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copied!<p><code>cvConvexityDefects</code> expects the <code>convexHull</code> sequence (second argument) to contain indices into the <code>contour</code> sequence (first argument):</p> <blockquote> <p>Convex hull obtained using ConvexHull2 that should contain pointers or indices to the contour points, not the hull points themselves </p> </blockquote> <ol> <li><p>In the most trivial case, where <code>cvFindContours</code> returns a single simple contour (your second image) you got lucky and your code would supply the correct sequence as the first parameter. </p></li> <li><p>In case <code>cvFindContours</code> finds holes in the contour (your third image), or if there are several simple contours or contours with holes (your first image) your code:</p> <ol> <li><p>finds a convex hull of each contour in turn, but only remembers the last one (since each iteration of the loop overwrites <code>retHulls</code> variable)</p></li> <li><p>passes the whole hierarchy of the contours, which doesn't correspond to indices in <code>retHulls</code>, to <code>cvConvexityDefects</code> as the first argument. </p></li> </ol></li> </ol> <p>Instead, you should have:</p> <ol> <li><p>passed <code>CV_RETR_EXTERNAL</code> to the <code>cvFindContour</code> to only get the outer contours (you don't care for defects of holes)</p></li> <li><p>moved the <code>cvConvexityDefects</code> inside the last loop. </p></li> </ol> <p>Something like:</p> <pre><code> /* ... */ if (argc < 2) { std::cerr << "Usage: convexity IMAGE\n"; exit(1); } cvNamedWindow( "original", 1 ); cvNamedWindow( "contours", 1 ); cvNamedWindow( "hull", 1 ); IplImage* original_img = NULL; original_img = cvLoadImage(argv[1], CV_LOAD_IMAGE_GRAYSCALE ); IplImage* img_edge = cvCreateImage( cvGetSize(original_img), 8, 1 ); IplImage* contour_img = cvCreateImage( cvGetSize(original_img), 8, 3 ); IplImage* hull_img = cvCreateImage( cvGetSize(original_img), 8, 3 ); cvThreshold( original_img, 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_EXTERNAL // Try all four values and see what happens ); cvCvtColor( original_img, contour_img, CV_GRAY2BGR ); for( CvSeq* c=first_contour; c!=NULL; c=c->h_next ) { cvDrawContours( contour_img, c, CVX_RED, CVX_BLUE, 0, 2, 8 ); } cvShowImage( "contours", contour_img ); //----------------------------------------------------------------------Convex Hull //-------------------------------------------------------------------Convex Defects CvMemStorage* hull_storage = cvCreateMemStorage(); CvSeq* retHulls = NULL; cvCvtColor( original_img, hull_img, CV_GRAY2BGR ); for(CvSeq* i = first_contour; i != NULL; i = i->h_next){ retHulls = cvConvexHull2(i,hull_storage,CV_CLOCKWISE,0); printf(" %d elements:\n", retHulls->total ); CvSeq* defect = NULL; defect = cvConvexityDefects(i,retHulls, NULL); // reuse storage of the contour printf(" %d defect:\n", defect->total ); // drawing hull.... you can't use the one returned above since it only // contains indices retHulls = cvConvexHull2(i,hull_storage,CV_CLOCKWISE,1); cvDrawContours( hull_img, retHulls, CVX_RED, CVX_BLUE, 0, 2, 8 ); } cvShowImage( "hull", hull_img ); /* ... */ </code></pre>

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