findcontours finds too many contours.

asked 2015-01-31 09:35:57 -0500

Dionysos gravatar image

Is it normal that find contours finds so many contours where there are obviously only 3 contours ? image description

image description

The code is :

Mat image;
    vector<vector<Point> > contours;
    vector<Vec4i> hierarchy;
    image = imread(argv[1], 0);   // Read the file
    findContours(image, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));
    cout << contours.size();

    RNG rng(12345);
    Mat drawing = Mat::zeros(image.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());
    /// Show in a window
    namedWindow("Contours", CV_WINDOW_AUTOSIZE);
    imshow("Contours", drawing);
namedWindow("Display window", WINDOW_AUTOSIZE);// Create a window for display.
    imshow("Display window", image);                   // Show our image inside it.
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Use CV_RETR_EXTERNAL instead of CV_RETR_TREE, which retrieves only the extreme outer contours.

Haris gravatar imageHaris ( 2015-01-31 10:08:13 -0500 )edit

I found the problem. It wasn't using CV_RETR_TREE. findContours only works properly on "edge images". So you have to run canny first

Dionysos gravatar imageDionysos ( 2015-01-31 14:13:10 -0500 )edit

@Dionysos, running canny first is kind of an overkill. Using the correct parameter solves it all. findContours wants binary images, of which edge images are a sort, but blobs is just fine if you use the external parameter.

StevenPuttemans gravatar imageStevenPuttemans ( 2015-02-01 02:21:51 -0500 )edit

Steve, that's what I thought but I was using a binary image (the top one). The bottom one is the contours drawn (where you see that there's too many).

Dionysos gravatar imageDionysos ( 2015-02-01 09:02:46 -0500 )edit