Suppose I have a video foo.mkv and an image bar.png (which happens to contains an alpha channel). I can blend this image over the video like this:
ffmpeg
-i foo.mkv
-i bar.png
-filter_complex "[0:v][1:v]overlay"
-vcodec libx264
myresult.mkv
(using multiple lines here for readability, normally this is one command line).
Now, besides the png image having an alpha channel of its own, I would also apply a custom overall transparency when blending this image over the video.
In the above example, the image would be visible 100% on top of the video — or at least the parts where its alpha channel is fully opaque.
Is there a way to add a custom overall opacity or transparency blend factor, something like opacity=0.5 or whatever, which would make the image only 50% visible?
Another option besides geq is colorchannelmixer.
[1:v]format=argb,colorchannelmixer=aa=0.5[zork]
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