[FFmpeg-user] encoding video from png diffs

[Joshua Grauman]

Hello everyone,

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.

Josh

> The compression level is great. I thought of this because I am doing a 
> screencast, so most screens are identical or very similar. So if I have an 
> initial .tga file with size ~3 megabytes, most of the diffs will be less than 
> 1k in size (some will be more like 15k, and some quite a bit more, but those 
> should just be single frames). 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. 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 :(.
>
> Josh
>
>>  On Thu, Sep 01, 2016 at 10:31:15 -0700, Joshua Grauman wrote:
>> >  So I know ffmpeg can encode video from a list of pngs. (img1.png, 
>> >  img2.png, img3.png, etc). But what if I have one png, and then a list of 
>> >  diffs (using vcdiff) like (img1.png, img2.diff, img3.diff, etc.).
>>
>>  So you basically created a lossless codec of your own. ;-) How good is
>>  the compression level for your use cases?
>> 
>> >  I could of course generate all the pngs, but it would take up a lot
>> >  of space and I'm wondering if anyone knows of any creative way to do
>> >  this? Thanks!
>>
>>  ffmpeg can handle image pipes. So what you would do is to pipe the
>>  first PNG to ffmpeg. Then create the seconds frame from img1.png and
>>  img2.diff, and pipe that to ffmpeg. Reuse that second frame to create
>>  the third one from img3.diff, and pipe that to ffmpeg. You can then
>>  always "reuse" or drop the previous result, and always have no more
>>  than two frames to keep for the moment.
>>
>>  It would be something like
>>  $ create_png.sh | ffmpeg -f png_pipe -i - ...
>>  (Untested. create_png.sh could be a complex command line instead of a
>>  script.)
>>
>>  I don't see any documentation right now, so just check
>>  $ ffmpeg -h demuxer=png_pipe
>>  I don't think you need most of the options, except the framerate
>>  perhaps.
>>
>>  Disclaimer: I haven't actually tested this. ;-)
>>
>>  Cheers,
>>  Moritz
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