Implementing OpenCV’s probabilistic Hough line transform in Java

Use the Hough line transform to detect straight lines in a given image. There are two Hough line transforms available in OpenCV, namely standard Hough line transform and probabilistic Hough line transform.

You can apply the Probabilistic Hough Line Transform using the HoughLinesP() method of the Imgproc class, which accepts the following parameters:

• Two Mat objects representing the source image and the vector storing the line parameters (r, Φ).

• Two double variables representing the resolution of parameters r (pixels) and Φ (radians).

• An integer representing the minimum number of intersections required to "detect" a line.

Example

The following Java example uses OpenCV’s probabilistic Hough line transform to detect lines in an image:

import java.awt.Image;
import java.awt.image.BufferedImage;
import java.io.IOException;
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.Point;
import org.opencv.core.Scalar;
import org.opencv.highgui.HighGui;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;
public class HoughLineProbabilisticTransform extends Application {
   public void start(Stage stage) throws IOException {
      //Loading the OpenCV core library
      System.loadLibrary( Core.NATIVE_LIBRARY_NAME );
      String file ="D:\Images\road4.jpg";
      Mat src = Imgcodecs.imread(file);
      //Converting the image to Gray
      Mat gray = new Mat();
      Imgproc.cvtColor(src, gray, Imgproc.COLOR_RGBA2GRAY);
      //Detecting the edges
      Mat edges = new Mat();
      Imgproc.Canny(gray, edges, 60, 60*3, 3, false);
      // Changing the color of the canny
      Mat cannyColor = new Mat();
      Imgproc.cvtColor(edges, cannyColor, Imgproc.COLOR_GRAY2BGR);
      //Detecting the hough lines from (canny)
      Mat lines = new Mat();
      Imgproc.HoughLinesP(edges, lines, 1, Math.PI/180, 50, 50, 10);
      for (int i = 0; i < lines.rows(); i++) {
         double[] data = lines.get(i, 0);
         //Drawing lines on the image
         Point pt1 = new Point(data[0], data[1]);
         Point pt2 = new Point(data[2], data[3]);
         Imgproc.line(cannyColor, pt1, pt2, new Scalar(0, 0, 255), 3);
      }
      //Converting matrix to JavaFX writable image
      Image img = HighGui.toBufferedImage(cannyColor);
      WritableImage writableImage= SwingFXUtils.toFXImage((BufferedImage) img, null);
      //Setting the image view
      ImageView imageView = new ImageView(writableImage);
      imageView.setX(10);
      imageView.setY(10);
      imageView.setFitWidth(575);
      imageView.setPreserveRatio(true);
      //Setting the Scene object
      Group root = new Group(imageView);
      Scene scene = new Scene(root, 595, 400);
      stage.setTitle("Hough Line Transform");
      stage.setScene(scene);
      stage.show();
   }
   public static void main(String args[]) {
      launch(args);
   }
}

Input image

Output

After execution, the above code will produce the following output−

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