shorten video by dropping frames

Question

I am trying to create a sample video that shows every half-second frame using ffmpeg.

For example, if a video is 24 fps, I would like to create a video show every 12, 24, 36, 48.. and so on frames and create video with those frames.

I found a way to do that by extracting image files and encoding the result to video.

ffmpeg -i input -vf select='not(mod(n\,12))' -vsync vfr image_%05d.jpg

and to create video,

ffmpeg -framerate 24 -i image_%05d.jpg -c:v libx264 -r 24 -pix_fmt yuv420p output.mpeg

Now, I want to know if there is a way to skip the step of saving image files and get re-encoded video in one process.

Answer 1

Use

ffmpeg -i input -vf select='not(mod(n\,12)),setpts=N/FRAME_RATE/TB' -r 24 output.mp4

The setpts makes the the selected frames have timestamps in continuous sequence as per the output frame rate.