How to change frame rate of MJPEG segments with/without re-encoding?

Question

I'm trying to record segments using a webcam and save these segments with a lower fps. I've tried this command, but it keeps giving me an error:

ffmpeg -f v4l2 -input_format mjpeg -video_size 800x600 -i /dev/video0 -c:v copy -c:a copy -f segment -segment_time 10 -reset_timestamps 1 -r 10 -strict -2 test_%04d.avi

The errors I get is:

[segment @ 0x15bf0e0] Non-monotonous DTS in output stream 0:0; previous: 86, current: 73; changing to 87. This may result in incorrect timestamps in the output file.
[segment @ 0x15bf0e0] Non-monotonous DTS in output stream 0:0; previous: 87, current: 74; changing to 88. This may result in incorrect timestamps in the output file.
[segment @ 0x15bf0e0] Non-monotonous DTS in output stream 0:0; previous: 88, current: 75; changing to 89. This may result in incorrect timestamps in the output file.

Anyone know how to do it right?

Answer 1

It's a warning, not an error (if it would be an error, FFmpeg would abort).

“Non-monotonous DTS” means that the DTS (Display Time Stamp) of some frame is less than the DTS of the previous frame.

This situation may cause many problems (with playback, seeking, etc.), so FFmpeg fixes timestamps by rewriting some of them.

Here are your 3 warning messages:

[...] Non-monotonous DTS ... ; previous: 86, current: 73; changing to 87. This may ...
[...] Non-monotonous DTS ... ; previous: 87, current: 74; changing to 88. This may ...
[...] Non-monotonous DTS ... ; previous: 88, current: 75; changing to 89. This may ...

in a graphical form: