Capture/Desktop
(If your alpha channel reflects the opacity of your windows, I presume
ffmpeg can't do that. But what do I know.)

Cheers,
Moritz