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How to remove the small blobs? [closed]

asked 2017-03-05 22:34:23 -0500

hoang anh tuan gravatar image

I want to erase the small white spots areas to keep the largest area (white colour) in the picture:



I used findContours and found the largest area then used drawContours. I got the following result:


My code

findContours(image, vtContours, vtHierarchy, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);

for (int i = 0; i < vtContours.size(); i++)
    double area = contourArea(vtContours[i]);

    if (area <= 100) {

    // Get only one
    if (area > dMaxArea)
        dMaxArea = area;
        nSavedContour = i;


if (nSavedContour == -1)
    return false;

image = Scalar::all(0);
drawContours(image, vtContours, nSavedContour, Scalar(255), CV_FILLED, 8);

Can you show me the other idea or my fault? Thank for supporting!

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Closed for the following reason the question is answered, right answer was accepted by hoang anh tuan
close date 2018-05-09 03:34:04.316314

3 answers

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answered 2017-03-05 23:30:01 -0500

LBerger gravatar image

updated 2017-03-06 03:27:38 -0500

You can use morphological operators like erode dilate or closing opening.

If you want to draw hole in your surface you have to use RETR_TREE instead of CV_RETR_EXTERNAL in findContours

About hierarchy : Original image is :

image description

Contours with hierarchy are

0 = [-1, -1, 1, -1] // contour 0 :-1 no next contour,-1  no previous contour, 1 contour #1 is a child, -1 no parent contour  
1 = [3, -1, 2, 0]  // contour 1 :-3 next contour,-1  no previous contour, 1 contour #2 is a child, 0 is parent contour  
2 = [-1, -1, -1, 1]
3 = [-1, 1, -1, 0]

image description

program is :

Mat img = imread("C:/Users/Laurent.PC-LAURENT-VISI/Desktop/14887743804853655.jpg", CV_LOAD_IMAGE_GRAYSCALE);
imshow("original", img);
Mat y;
threshold(img, y, 50, 255, THRESH_BINARY);
imshow("threshold", y);
double ccMin, ccMax;
findContours(y, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE);
Mat c = Mat::zeros(img.size(), CV_8U);
for (int i = 0; i <contours.size(); i++)
    cout<<i<<" = "<<hierarchy[i]<<"\n";
for (int i = 0; i <contours.size(); i++)
    drawContours(c, contours, i, Scalar(255), 1, LINE_8, hierarchy, 0);
    putText(c,format("%d",i),contours[i][0], FONT_HERSHEY_SIMPLEX,0.5,Scalar(255));
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Thank for supporting. I used morphological operators. But I want to use based on findContours. After change RETR_TREE instead of CV_RETR_EXTERNAL, what do I need to do? Tks!

hoang anh tuan gravatar imagehoang anh tuan ( 2017-03-06 00:08:24 -0500 )edit

answered 2017-03-06 00:26:37 -0500

Brandon212 gravatar image

updated 2017-03-06 00:44:17 -0500

If the problem you're asking for help with is the contour being filled in, probably the easiest way to make it how you want is to use that filled in contour as a mask on your image:

Mat mask = Mat::zeros(image.size(), CV_8U);
Mat newImage;
drawContours(mask, vtContours, nSavedContour, Scalar(255), CV_FILLED, 8);
image.copyTo(newImage, mask);

Then you'll have the contour you want in newImage.

Also if you want the largest contour, it's probably easiest to just sort the vector of contours. This is the function I always use for that:

void sortContours(vector<vector<Point>>& contours)
    auto contourComparator = [](vector<Point> a, vector<Point> b) { return contourArea(a) > contourArea(b); };
    sort(contours.begin(), contours.end(), contourComparator);

Then the largest one will just be the first one in the vector, so it would just be this to make the mask:

drawContours(mask, vtContours, 0, Scalar(255), CV_FILLED, 8);
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My result image is lost the hole. Your answer is similar to my way.

hoang anh tuan gravatar imagehoang anh tuan ( 2017-03-06 00:33:43 -0500 )edit

The hole will still be there if you use the code I provided at the top of the post. Did you imshow newImage?

Brandon212 gravatar imageBrandon212 ( 2017-03-06 00:38:56 -0500 )edit

answered 2017-03-06 04:46:55 -0500

I think."OPEN METHOD" can solove you problem. image description

     Mat src = imread("blob.jpg");
     Mat dst;
     Mat element = getStructuringElement(MORPH_ELLIPSE,Size(7,7));
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Asked: 2017-03-05 22:34:23 -0500

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Last updated: Mar 06 '17