Using cvApproxPoly() on the results of cvConvexHull2() in OpenCV

Question

I'm writing C (not C++) code to run convex hull on contours of rectangles. The (greatly simplified) code for that looks like this:

CvSeq* contours;
CvSeq* hull;

cvFindContours( img, &contours, sizeof(CvContour), CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE, CvPoint(0,0) );
hull = cvConvexHull2( contours, storage, CV_CLOCKWISE, 0 );

Where img and contours are both valid. There is a lot more between these lines, but the FindContours portion of it has been tested to work.

This code throws exception: Error: Bad argument (Unsupported sequence type) in cvApproxPoly.

I'm told that the problem is that the flag that identifies the sequence as a polyline is not set, and I have tried the suggestion hull->flags = hull->flags | 512 but presumably the flags have changed sometime between 2008 and now, since that does not work.

So the question is: how do I use cvApproxPoly() on the results of a cvConvexHull2()? What data type should I use, and what are the proper arguments for cvApproxPoly()?

`cvFindContours` has incorrect arguments. Show us full, actual calls to those three functions that produce this error. — Piotr Praszmo, Feb 08 '12 at 02:23

score 3 · Answer 1 · answered Dec 21 '12 at 10:26

I think you are missing the storage parameter:

cvFindContours(<CvArr *image>, <CvMemStorage *storage>, <CvSeq **first_contour>, <int header_size>, <int mode>, <int method>, <CvPoint offset>)

Test using

CvMemStorage *storage = cvCreateMemStorage(0);
cvFindContours( img, storage, &contours, sizeof(CvContour), CV_RETR_LIST, CV_CHAIN_APPROX_SIMPLE, CvPoint(0,0) );
hull = cvConvexHull2( contours, storage, CV_CLOCKWISE, 0 );

Using cvApproxPoly() on the results of cvConvexHull2() in OpenCV

1 Answers1