Color detection on HoughCircles using OpenCV

Question

I have detected 22 balls and am struggling to find a way to run a color detection algorithm on these circles to get their colors. I am using HoughCircles to detect the circles but don't know how to check what color these circles are? Source Code:

#include <stdio.h>
#include <cv.h>
#include <highgui.h>
#include <math.h>

int main(int argc, char** argv)
{
    //load image from directory
    IplImage* img = cvLoadImage("C:\\Users\\Nathan\\Desktop\\SnookerPic.png");


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

    //covert to grayscale
    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);

    //detect circles
    CvSeq* circles = cvHoughCircles(gray, storage, CV_HOUGH_GRADIENT, 1, 35.0, 75, 60,0,0);
    cvCvtColor(canny, rgbcanny, CV_GRAY2BGR);

    //draw all detected circles
    for (int 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]);
         cvScalar c = cvGet2D(center.x, center.y);//colour of circle

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

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

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

    //create window
    cvNamedWindow("circles", 1);
    cvNamedWindow("SnookerImage", 1);
    //show image in window
    cvShowImage("circles", rgbcanny);
    cvShowImage("SnookerImage", img);

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

    return 0;
}

Answer 1

If the balls each have a uniform color, you can check the color at the center:

CvMemStorage* storage = cvCreateMemStorage(0);
cvSmooth(image, image, CV_GAUSSIAN, 5, 5 );
CvSeq* results = cvHoughCircles(
image,
storage,
CV_HOUGH_GRADIENT,
2,
image->width/10
);
for( int i = 0; i < results->total; i++ ) 
{
float* p = (float*) cvGetSeqElem( results, i );
CvPoint center = cvPoint( cvRound( p[0] ), cvRound( p[1] ) );
CvScalar c = cvGet2D(image, center.x, center.y); //color of the center
}

Haven't tested the code but it should be ok.

EDIT:

Ooops, I forgot one parameter from the Get2D method, the actual image from which to get the color. Changed to the correct form.

Answer 2

We have written our own blob detection library in the open source vision framework: http://www.simplecv.org

The code to do what you want is as easy as:

img = Image("/path/to/image.png")
blobs = img.findBlobs()
circle_blobs = blobs.filter(blobs.isCircle() == True)
list_of_blobs_colors = circle_blobs.meanColor()