Histogram Equalization Algorithm and Implementation in Java

Histogram equalization is a technique used to enhance the contrast of image using the histogram of image. The histogram of image represents the frequency of gray levels in the image.The gray levels of image varying from 0 to 255, that is a gray scale image's pixel size is 8 bits(1 byte). So the histogram contains frequency of occurrence of values from 0 to 255.
The aim of histogram equalization is used in digital image processing is to generate an image with equally distributed brightness level over the whole brightness scale.

Histogram equalization can enhance contrast for brightness values close to histogram maxima and decrease contrast near minima.

ALGORITHM

Step 1. Image size: NxM, gray level from 0 to 255, create an array H of size 256 and initialise it with 0.
Step 2: Create image histogram by scan every pixel of image and increment the relevant member of array.
            H[grayval(pix)] = H[grayval(pix)]+1.
Step 3: Form a cumulative histogram CH of size 256.
            CH[0] = H[0]
            CH[i] =  CH[i-1] + H[i], i=1,2,3,...255.
Step 4: Set T[i] = Round((255*CH[i])/(NxM))
Step 4: Rescan image and create new image with gray level value,
            NewImg[x][y] = T[OldImg[x][y]]

SCREENSHOTS

Histogram Equalisation in Java: 1

Histogram Equalisation in Java: 2

SOURCE CODE

import java.awt.GridLayout;
import java.awt.image.BufferedImage;
import java.awt.image.WritableRaster;
import java.io.File;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;


/**
 *
 * @author AJITH KP (http://fb.com/ajithkp560)
 * http://www.terminalcoders.blogspot.com
 * 
 */
public class HistogramEqualization extends JFrame{
    HistogramEqualization(String in, String out){
        super("..:: Histogram Equalization ::..");
        try {
            this.setLayout(new GridLayout(1, 2, 10, 10));
            JPanel img1 = new JPanel();
            JPanel img2 = new JPanel();
            File f1 = new File(in);
            File f2 = new File(out);
            BufferedImage image1 = getGrayscaleImage(ImageIO.read(f1));
            img1.add(new JLabel(new ImageIcon(image1)));
            BufferedImage image2 = equalize(image1);
            ImageIO.write(image2, "png", f2);
            img2.add(new JLabel(new ImageIcon(image2)));
            this.add(img1);
            this.add(img2);
        } catch (Exception ex) {
            System.out.println(ex.getMessage());
        }
    }
    BufferedImage equalize(BufferedImage src){
        BufferedImage nImg = new BufferedImage(src.getWidth(), src.getHeight(), BufferedImage.TYPE_BYTE_GRAY);
        WritableRaster wr = src.getRaster();
        WritableRaster er = nImg.getRaster();
        int totpix= wr.getWidth()*wr.getHeight();
        int[] histogram = new int[256];

        for (int x = 1; x < wr.getWidth(); x++) {
            for (int y = 1; y < wr.getHeight(); y++) {
                histogram[wr.getSample(x, y, 0)]++;
            }
        }
        
        int[] chistogram = new int[256];
        chistogram[0] = histogram[0];
        for(int i=1;i<256;i++){
            chistogram[i] = chistogram[i-1] + histogram[i];
        }
        
        float[] arr = new float[256];
        for(int i=0;i<256;i++){
            arr[i] =  (float)((chistogram[i]*255.0)/(float)totpix);
        }
        
        for (int x = 0; x < wr.getWidth(); x++) {
            for (int y = 0; y < wr.getHeight(); y++) {
                int nVal = (int) arr[wr.getSample(x, y, 0)];
                er.setSample(x, y, 0, nVal);
            }
        }
        nImg.setData(er);
        return nImg;
    }
    BufferedImage getGrayscaleImage(BufferedImage src) {
        BufferedImage gImg = new BufferedImage(src.getWidth(), src.getHeight(), BufferedImage.TYPE_BYTE_GRAY);
        WritableRaster wr = src.getRaster();
        WritableRaster gr = gImg.getRaster();
        for(int i=0;i<wr.getWidth();i++){
            for(int j=0;j<wr.getHeight();j++){
                gr.setSample(i, j, 0, wr.getSample(i, j, 0));
            }
        }
        gImg.setData(gr);
        return gImg;
    }
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        System.out.println("#! TERMINALCODERS ::..\nhttp://www.terminalcoders.blogspot.com");
        if(args.length<2){
            System.out.println("Usage: java HistogramEqualization <input file name> <output file name>");
        }
        else{
            HistogramEqualization he = new HistogramEqualization(args[0], args[1]);
            he.setSize(1024, 500);
            he.setVisible(true);
            he.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        }
    }
    
}