OpenCv out of range error - opencv

Question

I am detecting circles like in the image. i am trying to read the pixel value of the centres of these circels to get the color of them. I am using cvPtr2D(img, center.x, center.y, NULL);to read the value of each circle in a for loop but i am getting an error the " one of arguments' value is out of range(index is out of range) in inknown function, line 179(which is the line of cvPtr2D(). i dont understand how these values sare out of range. anyone able to help? il post my 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]);
uchar* ptr;
ptr = cvPtr2D(img, center.x, center.y, NULL);
printf("B: %d G: %d R: %d\n", ptr[0],ptr[1],ptr[2]);
//CvScalar c;
//if(center.x > 0 && center.x < 1280 && center.y > 0 && center.y < 720)
//{
//c = cvGet2D(img,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", img);
//cvDestroyWindow("SnookerImage");
//cvDestroyWindow("circles");
//cvReleaseMemStorage("storage");
cvWaitKey(0);
return 0;
}

Answer 1

You're problem is that the line:
ptr = cvPtr2D(img, center.x, center.y, NULL);
Should be:
ptr = cvPtr2D(img, center.y, center.x, NULL);
Unfortunately, the documentation is not very clear on this. My intuition told me that this was what was happening, but I had to actually grep the OpenCV source code to see how it was used to get a confirmation on this.
You example looks to be working now.