[FFmpeg-user] Color Channel Expression

[Jonathan Baecker]

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.


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