How to fit an ellipse in an image to surround possible objects using OpenCV Java?

You can use the fitEllipse() method of the org.opencv.imgproc.Imgproc class to fit an ellipse in shape. This method accepts an object of class MatOfPoint2f, computes an ellipse that fits the given set of points, and returns a RotatedRect object.

Using this method you can draw an ellipse around possible objects in the image. To do this, read the image using the imread()

method of the Imgproc class.

• Use the cvtColor()

  method of the Imgproc class to convert it to a grayscale image.

• Use the threshold()

  method of the Imgproc class to convert the grayscale image to a binary image.

• Use the findContours()

  method of the Imgproc class to find contours in the image.

• Now, pass each contour value as a MatOfPoint2f to the fitEllipse()

  method to get the
• RotatedRec

  ## of the possible contours #Object. Finally, use the ellipse() method to draw an ellipse around the possible shape.

• Note − To fit an ellipse, the object should contain at least five points.

Example

import java.util.ArrayList;
import java.util.List;
import org.opencv.core.Core;
import org.opencv.core.Mat;
import org.opencv.core.MatOfPoint;
import org.opencv.core.MatOfPoint2f;
import org.opencv.core.RotatedRect;
import org.opencv.core.Scalar;
import org.opencv.highgui.HighGui;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;
public class FitEllipseExample {
   public static void main(String args[]) throws Exception {
      //Loading the OpenCV core library
      System.loadLibrary( Core.NATIVE_LIBRARY_NAME );
      //Reading the contents of the image
      String file ="D:\Images\javafx_graphical.jpg";
      Mat src = Imgcodecs.imread(file);
      //Converting the source image to binary
      Mat gray = new Mat(src.rows(), src.cols(), src.type());
      Imgproc.cvtColor(src, gray, Imgproc.COLOR_BGR2GRAY);
      Mat binary = new Mat(src.rows(), src.cols(), src.type(), new Scalar(0));
      Imgproc.threshold(gray, binary, 100, 255, Imgproc.THRESH_BINARY_INV);
      //Finding Contours
      List<MatOfPoint> contours = new ArrayList<>();
      Mat hierarchey = new Mat();
      Imgproc.findContours(binary, contours, hierarchey, Imgproc.RETR_TREE,
      Imgproc.CHAIN_APPROX_SIMPLE);
      //Empty rectangle
      RotatedRect[] rec = new RotatedRect[contours.size()];
      for (int i = 0; i < contours.size(); i++) {
         rec[i] = new RotatedRect();
         if (contours.get(i).rows() > 5) {
            rec[i] = Imgproc.fitEllipse(new MatOfPoint2f(contours.get(i).toArray()));
         }
         Scalar color_elli = new Scalar(190, 0, 0);
         Imgproc.ellipse(src, rec[i], color_elli, 5);
      }
      HighGui.imshow("Contours operation", src);
      HighGui.waitKey();
   }
}

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