OpenCV: How can I remove unwanted blobs or how can I copy wanted parts into an empty image?

Question

From the following image, how could I find the result image?

The images shown here are threshold images. I have tried using morphological operators but they even remove the blob I want. How could I solve this problem? Any hints?

Following is the result image I am interested to get/find:

import cv2
diff = cv2.imread('Image.png',0)
ret, thresh = cv2.threshold(diff, 12.5, 255, cv2.THRESH_BINARY)
thresh = cv2.dilate(thresh, None, iterations = 1)
cv2.imshow('img', thresh) # This is the  first picture I have shown
cv2.waitKey(0)

What is the difference between these 2 blobs that you have made basis upon for removing one and preserving the other ? — ZdaR, Mar 21 '18 at 08:47
Well, I haven't yet. I don't know what criteria to set as such. But I am always interested in the blob which is slightly bigger/longer than the other. — Jeremy, Mar 21 '18 at 08:50
You can compare the relative position by comparing the centroid of each blob, and you can compare the relative size of each blob by extracting its contour and obtaining a bounding box. However, if you can't specify what you want (the selection criteria) then naturally you also can't get what you want. — RAM, Mar 21 '18 at 09:19

score 1 · Accepted Answer · answered Mar 21 '18 at 09:11

You are most of the way there, all you need to do now is find the blobs, add some contours and find the biggest one. Easy! below is the code in C++, ill leave it up to you to work out how to convert it to Python:

cv::Mat mat = imread("g0cVU.png");
    Mat origImage = mat;
    Mat canny_output = mat;
    vector<vector<Point> > contours;
    vector<Vec4i> hierarchy;
    cv::Mat greyMat, colorMat;
    cv::cvtColor(mat, greyMat, CV_BGR2GRAY);
    int thresh = 100;
    RNG rng(12345);
    ///// Detect edges using canny
    Canny(greyMat, canny_output, thresh, thresh * 2, 3);
    /// Find contours
    findContours(canny_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));
    int largest_area = 0;
    int largest_contour_index = 0;
    Rect bounding_rect;
    /// Draw contours
    Mat drawing = Mat::zeros(canny_output.size(), CV_8UC3);
    for (int i = 0; i< contours.size(); i++)
    {
        Scalar color = Scalar(rng.uniform(0, 255), rng.uniform(0, 255), rng.uniform(0, 255));
        drawContours(drawing, contours, i, color, 2, 8, hierarchy, 0, Point());
        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
       }
    }
    rectangle(origImage, bounding_rect, Scalar(rng.uniform(0, 255), rng.uniform(0, 255), rng.uniform(0, 255)),2);
    /// Show in a window
    namedWindow("Contours", CV_WINDOW_AUTOSIZE);
    imshow("Contours", drawing);

    cv::namedWindow("img");
    cv::imshow("mat", mat);
    cv::imshow("mat", origImage);
    cv::imshow("mat123", drawing);
    cv::waitKey(0);

Which gives this results:

You can see in the bottom image the largest contor has a brown rectangle drawn around it.

o and obviously once you have the largest blob (or whatever blob you deem "the correct one") you can just set everything else to black which is fairly straightforward.

OpenCV: How can I remove unwanted blobs or how can I copy wanted parts into an empty image?

1 Answers1