OpenCV Contour and Centroid using Moments

 In this post, I am demonstrating how OpenCV contour and Moments work, below code will do, load image, convert to gray, smooth image , threshold image, find contour and find moment and centroid for the contour. Finally draw contour centre on source image and display the result.

Step 1

 1. Load Image in color mode

  Mat src = imread("shapes.jpg", 1);

Step 2

 2. Convert to grayscale
 3. Blur operation to remove the noise
 4. Threshold the image to process contour

  Mat gray, thr;
  cvtColor(src, gray, COLOR_BGR2GRAY);
  blur(gray, gray, Size(10, 10), Point(-1, -1), BORDER_DEFAULT);
  threshold(gray, thr, 50, 255, THRESH_BINARY);

Step 3

 5. FindContours to get all contour
 6. Get moment and Centroid of the contour

  vector< vector > contours; // Vector for storing contour
  vector hierarchy;
  findContours(thr, contours, hierarchy, RETR_CCOMP, CHAIN_APPROX_SIMPLE); // Find the contours in the image

  // get the moments
  vector mu(contours.size());
  for (int i = 0; i < contours.size(); i++) {
    mu[i] = moments(contours[i], false);
  }

  // get the centroid
  vector mc(contours.size());
  for (int i = 0; i < contours.size(); i++) {
    mc[i] = Point2f(mu[i].m10 / mu[i].m00, mu[i].m01 / mu[i].m00);
  }

  // draw centroid
  for (int i = 0; i < contours.size(); i++) {
    circle(src, mc[i], 5, Scalar(255, 255, 255), -1, 8,0); // draw centroid circle
    line(src, mc[i], Point(mc[i].x, mc[i].y - 100), Scalar(255, 255, 255), 1,8);
    String txt = format("Contour %d", i);
    putText(src, txt, Point(mc[i].x - 50, mc[i].y - 110), FONT_HERSHEY_SIMPLEX, 1, CV_RGB(255, 0, 0), 2);
  }

Full C++ Code

  Mat src = imread("shapes.jpg", 1);
  Mat gray, thr;
  cvtColor(src, gray, COLOR_BGR2GRAY);
  blur(gray, gray, Size(10, 10), Point(-1, -1), BORDER_DEFAULT);
  threshold(gray, thr, 50, 255, THRESH_BINARY);
  vector< vector > contours; // Vector for storing contour
  vector hierarchy;
  findContours(thr, contours, hierarchy, RETR_CCOMP, CHAIN_APPROX_SIMPLE); // Find the contours in the image

  // get the moments
  vector mu(contours.size());
  for (int i = 0; i < contours.size(); i++) {
    mu[i] = moments(contours[i], false);
  }

  // get the centroid
  vector mc(contours.size());
  for (int i = 0; i < contours.size(); i++) {
    mc[i] = Point2f(mu[i].m10 / mu[i].m00, mu[i].m01 / mu[i].m00);
  }

  // draw centroid
  for (int i = 0; i < contours.size(); i++) {
    circle(src, mc[i], 5, Scalar(255, 255, 255), -1, 8, 0); // draw centroid circle
    line(src, mc[i], Point(mc[i].x, mc[i].y - 100), Scalar(255, 255, 255), 1, 8);
    String txt = format("Contour %d", i);
    putText(src, txt, Point(mc[i].x - 50, mc[i].y - 110), FONT_HERSHEY_SIMPLEX, 1, CV_RGB(255, 0, 0), 2);
  }

  imshow("thr", thr);
  imshow("src", src);
  imwrite("contour.jpg", src);
  imwrite("contour_thr.jpg", thr);

  waitKey();