Simple object detection using OpenCV and machine learning

Well, basically you need to detect circles. Have you seen cvHoughCircles()? Are you allowed to use it?

This page has good info on how detecting stuff with OpenCV. You might be more interested on section 2.5.

This is a small demo I just wrote to detect coins in this picture. Hopefully you can use some part of the code to your advantage.

Input:

Outputs:

// compiled with: g++ circles.cpp -o circles `pkg-config --cflags --libs opencv`
#include <stdio.h>
#include <cv.h>
#include <highgui.h>
#include <math.h>

int main(int argc, char** argv)
{
    IplImage* img = NULL;

    if ((img = cvLoadImage(argv[1]))== 0)
    {
        printf("cvLoadImage failed\n");
    }

    IplImage* gray = cvCreateImage(cvGetSize(img), IPL_DEPTH_8U, 1);
    CvMemStorage* storage = cvCreateMemStorage(0);

    cvCvtColor(img, gray, CV_BGR2GRAY);

    // This is done so as to prevent a lot of false circles from being detected
    cvSmooth(gray, gray, CV_GAUSSIAN, 7, 7);

    IplImage* canny = cvCreateImage(cvGetSize(img),IPL_DEPTH_8U,1);
    IplImage* rgbcanny = cvCreateImage(cvGetSize(img),IPL_DEPTH_8U,3);
    cvCanny(gray, canny, 50, 100, 3);

    CvSeq* circles = cvHoughCircles(gray, storage, CV_HOUGH_GRADIENT, 1, gray->height/3, 250, 100);
    cvCvtColor(canny, rgbcanny, CV_GRAY2BGR);

    for (size_t i = 0; i < circles->total; i++)
    {
         // round the floats to an int
         float* p = (float*)cvGetSeqElem(circles, i);
         cv::Point center(cvRound(p[0]), cvRound(p[1]));
         int radius = cvRound(p[2]);

         // draw the circle center
         cvCircle(rgbcanny, center, 3, CV_RGB(0,255,0), -1, 8, 0 );

         // draw the circle outline
         cvCircle(rgbcanny, center, radius+1, CV_RGB(0,0,255), 2, 8, 0 );

         printf("x: %d y: %d r: %d\n",center.x,center.y, radius);
    }


    cvNamedWindow("circles", 1);
    cvShowImage("circles", rgbcanny);

    cvSaveImage("out.png", rgbcanny);
    cvWaitKey(0);

    return 0;
}

The detection of the circles depend a lot on the parameters of cvHoughCircles(). Note that in this demo I used Canny as well.