Read HSV value of pixel in opencv

Question

how would you go about reading the pixel value in HSV format rather than RGB? The code below reads the pixel value of the circles' centers in RGB format. Is there much difference when it comes to reading value in HSV?

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]);
         //uchar* ptr;
         //ptr = cvPtr2D(img, center.y, center.x, NULL);
         //printf("B: %d G: %d R: %d\n", ptr[0],ptr[1],ptr[2]);
         CvScalar s;

         s = cvGet2D(img,center.y, center.x);//colour of circle
        printf("B: %f G: %f R: %f\n",s.val[0],s.val[1],s.val[2]);

         // 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", img);
    //cvDestroyWindow("SnookerImage");
    //cvDestroyWindow("circles");
    //cvReleaseMemStorage("storage");
    cvWaitKey(0);

    return 0;
}

Answer 1

If you use the C++ interface, you can use

cv::cvtColor(img, img, CV_BGR2HSV);

See the documentation for cvtColor for more information.

Update:

Reading and writing pixels the slow way (assuming that the HSV values are stored as a cv::Vec3b (doc))

cv::Vec3b pixel = image.at<cv::Vec3b>(0,0); // read pixel (0,0) (make copy)
pixel[0] = 0; // H
pixel[1] = 0; // S
pixel[2] = 0; // V
image.at<cv::Vec3b>(0,0) = pixel; // write pixel (0,0) (copy pixel back to image)

Using the image.at<...>(x, y) (doc, scroll down a lot) notation is quite slow, if you want to manipulate every pixel. There is an article in the documentation on how to access the pixels faster. You can apply the iterator method also like this:

cv::MatIterator_<cv::Vec3b> it = image.begin<cv::Vec3b>(),
                    it_end = image.end<cv::Vec3b>();
for(; it != it_end; ++it)
{
    // work with pixel in here, e.g.:
    cv::Vec3b& pixel = *it; // reference to pixel in image
    pixel[0] = 0; // changes pixel in image
}