[FFmpeg-user] Blending two inputs with custom expression

[Gyan]

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.