Merge two videos with transparency in ffmpeg

Question

I have two videos merged into one file using below command. Unfortunately second video covers first video and it is not visible. How to make second video transparent (eg. 50%)?

ffmpeg
    -i in1.mp4 -i in2.mp4
    -filter_complex "nullsrc=size=480x360 [base];
        [0:v] setpts=PTS-STARTPTS, scale=480x360 [top];
        [1:v] setpts=PTS-STARTPTS, scale=480x360 [bottom];
        [base][top] overlay=shortest=1 [temp];
        [temp][bottom] overlay=shortest=1"
    -acodec libvo_aacenc -vcodec libx264 out.pm4

Solution

Use

ffmpeg \
    -i in1.mp4 -i in2.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

Set aa to the opacity value needed.

Comments

[Titan]

This almost works for me except the main video (in1.mp4) stops showing video after in2.mp4 has finished, however the audio from in1.mp4 does continue for the duration of the video, just the video is "paused"

[Titan]

can you also explain the top and bottom my overlay sits left aligned and I'd like it centered, even if I have to specify the number of pixels it needs to move over

[Janis S.]

@Mulvya this works good, any way to repeat the shortest video?

[Gyan]

If you know which input is shortest, its filterchain should be scale=480x360,loop=-1:99999:0,setpts=PTS-STARTPTS. where 99999 is a number equal or greater than its frame count.

[Tomilov Anatoliy]

How to mix sound in addition to video? With above command audio channel from input #0 is used.