Thresholding is a method of image segmentation, in general it is used to create binary images. Thresholding is of two types namely, simple thresholding and adaptive thresholding.

Simple Thresholding

In simple thresholding operation the pixels whose values are greater than the specified threshold value, are assigned with a standard value.

You can perform simple threshold operation on an image using the method threshold() of the Imgproc class, Following is the syntax of this method.

threshold(src, dst, thresh, maxval, type)

This method accepts the following parameters −

• src − An object of the class Mat representing the source (input) image.

• dst − An object of the class Mat representing the destination (output) image.

• thresh − A variable of double type representing the threshold value.

• maxval − A variable of double type representing the value that is to be given if pixel value is more than the threshold value.

• type − A variable of integer type representing the type of threshold to be used.

Example

The following program demonstrates how to perform simple thresholding operation on an image in OpenCV.

import org.opencv.core.Core;
import org.opencv.core.Mat;
import org.opencv.imgcodecs.Imgcodecs;
import org.opencv.imgproc.Imgproc;

public class Thresh {
   public static void main(String args[]) {
      // 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/chap14/thresh_input.jpg";
      Mat src = Imgcodecs.imread(file);

      // Creating an empty matrix to store the result
      Mat dst = new Mat();
      Imgproc.threshold(src, dst, 50, 255, Imgproc.THRESH_BINARY);

      // Writing the image
      Imgcodecs.imwrite("E:/OpenCV/chap14/thresh_trunc.jpg", dst);

      System.out.println("Image Processed");
   }
}

Assume that following is the input image thresh_input.jpg specified in the above program.

Output

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

Image Processed

If you open the specified path, you can observe the output image as follows −

Other types of simple thresholding

In addition to the THRESH_BINARY operation demonstrated in the previous example, OpenCV caters various other types of threshold operations. All these types are represented by predefined static fields (fixed values) of Imgproc class.

You can choose the type of the threshold operation you need, by passing its respective predefined value to the parameter named type of the threshold() method.

Imgproc.threshold(src, dst, 50, 255, Imgproc.THRESH_BINARY);

Following are the values representing various types of threshold operations and their respective outputs.