OpenCV cvblob - Render Blob

I'm trying to detect a object using cvblob. So I use cvRenderBlob() method. Program compiled successfully but when at the run time it is returning an unhandled exception. When I break it, the arrow is pointed out to CvLabel *labels = (CvLabel *)imgLabel->imageData + imgLabel_offset + (blob->miny * stepLbl); statement in the cvRenderBlob() method definition of the cvblob.cpp file. But if I use cvRenderBlobs() method it's working fine. I need to detect only one blob that is the largest one. Some one please help me to handle this exception. Here is my VC++ code,

CvCapture* capture = 0;
IplImage* frame = 0;
int key = 0;
CvBlobs blobs;
CvBlob *blob;

capture = cvCaptureFromCAM(0);

if (!capture) {
    printf("Could not initialize capturing....\n");
    return 1;
}

int screenx = GetSystemMetrics(SM_CXSCREEN);
int screeny = GetSystemMetrics(SM_CYSCREEN);

while (key!='q') {
    frame = cvQueryFrame(capture);
    if (!frame) break;

    IplImage* imgHSV = cvCreateImage(cvGetSize(frame), 8, 3);
    cvCvtColor(frame, imgHSV, CV_BGR2HSV);

    IplImage* imgThreshed = cvCreateImage(cvGetSize(frame), 8, 1);
    cvInRangeS(imgHSV, cvScalar(61, 156, 205),cvScalar(161, 256, 305), imgThreshed); // for light blue color

    IplImage* imgThresh = imgThreshed; 
    cvSmooth(imgThresh, imgThresh, CV_GAUSSIAN, 9, 9);开发者_Python百科

    cvNamedWindow("Thresh"); 
    cvShowImage("Thresh", imgThresh); 
    IplImage* labelImg = cvCreateImage(cvGetSize(imgHSV), IPL_DEPTH_LABEL, 1);
    unsigned int result = cvLabel(imgThresh, labelImg, blobs);

    blob = blobs[cvGreaterBlob(blobs)];
    cvRenderBlob(labelImg, blob, frame, frame);
    /*cvRenderBlobs(labelImg, blobs, frame, frame);*/
    /*cvFilterByArea(blobs, 60, 500);*/ 
    cvFilterByLabel(blobs, cvGreaterBlob(blobs));

    cvNamedWindow("Video");
    cvShowImage("Video", frame);
    key = cvWaitKey(1); 
 }

cvDestroyWindow("Thresh");
cvDestroyWindow("Video");
cvReleaseCapture(&capture);

First off, I'd like to point out that you are actually using the regular c syntax. C++ uses the class Mat. I've been working on some blob extraction based on green objects in the picture. Once thresholded properly, which means we have a "binary" image, background/foreground. I use

findContours() //this function expects quite a bit, read documentation

Descriped more clearly in the documentation on structural analysis. It will give you the contour of all the blobs in the image. In a vector which is handling another vector, which is handling points in the image; like so

vector<vector<Point>> contours;

I too need to find the biggest blob, and though my approach can be faulty to some extend, I won't need it to be different. I use

minAreaRect() // expects a set of points (contained by the vector or mat classes

Descriped also under structural analysis Then access the size of the rect

int sizeOfObject = 0;
int idxBiggestObject = 0; //will track the biggest object

if(contours.size() != 0) //only runs code if there is any blobs / contours in the image
{
    for (int i = 0; i < contours.size(); i++) // runs i times where i is the amount of "blobs" in the image.
    {
        myVector = minAreaRect(contours[i])
        if(myVector.size.area > sizeOfObject)
        {
             sizeOfObject = myVector.size.area; //saves area to compare with further blobs
             idxBiggestObject = i; //saves index, so you know which is biggest, alternatively, .push_back into another vector
        }
    }
}

So okay, we really only measure a rotated bounding box, but in most cases it will do. I hope that you will either switch to c++ syntax, or get some inspiration from the basic algorithm.

Enjoy.