CODEWITHSUNDEEP
c++copencvconvex-hull
silent
silent

Reputation: 2904

Finding the convex hull of an object in opencv?

I've written this based on the tutorial here but I'm unable to obtain the convex hull of the image (I'm using a similar hand image as shown in the tutorial). I get the source and edges output fine but the "Drawings" output which should draw the contour and convex hull lines don't show anything drawn and instead is completely black. Any ideas as to why this could be?

#include <opencv/cv.h>
#include <opencv/highgui.h>
#include <opencv/cxcore.h>

int main(int argc,char **argv)
{
    cvNamedWindow( "Source", 1 );
    cvNamedWindow( "edges window", 1 );
    cvNamedWindow( "Drawings", 1 );

    IplImage* src = cvLoadImage( "img.jpg", 0 );
    IplImage* edges = cvCreateImage( cvGetSize(src), 8, 1 );

    // Finding edges
    cvThreshold( src, edges, 150, 255, CV_THRESH_BINARY );

    CvMemStorage* storage = cvCreateMemStorage();
    CvSeq* first_contour = NULL;

    int Nc = cvFindContours(
        edges,
        storage,
        &first_contour,
        sizeof(CvContour),
        CV_RETR_LIST );

    // Finding convex Hull
    CvMemStorage* hull_storage = cvCreateMemStorage();
    CvSeq* retHulls = NULL;

    for(CvSeq* i = first_contour; i != 0; i = i->h_next){
    // note h_next is next sequence.
    retHulls = cvConvexHull2(first_contour,hull_storage,CV_CLOCKWISE,1);

    }

    // drawing contours and hull
    IplImage* draw = cvCreateImage(cvGetSize(edges), 8, 3 );

    for(CvSeq* i = first_contour; i != 0; i = i->h_next){
        cvDrawContours(draw,first_contour,cvScalar(255,0,0,0),cvScalar(255,0,0,0),0,1,8);
        cvDrawContours(draw,retHulls,cvScalar(255,0,0,0),cvScalar(255,0,0,0),0,1,8);

    }

    cvShowImage( "Source", src );
    cvShowImage( "edges window", edges );
    cvShowImage( "Drawings", draw );
    cvWaitKey();

    cvDestroyAllWindows();

    cvReleaseImage( &src );
    cvReleaseImage( &edges );
    cvReleaseImage( &draw );

    return 0;
}

Upvotes: 2

Views: 10182

Answers (2)

ArtemStorozhuk
ArtemStorozhuk

Reputation: 8725

You have to do the following changes:

  1. Change parameter from CV_RETR_LIST to CV_RETR_EXTERNAL in cvFindContours function.
  2. Change CV_THRESH_BINARY to CV_THRESH_OTSU in cvThreshold.

Here's proof (input/output):

enter image description here enter image description here

Upvotes: 3

plan9assembler
plan9assembler

Reputation: 2984

there is 2 methods,

  1. opencv convex hull

http://docs.opencv.org/doc/tutorials/imgproc/shapedescriptors/hull/hull.html

http://code.opencv.org/svn/opencv/trunk/opencv/samples/cpp/tutorial_code/ShapeDescriptors/hull_demo.cpp

  1. cvblob convex hull

https://code.google.com/p/cvblob/

https://code.google.com/p/cvblob/source/browse/trunk/cvBlob/cvblob.h?r=398

CvContourPolygon *cvPolygonContourConvexHull(CvContourPolygon const *p);

HTH

Upvotes: 0

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