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I'm trying to use FFMPEG to overlay one video on another. I'm using the following:

ffmpeg \
-i avi1.mp4 -i avi2.mp4 \
-filter_complex " \
    [0:v]setpts=PTS-STARTPTS, scale=480x360[top]; \
    [1:v]setpts=PTS-STARTPTS, scale=480x360, \
         format=yuva420p,colorchannelmixer=aa=0.5[bottom]; \
    [top][bottom]overlay=shortest=1" \
-acodec libvo_aacenc -vcodec libx264 out.mp4

The resulting video seem to have the colours of the original videos washed out.

Here is an image of the two videos before the merge: enter image description here

enter image description here

And here is an image of the output after merging them together:

enter image description here

I'm wondering why the resulting video has the colours altered in this way and how I can merge the two videos together without the colours being altered in this way.

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    The input videos do not have transparency/alpha. When you overlay one on top of another, the white background goes on top with 50% opacity and hides the bottom one 50%.
    – Rajib
    Oct 18, 2021 at 6:48
  • Can I overcome this by having the videos exported with transparency/alpha before layering them together?
    – Kurt
    Oct 18, 2021 at 14:57
  • Absolutely. If this is coming from a 3D application export the frames in 32 bits at least- that is 8 bits per RGB and 8 bits for alpha. Compositing will then be possible. Note that this is the minimum expected. You can increase bit depth etc.
    – Rajib
    Oct 18, 2021 at 15:08
  • I tried again using the same command as above after exporting the videos from Apple ProRes 4444. I can see the backgrounds of each video are transparent but the merged result looks the same as the image above.
    – Kurt
    Oct 18, 2021 at 16:12
  • The complete log is missing from the ffmpeg command, which is needed to answer, but I can see that your ffmpeg must be super old because you're using libvo_aacenc which was removed in 2016. See the FFmpeg Download page for links to compiled ffmpeg for Windows, Linux, and macOS. Upgrade before doing anything else.
    – llogan
    Oct 18, 2021 at 18:02

1 Answer 1

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If your movies indeed have alpha, you do not need to use colorchannelmixer if you want full transparency. Simply use:

ffmpeg -i movie_for_bottom.mov -i movie_for_top.mov -filter_complex "[0:v]setpts=PTS-STARTPTS, scale=480x360[bottom];[1:v]setpts=PTS-STARTPTS, scale=480x360[top];[bottom][top]overlay=shortest=1" -vcodec libx264 out.mp4

If alpha is present in the movie ffmpeg will use it in overlay filter.

The resultant movie will NOT have alpha(transparency).

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