[FFmpeg-user] Color Channel Expression

[Paul B Mahol]

On 7/31/17, Jonathan Baecker <jonbae77 at gmail.com> wrote:
>
>
> 2017-07-31 14:41 GMT+02:00 Paul B Mahol <onemda at gmail.com
> <mailto:onemda at gmail.com>>:
>
>     On 7/31/17, Jon bae <jonbae77 at gmail.com <mailto:jonbae77 at gmail.com>>
>     wrote:
>      > 2017-07-31 14:00 GMT+02:00 Paul B Mahol <onemda at gmail.com
>     <mailto:onemda at gmail.com>>:
>      >
>      >> On 7/31/17, Jon bae <jonbae77 at gmail.com
>     <mailto:jonbae77 at gmail.com>> wrote:
>      >> > 2017-07-31 10:30 GMT+02:00 Paul B Mahol <onemda at gmail.com
>     <mailto: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?
>
>     Try harder to explain your use case?
>
> Sorry, my english is not so good, but I will try it. In the attachment
> you found 3 images. Universum.png is my original lower third, it comes
> out from blackmagic fusion. Universum-comp.jpg show the composite of a
> background video with the lower third, this is the correct result. The
> Universium-ffmpeg is the version from ffmpeg, as you see ffmpeg handles
> the alpha channel different so the lower third is more dark.
>
> I can simulate the same effect, from ffmpeg, in my compositing program,
> when I load the lower third image with the option "Post-Multiply by
> Alpha". I guess this is what ffmpeg does in Background, when it load
> images with alpha channel.
> To get rid of this effect now in my composition program I have to divide
> the color channel from the lower third with its own alpha:
> red/alpha;green/alpha;blue/alpha. Now I can overlay the lower third and
> the result is correct.
>
> This division now I would need in ffmpeg. I though I can do it with your
> premultiply filter, but maybe not.

Perhaps you want overlay filter?