gray1

import java.awt.*;
import java.awt.image.*;
import java.awt.color.*;
import java.io.*;
import javax.imageio.*;
import javax.swing.*;

public class gray1 {

    public static void main(String [] args) {

	String filename = "bld.jpg";
	if (args.length>0) filename = args[0];

	showImage f1 = new showImage(filename);
	f1.setLocation(100,100);
	f1.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
	int w = f1.getWidth();
	int h = f1.getHeight();

	long start = System.nanoTime();
	BufferedImage grayImage = rgb2gray(f1.img);
	long etime = System.nanoTime() - start;
	double elapsed_time = 1e-6*(double)etime;
	System.out.format("Elapsed time %g milliseconds%n",elapsed_time);
	double area = f1.img.getWidth()*f1.img.getHeight()*1e-6;
	System.out.format("%d x %d Area %g megapixels%n",f1.img.getWidth(),
		f1.img.getHeight(), area);
	System.out.format("Time per megapixel %g milliseconds%n",elapsed_time/area);	

	showImage f2 = new showImage(grayImage,"gray version");
	f2.setLocation(120+w,100);
	f2.writeImage("gray","png");	
    }


    public static BufferedImage rgb2gray(BufferedImage img) {
        int w1 = img.getWidth();
        int h1 = img.getHeight();
        // int value[][] = new int[w1][h1];
        BufferedImage gray = new BufferedImage(w1, h1, 1);// new image
        int value, alpha, r, g, b;
        for (int i = 0; i < w1; i++) {
            for (int j = 0; j < h1; j++) {
                value = img.getRGB(i, j); // read and store pixel value
                alpha = get_alpha(value);
                r = get_red(value);
                g = get_green(value);
                b = get_blue(value);

                value = (r + g + b) / 3; // grey by averaging the pixels
                r = g = b = value;
                value = create_rgb(alpha, r, g, b);
                gray.setRGB(i, j, value);
            }
        }
        return gray;
    }

    public static int create_rgb(int alpha, int r, int g, int b) {
        int rgb = (alpha << 24) + (r << 16) + (g << 8) + b;
        return rgb;
    }

    public static int get_alpha(int rgb) {
        return (rgb >> 24) & 0xFF;
        // return rgb & 0xFF000000;
    }

    public static int get_red(int rgb) {
        return (rgb >> 16) & 0xFF;
        // return rgb & 0x00FF0000;
    }

    public static int get_green(int rgb) {
        return (rgb >> 8) & 0xFF;
    }

    public static int get_blue(int rgb) {
        return rgb & 0xFF;
    }
}

Results