What's right way to change playback speed with ffmpeg?

Question

In my project i'm using ffmpeg to playback media. Currently I'm trying to implement changing of playback speed. Will it be right to drop certain packets at high rates, for example not keyframes ? Or I should rely only on changing timestamps and duration, even if performance is low(for example 4k video) and as a consequence increased speed isn't noticable ?

Answer 1

If your file contains standard PTS reference information, I think the best way to change back playback speed will be using the setpts filter.

For example, to speed up the video by x2 try:

ffplay [INPUT] -vf setpts=0.5*PTS

The filter works in FFmpeg as well.

Answer 2

ffplay [INPUT] -vf setpts=0.5*PTS will drop frames to achieve the desired speed. You can avoid dropped frames by specifying a higher output frame rate than the input.

To preserve all the frames and just increase the frame rate by 4 times and speed as consequence exec:

ffmpeg -i input.mkv -r NEW_FPS -filter:v "setpts=0.25*PTS" output.mkv

where NEW_FPS = old_fps * 4

• To check the frame rate: ffprobe video_name

• To check number of frames:

  ffprobe -v error -count_frames -select_streams v:0   -show_entries stream=nb_read_frames -of default=nokey=1:noprint_wrappers=1 video_name