Learning OpenCV

Another Contour Example

In this example, we find contours on an input image and then proceed to draw them one by one. This is a good example to tinker with on your own to explore the effects of changing either the contour finding mode (CV_RETR_LIST in the code) or the max_depth that is used to draw the contours (0 in the code). If you set max_depth to a larger number, notice that the example code steps through the contours returned by cvFindContours() by means of h_next. Thus, for some topologies (CV_RETR_TREE, CV_RETR_CCOMP, etc.), you may see the same contour more than once as you step through. See Example 8-3.

Example 8-3. Finding and drawing contours on an input image

int main(int argc, char* argv[]) { cvNamedWindow( argv[0], 1 ); IplImage* img_8uc1 = cvLoadImage( argv[1], CV_LOAD_IMAGE_GRAYSCALE ); IplImage* img_edge = cvCreateImage( cvGetSize(img_8uc1), 8, 1 ); IplImage* img_8uc3 = cvCreateImage( cvGetSize(img_8uc1), 8, 3 ); cvThreshold( img_8uc1, img_edge, 128, 255, CV_THRESH_BINARY ); CvMemStorage* storage = cvCreateMemStorage(); CvSeq* first_contour = NULL; int Nc = cvFindContours( img_edge, storage, &first_contour, sizeof(CvContour), CV_RETR_LIST // Try all four values and see what happens ); int n=0; printf( "Total Contours Detected: %d\n", Nc ); for( CvSeq* c=first_contour; c!=NULL; c=c->h_next ) { cvCvtColor( img_8uc1, img_8uc3, CV_GRAY2BGR ); cvDrawContours( img_8uc3, c, CVX_RED, CVX_BLUE, 0, // Try different values of max_level, and see what happens 2, 8 ); printf("Contour ...