[FFmpeg-user] Blending two inputs with custom expression

[Paul B Mahol]

On 5/11/17, SviMik <svimik at gmail.com> wrote:
> 2017-05-11 19:10 GMT+03:00 Gyan <gyandoshi at gmail.com>:
>>
>> Loop the image input i.e. `-loop 1 -i in.png`
>>
>> Unfortunately, there's a bug at present which prevents the use of
>> shortest=1 in the blend filter. (https://trac.ffmpeg.org/ticket/6292)
>>
>> So, you'll have to terminate using -t N where N is the duration of the
>> video, or if the video has an audio stream, by adding -shortest.
>
> Works. But the result was still wrong. Then I figured out what was wrong,
> and my idea with swapping layers was actually right. The final solution is
> to change the video format to rgba to fix the wrong result, and swap the
> blend layers to fix the drops, then we don't need that loop. I have also
> stored the premult png separately trying to optimize the speed:
>
> ffmpeg -i mask.png -filter_complex
> "geq=r='p(X,Y)*alpha(X,Y)/255':g='p(X,Y)*alpha(X,Y)/255':b='p(X,Y)*alpha(X,Y)/255':a='p(X,Y)'"
> mask_premult.png
>
> ffmpeg -i tmp.flv -i mask_premult.png -filter_complex
> "[0:v]format=rgba[rgbv];[rgbv][1:v]blend=all_expr=A-B:c3_expr=B,lutrgb=a=maxval-val,geq=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))'"
> tmp.mkv
>
> But the fps=1.0 makes me sad (comparing with fps=40 without filters when
> just encoding). Perhaps writing the filter in C is the right way if I want
> something usable in real-time application...

If this is about alpha premultiply, there is premultiply filter.