OpenCV - find bounding box of largest blob in binary image

Question

What is the most efficient way to find the bounding box of the largest blob in a binary image using OpenCV? Unfortunately, OpenCV does not have specific functionality for blob detection. Should I just use findContours() and search for the largest in the list?

Answer 1

Here. It. Is. (FYI: try not to be lazy and figure out what happens in my function below.

cv::Mat findBiggestBlob(cv::Mat & matImage){
    int largest_area=0;
    int largest_contour_index=0;

    vector< vector<Point> > contours; // Vector for storing contour
    vector<Vec4i> hierarchy;

    findContours( matImage, contours, hierarchy,CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE ); // Find the contours in the image

    for( int i = 0; i< contours.size(); i++ ) {// iterate through each contour. 
        double a=contourArea( contours[i],false);  //  Find the area of contour
        if(a>largest_area){
            largest_area=a;
            largest_contour_index=i;                //Store the index of largest contour
            //bounding_rect=boundingRect(contours[i]); // Find the bounding rectangle for biggest contour
        }
    }

    drawContours( matImage, contours, largest_contour_index, Scalar(255), CV_FILLED, 8, hierarchy ); // Draw the largest contour using previously stored index.
    return matImage;
}

Answer 2

If you want to use OpenCV libs, check out OpenCVs SimpleBlobDetector. Here's another stack overflow showing a small tutorial of it: How to use OpenCV SimpleBlobDetector

This only gives you key points though. You could use this as an initial search to find the blob you want, and then possibly use the findContours algorithm around the most likely blobs.

Also the more information you know about your blob, you can provide parameters to filter out the blobs you don't want. You might want to test out the area parameters of the SimpleBlobDetector. Possibly could could compute the area based on the size of the area of the image and then iteratively allow for a smaller blob if the algorithm does not detect any blobs.

Here is the link to the main OpenCV documentation: http://docs.opencv.org/modules/features2d/doc/common_interfaces_of_feature_detectors.html#simpleblobdetector