Question

I’m trying to draw contours around blobs in a binary image, however, sometimes, openCV draws a single contour around two distinct blobs. below is an example. How can i solve this issue?

Here it should draw two bounding boxes for the blob on the right and separately for the one of the left. I agree they are close but enough distance in between them. I’m only drawing External contours instead of the tree or list. I’m also using cvFindNextContour(contourscanner) as this is a easier implementation for my case.

Thanks

EDIT:
Image displayed in the “output” window is from a different function which does just image subtraction. Image displayed in the “contours” window is in the function pplfind(). “output” image is passed to img_con().

IplImage* img_con(IplImage* image){
    int ppl;
    CvMemStorage* memstr = cvCreateMemStorage();
    IplImage* edges = cvCreateImage(cvGetSize(image),8,1);
    cvCanny(image,edges,130,255);
    CvContourScanner cscan = cvStartFindContours(image,memstr,sizeof(CvContour),CV_RETR_EXTERNAL,CV_CHAIN_APPROX_NONE,cvPoint(0,0));


ppl = pplfind(cscan,cvGetSize(image));
if (ppl !=0 )
    printf("Estimated number of people: %d\n",ppl);
cvEndFindContours(&cscan);
cvClearMemStorage(memstr);

return edges;

}
int pplfind(CvContourScanner cscan, CvSize frSize){

    ofstream file; char buff[50];

    file.open("box.txt",ofstream::app);

    int ppl =0;

    CvSeq* c;

    IplImage *out = cvCreateImage(frSize,8,3);

    while (c = cvFindNextContour(cscan)){

        CvRect box = cvBoundingRect(c,1);

        if ((box.height > int(box.width*1.2))&&(box.height>20)){//&&(box.width<20)){//

            ppl++;

            cvRectangle(out,cvPoint(box.x,box.y),cvPoint(box.x+box.width,box.y+box.height),CV_RGB(255,0,50),1);
        cvShowImage("contours",out);
        //cvWaitKey();
    }
    //printf("Box Height: %d , Box Width: %d ,People: %d\n",box.height,box.width,ppl);
    //cvWaitKey(0);
    int coord = sprintf_s(buff,"%d,%d,%d\n",box.width,box.height,ppl);
    file.write(buff,coord);
}
file.close();
cvReleaseImage(&out);
return ppl;

}


												

Answer 1

I’ve never used cvFindNextContour, but running cvFindContours with CV_RETR_EXTERNAL on your image seems to work fine:

I use OpenCV + Python, so this code might not be useful for you, but for the sake of completeness here it goes:

contours = cv.findContours(img, cv.CreateMemStorage(0), mode=cv.CV_RETR_EXTERNAL)
while contours:
    (x,y,w,h) = cv.BoundingRect(contours)
    cv.Rectangle(colorImg, (x,y), (x+w,y+h), cv.Scalar(0,255,255,255))
    contours = contours.h_next()

Edit: you asked how to draw only those contours with certain properties; it would be something like this:

contours = cv.findContours(img, cv.CreateMemStorage(0), mode=cv.CV_RETR_EXTERNAL)
while contours:
    (x,y,w,h) = cv.BoundingRect(contours)
    if h > w*1.2 and h > 20:
        cv.Rectangle(colorImg, (x,y), (x+w,y+h), cv.Scalar(0,255,255,255))
    contours = contours.h_next()