[SOLVED] Subsampling videos to save every Nth frame

Subsampling videos to save every Nth frame

I have thousands of 30sec/20fps/.avi videos (so 600 frames total per video). I need to automate subsampling these videos in order to save every 100th frame (every 5 seconds). Any picture format is fine.

Is there an easy way to do this in either Matlab (R2015b) or Python+libraries?

Solution

In MATLAB:

you can use VideoWriter object or imwrite, depends on the desired output format:

vin = VideoReader('vid1.mp4');
vout = VideoWriter('vid-out.mp4');
framenum = 0;
everyNframe = 100;
vout.open();
while vin.hasFrame
    frame = vin.readFrame;
    if rem(framenum,everyNframe) == 0
        vout.writeVideo(frame);
        % OR
        imwrite(frame, [num2str(framenum,'%04i') '.jpg']);
        disp(framenum)
    end
    framenum = framenum + 1;
end
vout.close();

another option the ffwd the input video to the next desired frame is by setting vin.CurrentTime, but for some reason it is slower than simply read 100 frames.