Question

Input Image — https://i.stack.imgur.com/hIqEd.png
Output Image with Convex Hull of contours drawn — https://i.stack.imgur.com/pEKPP.png

Also any help on how to get one contour will be much appreciated?

Answer 1

The trick to retrieve the image as one contour seems to be processing the image with Canny before executing cvFindContours.

IplImage* src = cvLoadImage(argv[1], CV_LOAD_IMAGE_GRAYSCALE);

IplImage* cc_img = cvCreateImage( cvGetSize(src), src->depth, 3 );
cvSetZero(cc_img);
CvScalar(ext_color);

CvMemStorage *mem;
mem = cvCreateMemStorage(0);
CvSeq *contours = 0;

// edges returned by Canny might have small gaps between them, which causes some problems during contour detection
// Simplest way to solve this s to "dilate" the image.
cvCanny(src, src, 10, 50, 3); 
cvShowImage("Tutorial", src);
cvWaitKey(0);

int n = cvFindContours( src, mem, &contours, sizeof(CvContour), CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE, cvPoint(0,0));

CvSeq* ptr = 0;
for (ptr = contours; ptr != NULL; ptr = ptr->h_next)
{   
    ext_color = CV_RGB( rand()&255, rand()&255, rand()&255 ); //randomly coloring different contours
    cvDrawContours(cc_img, ptr, ext_color, CV_RGB(0,0,0), -1, CV_FILLED, 8, cvPoint(0,0));
}   

cvNamedWindow("Tutorial");
cvShowImage("Tutorial", cc_img);
//cvSaveImage("out.png", cc_img);

cvWaitKey(0);

Outputs: