OpenCV slowing down WebCam capture

Question

I'm capturing frames from a Webcam using OpenCV in a C++ app both on my Windows machine as well as on a RaspberryPi (ARM, Debian Wheezy). The problem is the CPU usage. I only need to process frames like every 2 seconds - so no real time live view. But how to achieve that? Which one would you suggest?

Grab each frame, but process only some: This helps a bit. I get the most recent frames but this option has no significant impact on the CPU usage (less than 25%)
Grab/Process each frame but sleep: Good impact on CPU usage, but the frames that I get are old (5-10sec)
Create/Destroy VideoCapture in each cycle: After some cycles the application crashes - even though VideoCapture is cleaned up correctly.
Any other idea?

Thanks in advance

#include <opencv2/opencv.hpp>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <iostream>
#include <vector>
#include <unistd.h>
#include <stdio.h>

using namespace std;

int main(int argc, char *argv[])
{
    cv::VideoCapture cap(0); //0=default, -1=any camera, 1..99=your camera

    if(!cap.isOpened()) 
    {
        cout << "No camera detected" << endl;
        return 0;
    }

    // set resolution & frame rate (FPS)
    cap.set(CV_CAP_PROP_FRAME_WIDTH, 320);
    cap.set(CV_CAP_PROP_FRAME_HEIGHT,240);
    cap.set(CV_CAP_PROP_FPS, 5);

    int i = 0;
    cv::Mat frame;

    for(;;)
    {
        if (!cap.grab())
            continue;

        // Version 1: dismiss frames
        i++;
        if (i % 50 != 0)
            continue;

        if( !cap.retrieve(frame) || frame.empty() )
            continue;

        // ToDo: manipulate your frame (image processing)

        if(cv::waitKey(255) ==27) 
            break;  // stop on ESC key

        // Version 2: sleep
        //sleep(1);
    }

    return 0;
}

cyriel cyriel · Accepted Answer · 2013-06-10T23:32:15

Create/Destroy VideoCapture in each cycle: not yet tested

It may be a bit troublesome on Windows (and maybe on other operating systems too) - First frame grabbed after creating VideoCapture is usually black or gray. Second frame should be fine :)

Other ideas:
- modified idea nr 2 - after sleep grab 2 frames. First frame may be old, but second should be new. It's not tested and generally i'm not sure about that, but it's easy to check it.
- Eventually after sleep you may grab frames in while loop (without sleep) waiting till you grab the same frame twice (but it may be hard to achieve especially on RasberryPi).

OpenCV slowing down WebCam capture

1 Answers