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Modify image speed
The projection or viewing speed of a video should be changed from 25 fps as digitised to 18 fps as shot.
ffmpeg \ -i input_file \ -filter:v "setpts=25/18*PTS" \ -an \ output_file
ffmpeg \ -i input_file \ -filter:v "setpts=input_fps/output_fps*PTS" \ -an \ output_file
During film digitisation the speed of sound video is usually used, witch is 25 fps (frames per second) in «Europe». Yet silent and amateur films haven often been shot at a lower frame rate, like 16 or 18 fps.
The video filter is specified by