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you have to call cap.read() once for each image you want to read from the video, not only once.

also, you probably should not rely on values from CAP_PROP_POS_FRAMES or CAP_PROP_FRAME_COUNT, those might not be correct (or even supported), depending on the codec. then, while(frame_number <= video_length) is one too many. so:

int frame_number=0;
if (cap.isOpened())
{
    while(cap.read(frame)) //the last frame of the movie will be invalid. check for it !
    {
        Imgcodecs.imwrite(output + "/" + frame_number +".jpg", frame);
        frame_number++;
    }
    cap.release();
}