display a video from my webcam trying to implement circle detection - opencv

Question

I'm trying to display a video from my webcam (which was working grand) and now I'm trying to implement circle detection into this video stream.
Unhandled exception at 0x001a1a4d in test.exe: 0xC0000005: Access violation reading location 0x00000004.)
The error is linked to the line of code:
cvCircle( img, cvPoint(cvRound(p[0]),cvRound(p[1])), 3, CV_RGB(255,0,0), -1, 8, 0 );.
Can anyone help please?
#include <stdio.h>
#include "cv.h"
#include "highgui.h"
#include <iostream>
#include <math.h>
#include <string.h>
#include <conio.h>
using namespace std;
IplImage* img = 0;
CvMemStorage * cstorage;
CvMemStorage * hstorage;
void detectCircle( IplImage *frame );
int main( int argc, char **argv )
{
CvCapture *capture = 0;
IplImage *frame = 0;
int key = 0;
hstorage = cvCreateMemStorage( 0 );
cstorage = cvCreateMemStorage( 0 );
//CvVideoWriter *writer = 0;
//int colour = 1;
//int fps = 25;
//int frameW = 640;
//int frameH = 480;
//writer = cvCreateVideoWriter("test.avi",CV_FOURCC('P', 'I', 'M', '1'),fps,cvSize(frameW,frameH),colour);
//initialise camera
capture = cvCaptureFromCAM( 0 );
//check if camera present
if ( !capture )
{
fprintf( stderr, "cannot open webcam\n");
return 1;
}
//create a window
cvNamedWindow( "Snooker", CV_WINDOW_AUTOSIZE );
while(key !='q')
{
//get frame
frame = cvQueryFrame(capture);
//int nFrames = 50;
//for (int i=0; i<nFrames;i++){
//cvGrabFrame(capture);
//frame = cvRetrieveFrame(capture);
//cvWriteFrame(writer, frame);
//}
//check for frame
if( !frame ) break;
detectCircle(frame);
//display current frame
//cvShowImage ("Snooker", frame );
//exit if Q pressed
key = cvWaitKey( 20 );
}
// free memory
cvDestroyWindow( "Snooker" );
cvReleaseCapture( &capture );
cvReleaseMemStorage( &cstorage);
cvReleaseMemStorage( &hstorage);
//cvReleaseVideoWriter(&writer);
return 0;
}
void detectCircle( IplImage * img )
{
int px;
int py;
int edge_thresh = 1;
IplImage *gray = cvCreateImage( cvSize(img->width,img->height), 8, 1);
IplImage *edge = cvCreateImage( cvSize(img->width,img->height), 8, 1);
cvCvtColor(img, gray, CV_BGR2GRAY);
gray->origin = 1;
// color threshold
cvThreshold(gray,gray,100,255,CV_THRESH_BINARY);
// smooths out image
cvSmooth(gray, gray, CV_GAUSSIAN, 11, 11);
// get edges
cvCanny(gray, edge, (float)edge_thresh, (float)edge_thresh*3, 5);
// detects circle
CvSeq* circle = cvHoughCircles(gray, cstorage, CV_HOUGH_GRADIENT, 1, gray->height/50, 5, 35);
// draws circle and its centerpoint
float* p = (float*)cvGetSeqElem( circle, 0 );
cvCircle( img, cvPoint(cvRound(p[0]),cvRound(p[1])), 3, CV_RGB(255,0,0), -1, 8, 0 );
cvCircle( img, cvPoint(cvRound(p[0]),cvRound(p[1])), cvRound(p[2]), CV_RGB(200,0,0), 1, 8, 0 );
px=cvRound(p[0]);
py=cvRound(p[1]);
cvShowImage ("Snooker", img );
}