OpenCV#6: Find the cross correlation of two opencv images

This function calculates the cross correlation between two images of the type IplImage*. Returns the resulting value.

double cross_correlation( IplImage* img1, IplImage* img2 )
{
double corr;

int M = img1->width;
int N = img1->height;

BwImage img_1( img1 ); // using opencv wrapper for accessing image pixels
BwImage img_2( img2 );

CvScalar img1_avg = cvAvg( img1, NULL );
CvScalar img2_avg = cvAvg( img2, NULL );

double sum_img1_img2 = 0;
double sum_img1_2 = 0;
double sum_img2_2 = 0;

for( int m=0; m<M; ++m )
{
   for( int n=0; n<N; ++n )
   {
       sum_img1_img2   = sum_img1_img2 + (img_1[m][n]-img1_avg.val[0])*(img_2[m][n]-img2_avg.val[0]);  
       sum_img1_2      = sum_img1_2 + (img_1[m][n]-img1_avg.val[0])*(img_1[m][n]-img1_avg.val[0]);
       sum_img2_2      = sum_img2_2 + (img_2[m][n]-img2_avg.val[0])*(img_2[m][n]-img2_avg.val[0]);
   }
}

corr = sum_img1_img2/sqrt(sum_img1_2*sum_img2_2);

return corr;
}