OpenCV - Histogram Equalization


The histogram of an image shows the frequency of pixels’ intensity values. In an image histogram, the X-axis shows the gray level intensities and the Y-axis shows the frequency of these intensities.

Histogram equalization improves the contrast of an image, in order to stretch out the intensty range. You can equalize the histogram of a given image using the method equalizeHist() of the Imgproc class. Following is the syntax of this method.

equalizeHist(src, dst)

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 output. (Image obtained after equalizing the histogram)


The following program demonstrates how to equalize the histogram of a given image.

import java.util.ArrayList;
import java.util.List;

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

public class HistoTest {
   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/chap20/histo_input.jpg";

      // Load the image
      Mat img = Imgcodecs.imread(file);

      // Creating an empty matrix
      Mat equ = new Mat();

      // Applying blur
      Imgproc.blur(equ, equ, new Size(3, 3));

      // Applying color
      Imgproc.cvtColor(equ, equ, Imgproc.COLOR_BGR2YCrCb);
      List<Mat> channels = new ArrayList<Mat>();

      // Splitting the channels
      Core.split(equ, channels);

      // Equalizing the histogram of the image
      Imgproc.equalizeHist(channels.get(0), channels.get(0));
      Core.merge(channels, equ);
      Imgproc.cvtColor(equ, equ, Imgproc.COLOR_YCrCb2BGR);

      Mat gray = new Mat();
      Imgproc.cvtColor(equ, gray, Imgproc.COLOR_BGR2GRAY);
      Mat grayOrig = new Mat();
      Imgproc.cvtColor(img, grayOrig, Imgproc.COLOR_BGR2GRAY);

      Imgcodecs.imwrite("E:/OpenCV/chap20/histo_output.jpg", equ);
      System.out.println("Image Processed");

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

Histo Input


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 −

Histo Output