finding convexity defects in opencv? [crashes depending on the given input image..]

Question

I have a program that calculates the convex hull of an image. I'm trying to use this information in order to count the number of fingers that are present in an input image. From some surfing I found out that the way to do this (count fingers) is by

1. Finding contours
2. Convex Hull
3. Convexity defects

But I'm having trouble using the convexity defects function. It compiles fine but at runtime the program crashes with certain input images but not with others and I can't seem to figure out why.

These are the input images

1. this image causes a crash
2. but this does not.
3. this also causes a crash even though its similar to the above

code..

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

#define CVX_RED     CV_RGB(0xff,0x00,0x00)
#define CVX_GREEN   CV_RGB(0x00,0xff,0x00)
#define CVX_BLUE    CV_RGB(0x00,0x00,0xff)

int main(int argc, char* argv[]) {

  cvNamedWindow( "original", 1 );
  cvNamedWindow( "contours", 1 );
  cvNamedWindow( "hull", 1 );
  IplImage* original_img = NULL;

  original_img = cvLoadImage("img.jpg", 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_LIST // Try all four values and see what happens
  );

  for( CvSeq* c=first_contour; c!=NULL; c=c->h_next ) {
     cvCvtColor( original_img, contour_img, CV_GRAY2BGR );
     cvDrawContours(
        contour_img,
        c,
        CVX_RED,  
        CVX_BLUE,
        0,     
        2,
        8
     );
  }

  //----------------------------------------------------------------------Convex Hull

  CvMemStorage* hull_storage = cvCreateMemStorage();
  CvSeq* retHulls = NULL;

  for(CvSeq* i = first_contour; i != NULL; i = i->h_next){
    retHulls = cvConvexHull2(i,hull_storage,CV_CLOCKWISE,0); 
    // with 1 it draws the Hull image but not with 0..?
    // however it needs to be 0 for convexitydefects to work?
  }

  printf(" %d elements:\n", retHulls->total );

  // drawing hull

  for( CvSeq* j=retHulls; j!=NULL; j=j->h_next ) {
    cvCvtColor( original_img, hull_img, CV_GRAY2BGR );
    cvDrawContours(
        hull_img,
        j,
        CVX_RED,  
        CVX_BLUE,
        0,        
        2,
        8
     );  

  }


  //----------------------------------------------------------------------Convexity Defects??

  CvMemStorage* convexStorage = cvCreateMemStorage();
  CvSeq* defect = NULL;
  defect = cvConvexityDefects(first_contour,retHulls, convexStorage);
  printf(" %d defect:\n", defect->total );


  cvShowImage( "contours", contour_img );
  cvShowImage( "original", original_img );
  cvShowImage( "hull", hull_img );
  cvWaitKey(0);
  cvDestroyWindow( "contours" );
  cvDestroyWindow( "original" );
  cvDestroyWindow( "hull" );
  cvReleaseImage( &original_img );
  cvReleaseImage( &contour_img );
  cvReleaseImage( &hull_img );
  cvReleaseImage( &img_edge );
  return 0;
}

Answer 1

cvConvexityDefects expects the convexHull sequence (second argument) to contain indices into the contour sequence (first argument):

Convex hull obtained using ConvexHull2 that should contain pointers or indices to the contour points, not the hull points themselves

1. In the most trivial case, where cvFindContours returns a single simple contour (your second image) you got lucky and your code would supply the correct sequence as the first parameter.

2. In case cvFindContours finds holes in the contour (your third image), or if there are several simple contours or contours with holes (your first image) your code:

   1. finds a convex hull of each contour in turn, but only remembers the last one (since each iteration of the loop overwrites retHulls variable)

   2. passes the whole hierarchy of the contours, which doesn't correspond to indices in retHulls, to cvConvexityDefects as the first argument.

Instead, you should have:

1. passed CV_RETR_EXTERNAL to the cvFindContour to only get the outer contours (you don't care for defects of holes)

2. moved the cvConvexityDefects inside the last loop.

Something like:

  /* ... */

  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 );
  /* ... */