Combining videos with ffmpeg using crossfades and plain cuts

I am writing a script to combine/slice arbitrary video files from S3 into one output video. So far, I am doing this by first trimming the videos to their proper length using ffmpeg -i input-X.mp4 -ss start -t duration slice-X.mp4 and recombining the resultant slices with the ffmpeg concat filter.

I want to be able to crossfade and cut between videos. concat does not support transitions. What is the best way to combine videos with crossfades and cuts on the Linux command line? Is ffmpeg the best tool for the job?

My question is similar to “How do you create a crossfade transition between multiple videos in FFMPEG?” but I do not necessarily need to use ffmpeg. Also, I want to be able to fade in between some slices and cut between others.

Answer

Below is the single step command template, assuming five slices.

ffmpeg -i input.mp4 -i input.mp4 -i input.mp4 -i input.mp4 -i input.mp4 
-filter_complex 
"[0:v]trim=0.5:4.5,setpts=PTS-STARTPTS[1]; 
 [1:v]trim=12:17,setpts=PTS-STARTPTS+(3/TB),format=yuva420p,fade=in:st=3:d=1:alpha=1[2]; 
 [2:v]trim=34.1:36,setpts=PTS-STARTPTS+(7/TB),format=yuva420p,fade=in:st=7:d=1:alpha=1[3]; 
 [3:v]trim=21:25,setpts=PTS-STARTPTS[4]; 
 [4:v]trim=27:31,setpts=PTS-STARTPTS+(3/TB),format=yuva420p,fade=in:st=0:d=1:alpha=1[5]; 
 [0:a]atrim=0.5:4.5,asetpts=PTS-STARTPTS[1a]; 
 [1:a]atrim=12:17,asetpts=PTS-STARTPTS[2a]; 
 [2:a]atrim=34.1:36,asetpts=PTS-STARTPTS[3a]; 
 [3:a]atrim=21:25,asetpts=PTS-STARTPTS[4a]; 
 [4:a]atrim=27:31,asetpts=PTS-STARTPTS[5a]; 
 [1][2]overlay,format=yuv420p[12]; 
 [12][3]overlay,format=yuv420p[123]; 
 [4][5]overlay,format=yuv420p[45]; 
 [1a][2a]acrossfade=d=1[12a]; 
 [12a][3a]acrossfade=d=1[123a]; 
 [4a][5a]acrossfade=d=1[45a]; 
 [123][123a][45][45a]concat=n=2:v=1:a=1[v][a]" 
-map [v] -map [a] SingleStepOutput.mp4

I have inputted the video multiple times, once for each slice, because using a single input pad (even with split/asplit) leads to buffer overflows.

The setpts/asetpts filters are used because trim/atrim carry over the original timestamps. The setpts filters are offset for the slices which have to fade in. The offset value is the preceding slide duration - crossfade duration. The yuva420p is needed to create an alpha channel whose value is actually modulated by the fade filter.