Pretty much the exact same issue was described here several years ago, but the 'solution' given, while it works, is more inconvenient and doesn't explain why the actual command in question does not work.
I'm using the following command to mix the audio of input.mp4 with song.m4a and ensure that the dimensions of my output file are even numbers:
ffmpeg -i input.mp4 -i song.m4a -filter_complex \
[0:a][1:a]amix[s0];[0:v]pad=height=ceil(ih/2)*2:width=ceil(iw/2)*2[s1] \
-map [s0] -map [s1] -shortest output.mp4
input.mp4 is only about 20 seconds long, but song.m4a is about 8 minutes long. I want to mix both the audio streams together, so that the first 20 seconds of song.m4a plays alongside the original audio of input.m4a. To ensure that the output file does not become 8 minutes long, I'm using the -shortest option, which should make FFmpeg stop encoding once it hits the shortest audio/video stream. However, in the command above, -shortest seems to be not working entirely.
When I separate the audio and video filters into 2 separate -filter_complexs, it works as intended (doing this was also the solution given in the issue a linked at the top):
ffmpeg -i input.mp4 -i song.m4a -filter_complex \
[0:a][1:a]amix[s0] -filter_complex [0:v]pad=height=ceil(ih/2)*2:width=ceil(iw/2)*2[s1] \
-map [s0] -map [s1] -shortest output.mp4
Same with if I remove the video filters entirely:
ffmpeg -i input.mp4 -i song.m4a -filter_complex [0:a][1:a]amix[s0] -map [s0] -map 0:v -shortest output.mp4
Using the above 2 commands results in a 20-second long video as intended.
I guess my question is why is -shortest made redundant when combining both audio and video filters in a single -filter_complex argument? Is this a bug, or the intended behaviour? How can I get it to work as intended with just one -filter_complex?
I'm using a self-compiled FFmpeg build on Linux (using the most recent Git version as of writing this) but I can confirm I can also recreate the issue with the official Linux AND Windows builds as well, even when using different audio and video files too.
The issue can be easily recreated using these commands:
// Create video file
ffmpeg -f lavfi -i testsrc2 -f lavfi -i sine=2000 -t 5 -y input.mp4
// Create song file
ffmpeg -f lavfi -i sine=500 -t 10 -y song.m4a
// Combine both files (-shortest should work here but it does not)
ffmpeg -i input.mp4 -i song.m4a -lavfi [0:a][1:a]amix[s0];[0:v]null[s1] -map [s0] -map [s1] -shortest -y output.mp4
I've also opened a ticket on the FFmpeg Trac which give more useful information in it's replies.
