OpenCV exercises

Motion detection can also be achieved through the use of histograms. In this example, the average intensities of consecutive histograms are compared in order to determine if there was some movement in the camera stream.

This program can be run by ./motiondetector <threshold>, where <threshold> is the relative difference to consider a moviment (value from 0 to 100). A delay of 1 second is put between each frame, because in case a too short interval was used, the difference between frames would be too little and the algorithm would not detect.

In a surveilance system, the ideal I think would be to compare frames with a background frame (empty scene) in order to determine suspecius activity with a high frame rate.

#include <iostream>
#include <cstdlib>
#include <opencv2/opencv.hpp>

using namespace cv;
using namespace std;

int main(int argc, char** argv){
    if (argc != 2) {
        cout << "usage: " << argv[0] << " <thresh_motion>" << endl
             << "Where thresh_motion is a threshold for "
             << "relative difference of histograms between 0 and 100." << endl;
        exit(1);
    }

    float thresh_motion = atof(argv[1]);

    Mat image, grey;
    int width, height;
    VideoCapture cap;

    int histSize = 256;
    float range[] = {0, 256};
    const float* histRange = {range};
    Mat hist;

    float avg = -1, prev_avg = -1;

    cap.open(0);

    if (!cap.isOpened()){
        cout << "No cameras available" << endl;
        exit(1);
    }

    width  = cap.get(CV_CAP_PROP_FRAME_WIDTH);
    height = cap.get(CV_CAP_PROP_FRAME_HEIGHT);
    
    cout << "###### Image dimensions ######" << endl    
         << "Width = " << width << endl
         << "Height  = " << height << endl
         << "##############################" << endl;

    namedWindow("grey", WINDOW_NORMAL);

    while (1) {
        cap >> image;
        if(image.empty()) exit(1);

        cvtColor(image, grey, CV_BGR2GRAY);

        calcHist(&grey, 1, 0, Mat(), hist, 1, &histSize, &histRange);

        avg = 0;
        for (int i = 0; i < 256; ++i) {
            avg += i * hist.at<float>(i) / ( (float)(width*height) );
        }

        if (prev_avg > 0 && // First iteration and not zero for division
            abs(avg-prev_avg)/prev_avg > thresh_motion/100.0 ) {

            cout << "Detected movement!!" << endl;
        }

        prev_avg = avg;
    
        imshow("grey", grey);
        
        if(waitKey(1000) >= 0) break;
    }
    exit(0);
}