[FFmpeg-user] Blending two inputs with custom expression

[SviMik]

2017-05-11 17:38 GMT+03:00 SviMik <svimik at gmail.com>:
>
> 2017-05-10 8:07 GMT+03:00 Gyan <gyandoshi at gmail.com>:
> >
> > On Wed, May 10, 2017 at 5:41 AM, SviMik <svimik at gmail.com> wrote:
> >
> > >
> > > Great answer, thank you for the idea! I have tried to run it, and it
> > > produced me a black screen (all zeroes), but I think it may be my
fault
> > > somewhere.
> > >
> >
> > In the first geq filter, add a='p(X,Y)' after the b expression. Turns
out
> > the geq initializes alpha to 1 if no expression is provided. Ideally, it
> > ought to pass through.
>
> It works! Thank you so much!
>
> By the way, I got few pixels with >255 value, and it turns out the ffmpeg
can't handle it itself - the pixel with r=266 value became r=10. So I had
to add min() to the last geq to avoid overflow:
>
r='min(255,255*p(X,Y)/alpha(X,Y))':g='min(255,255*p(X,Y)/alpha(X,Y))':b='min(255,255*p(X,Y)/alpha(X,Y))'
>
> Now it works like a charm.

I was too fast. I thought that if ffmpeg filter works on an image, then it
should work on the video too. I was wrong. The blend filter makes ffmpeg to
simply drop frames:
frame=    2 fps=0.5 q=-1.0 Lsize=     223kB time=00:00:06.78 bitrate=
268.8kbits/s dup=0 drop=194

I think it tries to process the first frame then fails because the top
layer is png and the bottom is a video. I tried to switch top and bottom
and swap A/B in the expression. Now it doesn't drop the frames, but I
couldn't manage to get the same result on the output. Seems that layer
position is important here, and swapping the layers made the result of the
blending different.