how to detect circle in image with openCV? - c

Question

I want to detect circles in the image.this is the code i tried for detecting number of circles in this image:
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <stdio.h>
/** #function main */
int main(int argc, char** argv)
{
//load image from directory
IplImage* gray = cvLoadImage("comb.png",0);
// IplImage* gray = cvCreateImage(cvGetSize(img), IPL_DEPTH_8U, 1);
CvMemStorage* storage = cvCreateMemStorage(0);
//covert to grayscale
cvCvtColor(gray, gray, CV_RGB2GRAY);
// 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(gray),IPL_DEPTH_8U,1);
//IplImage* rgbcanny = cvCreateImage(cvGetSize(gray),IPL_DEPTH_8U,3);
cvCanny(gray, canny, 10, 20, 3);
//detect circles
CvSeq* circles = cvHoughCircles(gray, storage, CV_HOUGH_GRADIENT, 1, gray->width/10);
if(circles->total>0)
printf("circles found: %d\n",circles->total);
else
printf("circles not found");
return 0;
}
but the following error occurs :-
OpenCV Error: Assertion failed (dst.data == dst0.data) in cvCvtColor, file /OpenCV/OpenCV-2.4.3/modules/imgproc/src/color.cpp, line 3811
terminate called after throwing an instance of 'cv::Exception'
what(): /OpenCV/OpenCV-2.4.3/modules/imgproc/src/color.cpp:3811: error: (-215) dst.data == dst0.data in function cvCvtColor
please tell me what is wrong with the program.i am using ubuntu linux.

Answer 1

You open gray as a gray image (cvLoadImage("comb.png",0), with the "0" meaning "grayscale"). Then you try to convert this gray frame from RGB to Gray, and of course this cannot work since your image is already gray.
Simply try to comment out the following line:
//covert to grayscale
cvCvtColor(gray, gray, CV_RGB2GRAY);
By the way, you should use
IplImage* gray = cvLoadImage("comb.png", CV_LOAD_IMAGE_GRAYSCALE);
instead of
IplImage* gray = cvLoadImage("comb.png", 0);
That is clearer.