OpenCV - Drawing a Line


You can draw a line on an image using the method line() of the imgproc class. Following is the syntax of this method.

line(img, pt1, pt2, color, thickness)

This method accepts the following parameters −

  • mat − A Mat object representing the image on which the line is to be drawn.

  • pt1 and pt2 − Two Point objects representing the points between which the line is to be drawn.

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

  • thickness − An integer representing the thickness of the line; by default, the value of thickness is 1.


The following program demonstrates how to draw a line 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.Point;
import org.opencv.core.Scalar;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;

public class DrawingLine extends Application {
   Mat matrix = null;

   public void start(Stage stage) throws Exception {
      // Capturing the snapshot from the camera
      DrawingLine obj = new DrawingLine();
      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 a line 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);

      // Drawing a line
      Imgproc.line (
         matrix,                    //Matrix obj of the image
         new Point(10, 200),        //p1
         new Point(300, 200),       //p2
         new Scalar(0, 0, 255),     //Scalar object for color
         5                          //Thickness of the line
      // 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 Line

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