[FFmpeg-user] Color Channel Expression

[Jon bae]

2017-07-31 22:16 GMT+02:00 Paul B Mahol <onemda at gmail.com>:

> 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?
>

No the overlay filter don't handle the alpha channel correct, that is what
I was trying to say.

You can try it by your own - get this 3 images and run this command:

ffmpeg -loop 1 -f image2 -i backgroud.png -i multiplied-alpha.png
-filter_complex overlay -pix_fmt rgb24 -f sdl "multiplied alpha"

ffmpeg -loop 1 -f image2 -i backgroud.png -i pre-divide-alpha.png
-filter_complex overlay -pix_fmt rgb24 -f sdl "divide alpha"

The second one overlay the image correct. The first one overlay the image
different, because the alpha channel is not the same.
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