OpenCV. Drawing rectangle when matching

Question

i use OpenCv to find an area witch match a template on a reference image. When the code find the area witch match a template draw a rectangle around the area but what i want is when the code doesnt find the area the code doesnt draw any rectangle

code:

IplImage    *res;
    CvPoint     minloc, maxloc;
    double      minval, maxval;
    int         img_width, img_height;
    int         tpl_width, tpl_height;
    int         res_width, res_height;

    NSString *path = [[NSBundle mainBundle] pathForResource:@"reference" ofType:@"jpg"];
    reference.image = [UIImage imageWithContentsOfFile:path];

    NSString *pathPatron = [[NSBundle mainBundle] pathForResource:@"template" ofType:@"jpg"];
    template.image = [UIImage imageWithContentsOfFile:pathPatron];

    IplImage *img = [self CreateIplImageFromUIImage:original.image];//
    IplImage *tpl = [self CreateIplImageFromUIImage:patron.image];

    img_width  = img->width;
    img_height = img->height;
    tpl_width  = tpl->width;
    tpl_height = tpl->height;
    res_width  = img_width - tpl_width + 1;
    res_height = img_height - tpl_height + 1;    
    res = cvCreateImage( cvSize( res_width, res_height ), IPL_DEPTH_32F, 1 );

    /* choose template matching method to be used */
    cvMatchTemplate( img, tpl, res, CV_TM_SQDIFF );

      cvMinMaxLoc( res, &minval, &maxval, &minloc, &maxloc, 0 );

        /* draw red rectangle */

    cvRectangle( img, 
                cvPoint( minloc.x, minloc.y ), 
                cvPoint( minloc.x + tpl_width, minloc.y + tpl_height ),
                cvScalar( 0, 0, 255, 0 ), 1, 0, 0 );    

    /* display images */
    reference.image = [self UIImageFromIplImage:img];

    cvReleaseImage(&img);
    cvReleaseImage(&tpl);
    cvReleaseImage(&res);

Thanks in advance

Answer 1

cvMatchTemplate does not really find the match - it just calculates similarity map for your image and template. Next you need to decide if there is a real match or there are no matches. The simplest solution is a comparison with some threshold:

if (minval < THRESHOLD)
{
    //draw rectangle
}

You need to make some experiments to find a working value of the THRESHOLD.