Sampling video every N frames starting at n0 initial frame

Question

Question

I am trying to figure out how to sample a video stream x[n] every N frames, starting at frame n_i, i < N, so that I end up with N new videos of length len(x) / N.

In formula this is simply: y_i[n] = x[n_i + n * N].

Here there is a diagram of what I am trying to achieve:

The greedy solution would be simply dumping the frames to a folder and then create new videos out of appropriately indexed frames. I was hoping there were some more elegant solution with ffmpeg since I have to process hundreds of video.

Implementation

Finally, I managed to write the final implementation, which I am reporting here for completeness.
It does scale the minimum dimension to 256, does not process more than max_frames; performs the sampling every k frames, send the first k - 1 samples to one folder and the k-th one to another one. It also set the output frame rate to the input average frame rate, since otherwise some videos will go at 120 Hz...

k=5
kk=$(awk "BEGIN{print 1/$k}")
ffmpeg \
    -i $src_video_path \
    -an \
    -loglevel error \
    -filter_complex \
        "setpts=$kk*PTS, \
        scale=w=2*trunc(128*max(1\, iw/ih)):h=2*trunc(128*max(1\, ih/iw))[m]; \
        [m]select=n=$k:e=(mod(n\,$k)+1)*lt(n\,$max_frames) \
        $(for ((i=1; i<=$k; i++)); do
            echo -n "[a$i]"
        done)" \
    $(for ((i=1; i<$k; i++)); do
        echo -n "-r $fps -map [a$i] $dst_video_path/$i.mp4 "
    done
    echo -n "-r $fps -map [a$k] $val_video_path/$k.mp4"
    )

Answer 1

One-by-one solution

You can use ffmpeg's select filter, which allows you to evaluate an expression using the sequential frame number, and decide whether or not to skip it (and which output to send it to if it isn't skipped).

To generate, for example, the second video where you are splitting into four each time, you would do the following:

ffmpeg -i input_video -vf select='not(mod(n-1\,4)), setpts=0.25*PTS' -an output_video

Change the 4 if you are splitting into a different number, and the 1 if you want a different selection (so for four you would run it four times, starting without subtracting anything, and ending with subtracting three.

The -vf ... -an segment is to get the video to play back at the right speed, and have the right length (which requires removing any audio tracks).

All-at-once solution

The select filter can also split the file and save each output separately. Here is a command that will split the file into k outputs, (change .mp4 if you want a different extension).

k=4;ffmpeg -i INPUT_FILE -an -filter_complex "setpts=`bc -l <<< 1/$k`*PTS[m];[m]select=n=$k:e=mod(n\,$k)+1`for ((i=1; i<=$k; i++)); do echo -n "[a$i]"; done`" `for ((i=1; i<=$k; i++)); do echo -n "-map [a$i] out$i.mp4 "; done`

The loops within the command are just to write out [a1][a2]... and -map [a1] out1.mp4.... Other than that the modification is to use filter_complex, which is required when dealing with multiple outputs, and changing the filter to direct each frame to its own output mod k.