2017-07-31 14:41 GMT+02:00 Paul B Mahol <one...@gmail.com
<mailto:one...@gmail.com>>:
On 7/31/17, Jon bae <jonba...@gmail.com <mailto:jonba...@gmail.com>>
wrote:
> 2017-07-31 14:00 GMT+02:00 Paul B Mahol <one...@gmail.com
<mailto:one...@gmail.com>>:
>
>> On 7/31/17, Jon bae <jonba...@gmail.com
<mailto:jonba...@gmail.com>> wrote:
>> > 2017-07-31 10:30 GMT+02:00 Paul B Mahol <one...@gmail.com
<mailto:one...@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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