Why trimming in ffmpeg eats up so much memory?

Question

Basically, I want to take two rectangles ouf of sixth minute of the video and overlay them at an earlier point in the video.

If I use ffmpeg three times like this:

ffmpeg -i a.mp4 -filter_complex "[0:v]trim=start_frame=9000:end_frame=9001,crop=1280:100:0:0,null,setpts=PTS-STARTPTS[v]" -map [v] -c:v libx264 -c:a null -crf 15 -preset slow -y -t 0.1 still1.mp4
ffmpeg -i a.mp4 -filter_complex "[0:v]trim=start_frame=9000:end_frame=9001,crop=100:100:600:500,null,setpts=PTS-STARTPTS[v]" -map [v] -c:v libx264 -c:a null -crf 15 -preset slow -y -t 0.1 still2.mp4
ffmpeg -i a.mp4 -i still1.mp4 -i still2.mp4 -filter_complex \
"[0:v][1:v]overlay=0:0:enable='between(t,60,61)'[v0]; \
[v0][2:v]overlay=600:500:enable='between(t,60,61)'[v1]" \
-map [v1] -c:v libx264 -crf 17 -preset slow -y -t 362 p.mp4

The consumption of RAM as measured by top stays around 3 GB. However, when I try it in one command, it quickly eats up 13GB of my free memory and the process quickly gets killed by OOM killer:

ffmpeg -i a.mp4 -filter_complex \
"[0:v]trim=start_frame=9000:end_frame=9001,crop=1280:100:0:0,setpts=PTS-STARTPTS[still1]; \
[0:v]trim=start_frame=9000:end_frame=9001,crop=100:100:600:500,setpts=PTS-STARTPTS[still2]; \
[0:v][still1]overlay=0:0:enable='between(t,61,62)'[v0]; \
[v0][still2]overlay=600:500:enable='between(t,61,62)'[v1]" \
-map [v1] -c:v libx264 -crf 17 -preset slow -y  p.mp4

I suspect ffmpeg keeps all the frames decoded in memory when trimming, but is there any way around this? It sounds wastefull, I am interested in two little frames, that should not eat up GB of memory!

Answer 1

Use better command: ffmpeg -i a.mp4 -i a.mp4 -i a.mp4 -filter_complex \ "[0:v]trim=start_frame=9000:end_frame=9001,crop=1280:100:0:0,setpts=PTS-STARTPTS[still1]; \ [1:v]trim=start_frame=9000:end_frame=9001,crop=100:100:600:500,setpts=PTS-STARTPTS[still2]; \ [2:v][still1]overlay=0:0:enable='between(t,61,62)'[v0]; \ [v0][still2]overlay=600:500:enable='between(t,61,62)'[v1]" \ -map [v1] -c:v libx264 -crf 17 -preset slow -y p.mp4

This way ffmpeg will not keep frames it still needs in memory.