OpenCV - Drawing Convex Polylines


You can draw convex polylines on an image using the method fillconvexPoly() of the imgproc class. Following is the syntax of this method.

fillConvexPoly(Mat img, MatOfPoint points, Scalar color)

This method accepts the following parameters −

  • mat − A Mat object representing the image on which the convex Polylines are to be drawn.

  • points − A MatOfPoint object representing points between which the convex polylines are to be drawn.

  • scalar − A Scalar object representing the color of the convex Polylines. (BGR)

The constructor of the MatOfPoint class accepts objects of the class Point.

MatOfPoint(Point... a)


The following program demonstrates how to draw convex polylines on an image and display it using JavaFX window.

import java.awt.image.BufferedImage;

import javax.imageio.ImageIO;

import javafx.application.Application;
import javafx.embed.swing.SwingFXUtils;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.image.ImageView;
import javafx.scene.image.WritableImage;
import javafx.stage.Stage;

import org.opencv.core.Core;
import org.opencv.core.Mat;
import org.opencv.core.MatOfByte;
import org.opencv.core.MatOfPoint;
import org.opencv.core.Point;
import org.opencv.core.Scalar;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;

public class FillConvexPoly extends Application {
   Mat matrix = null;

   public void start(Stage stage) throws Exception {
      // Capturing the snapshot from the camera
      FillConvexPoly obj = new FillConvexPoly();
      WritableImage writableImage = obj.LoadImage();

      // Setting the image view
      ImageView imageView = new ImageView(writableImage);

      // setting the fit height and width of the image view

      //Setting the preserve ratio of the image view

      // Creating a Group object
      Group root = new Group(imageView);

      // Creating a scene object
      Scene scene = new Scene(root, 600, 400);

      // Setting title to the Stage
      stage.setTitle("Drawing convex Polylines (fill) on the image");

      // Adding scene to the stage

      // Displaying the contents of the stage;
   public WritableImage LoadImage() throws Exception {
      // Loading the OpenCV core library
      System.loadLibrary( Core.NATIVE_LIBRARY_NAME );

      // Reading the Image from the file and storing it in to a Matrix object
      String file ="E:/OpenCV/chap8/input.jpg";
      Mat matrix = Imgcodecs.imread(file);

      MatOfPoint matOfPoint = new MatOfPoint (
         new Point(75, 100), new Point(350, 100),
         new Point(75, 150), new Point(350, 150),
         new Point(75, 200), new Point(350, 200),
         new Point(75, 250), new Point(350, 250)
      // Drawing polylines
      Imgproc.fillConvexPoly (
         matrix,                       // Matrix obj of the image
         matOfPoint,                   // java.util.List<MatOfPoint> pts
         new Scalar(0, 0, 255)         // Scalar object for color
      // Encoding the image
      MatOfByte matOfByte = new MatOfByte();
      Imgcodecs.imencode(".jpg", matrix, matOfByte);

      // Storing the encoded Mat in a byte array
      byte[] byteArray = matOfByte.toArray();

      // Displaying the image
      InputStream in = new ByteArrayInputStream(byteArray);
      BufferedImage bufImage =;
      this.matrix = matrix;
      // Creating the Writable Image
      WritableImage writableImage = SwingFXUtils.toFXImage(bufImage, null);
      return writableImage;
   public static void main(String args[]) {

On executing the above program, you will get the following output −

Drawing Convex Polylines