[FFmpeg-user] Color Channel Expression

[Paul B Mahol]

On 7/31/17, Jon bae <jonbae77 at gmail.com> wrote:
> 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?

Try harder to explain your use case?

Like providing input and wanted output images and explaining in more detail how
alpha should be manipulated.