Question

I build opencv with ffmpeg support and this is a snippet of my code to read a video and iterate through the frames.

if (argc != 3) {
    std::cout<<"Usage: exe input_video_filename skip_frames \n"<<std::endl;
    return 0;
}
char* video_fn = argv[1];
int skip_frames = atoi(argv[2]);
VideoCapture cap;
if(!cap.open(video_fn))
{
    std::cout<<"error reading video"<<video_fn<<std::endl;
    return -1;
}
Mat frame,gray;
int i = 0;
double score = 0;
vector<double> scores;
while(true)
{
    if(!**cap.retrieve(frame)**)
    {
        std::cout<<"error retrieve frame"<<std::endl;
        break;
    }
    if(i%skip_frames)
    {
        cvtColor(frame,gray,CV_BGR2GRAY);
        double sc = compute_cpbdm(gray);
        std::cout<<sc<<std::endl;
        scores.push_back(sc);
    }

}

The program succeed to read the video at line cap.open(video_fn) but fail to retrieve frame from VideoCapture at the line if(!cap.retrieve(frame)). I tried with mp4 and avi but it always fails.Does anyone encountered similar problem? How to solve it? Many thanks!

Answer 1

you’re using capture.retrieve(frame), then there should be a capture.grab() before that.

alternatively, you can do: capture.read(frame) , which does both at the same time,

or just capture >> frame; (same thing as read)