How to find the coordinates of a point w.r.t another point on an image using OpenCV

[silentassasin]

Hi,
I wrote a program for detecting circles using Hough Transform using OpenCV in C. The program inputs 3 images, each image contains a fixed small circle and a big circle with variable position. The program then recognizes both the circles and marks the centres of both the circles. Now what I want to do is that in the output image the (x,y) coordinates of the centre of the bigger circle should be displayed with respect to the centre of the fixed smaller circle . Here's the code for 'circle.cpp'

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

int main(int argc, char** argv)
{
    IplImage* img;
    int n=3;
    char input[21],output[21];    

    for(int l=1;l<=n;l++)
    {     
      sprintf(input,"Frame%d.jpg",l);  // Inputs Images

      if(  (img=cvLoadImage(input))!= 0)
    {
        IplImage* gray = cvCreateImage( cvGetSize(img), IPL_DEPTH_8U, 1 );
        IplImage* canny=cvCreateImage(cvGetSize(img),IPL_DEPTH_8U,1);
        IplImage* rgbcanny=cvCreateImage(cvGetSize(img),IPL_DEPTH_8U,3);
        CvMemStorage* storage = cvCreateMemStorage(0);
        cvCvtColor( img, gray, CV_BGR2GRAY );
        cvSmooth( gray, gray, CV_GAUSSIAN, 9, 9 ); // smooth it, otherwise a lot of false circles may be detected
        cvCanny(gray,canny,50,100,3);

        CvSeq* circles = cvHoughCircles( canny, storage, CV_HOUGH_GRADIENT, 2, gray->height/4, 200, 100 );
        int i;
        cvCvtColor(canny,rgbcanny,CV_GRAY2BGR);
        for( i = 0; i < circles->total; i++ )
        {
             float* p = (float*)cvGetSeqElem( circles, i );
             cvCircle( rgbcanny, cvPoint(cvRound(p[0]),cvRound(p[1])), 3, CV_RGB(0,255,0), -1, 8, 0 );
             cvCircle( rgbcanny, cvPoint(cvRound(p[0]),cvRound(p[1])), cvRound(p[2]), CV_RGB(255,0,0), 3, 8, 0 );
        }
        cvNamedWindow( "circles", 1 );
        cvShowImage( "circles", rgbcanny );

        //Displays Output images
        sprintf(output,"circle%d.jpg",l);   
        cvSaveImage(output,rgbcanny);    
        cvWaitKey(0);
    }
}
    return 0;
}

And here are the input and output images:
[IMG]http://i.imgur.com/0vCe0.png[/IMG] [IMG]http://i.imgur.com/7RlHP.jpg[/IMG] [IMG]http://i.imgur.com/KxMei.jpg[/IMG] [IMG]http://i.imgur.com/y9aGq.jpg[/IMG] [IMG]http://i.imgur.com/cuAva.jpg[/IMG] [IMG]http://i.imgur.com/TscrG.jpg[/IMG]

Please suggest what changes should I make in the code to display the desired (x,y)coordinates. Thanx a lot :)