By Nguyen Minh Binh

2013-05-22 03:08:23 8 Comments

Could OpenCV detect the geometric shapes which is drawn by hand as below? The shape can be a rectangle, triangle, circle, curve, arc,polygon,... I am going to develop an android application which detect these shapes. enter image description here


@William 2013-05-22 15:18:20

Well, I tried it in a harry. Normally you need to skeletonize the input. Anyway. You can reason about the shapes based on their points. Normally a square has 4, a triangle 3, etc. Effort results:

Canny results: enter image description here

Polygonal approximation: enter image description here

Console output:

contour points:11
contour points:6
contour points:4 
contour points:5

Here is the code:

    Mat src=imread("WyoKM.png");
    Mat src_gray(src.size(),CV_8UC1);
    if (src.empty()) exit(-10);

    /// Convert image to gray and blur it
    cvtColor( src, src_gray, CV_BGR2GRAY );

    Mat canny_output;
    vector<vector<Point> > contours;
    vector<Vec4i> hierarchy;

    /// Detect edges using canny
    int thresh=100;
    Canny( src_gray, canny_output, thresh, thresh*2, 3 );

    /// Find contours
    findContours( canny_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );

    // testing the approximate polygon
    cv::Mat result(src_gray.size(),CV_8U,cv::Scalar(255));
    for(int i=0;i<contours.size();i=i+4) //for testing reasons. Skeletonize input.
        std::vector<cv::Point> poly;
            5, // accuracy of the approximation
            true); // yes it is a closed shape

        // Iterate over each segment and draw it
        std::vector<cv::Point>::const_iterator itp= poly.begin();
        cout<<"\ncontour points:"<<poly.size();
        while (itp!=(poly.end()-1)) {
        // last point linked to first point



@Nguyen Minh Binh 2013-05-22 16:03:03

Based on the contour points, can I determine current shape is a rectangle/circle/... ? Is there any rule to determine this? And then, for example, If the shape is a circle, Can I get more information about this shape? e.g. Could I do some thing to get the center point and radius?

@William 2013-05-22 20:30:40

Well, yes you can.Search about cvhoughcircles, contours, cvmoments... Good luck.

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