[FFmpeg-user] Color Channel Expression

[Jon bae]

2017-07-31 14:00 GMT+02:00 Paul B Mahol <onemda at gmail.com>:

> On 7/31/17, Jon bae <jonbae77 at gmail.com> wrote:
> > 2017-07-31 10:30 GMT+02:00 Paul B Mahol <onemda at gmail.com>:
> >>
> >>>
> >>> What exactly you tried?
> >>>
> >>> Perhaps you want premultiply filter?
> >>>
> >>> Yes I have a video and a lower third, and I want to overlay the lower
> >> third. But my alpha channel from the lower third is not pre-devided with
> >> the alpha channel, so  I need to do a channel division in ffmpeg.
> >> Something like:
> >>
> >> ffmpeg -i video.mp4 -i lowerthird.png -filter_complex
> >> [1:v]geq=r=r/a:g=g/a:b=b/a[gq];[0:v][gq]overlay ... output.mp4
> >>
> >> I see that you wrote a filter for this, but can you please give me an
> > example of how it works? I don't get it to run.
>
> "I don't get it to run" means nothing to me.
>
> I'm not sure if I use the filter correct... But with this:

ffmpeg -i video.mp4 -i image.png -filter_complex
"[0:v]format=rgba[a];[1:v]format=rgba[b];[a][b]premultiply"

My result is, that everything what had alpha = 0, in my image, is now
black. Mybe I understand the use case wrong, but how you would use this
filter?