• example that reads each pixel of the screen
• input: each pixed
• output: manipulated pixels (overlay structure)
  • converts pixel into black and white.
  • red border to show manipulated pixels!

import java.awt.*;
import java.awt.image.*;

public class Test {

    public static void main(String[] args) throws AWTException {
        int xLen = 1000;    // width of the layer
        int yLen = 1000;    // height of the layer
        Robot r = new Robot();
        BufferedImage img = r.createScreenCapture(new Rectangle(0,0,xLen,yLen));

        // create overlayer Window
        Window w=new Window(null) {
            @Override
            public void update(Graphics g) {
                paint(g);
            }
            @Override
            public void paint(Graphics g) {
                // compute for each pixel corresponding greyscale
                for (int x=0; x<xLen; x++) {
                    for (int y=0; y<yLen; y++) {
                        // search for border-pixels!
                        if (x==0 || y==0 || x==xLen-1 || y==yLen-1) {
                            g.setColor(Color.RED);  // set borderColor to RED (255,0,0)
                        } else {
                            int rgb   = img.getRGB(x, y);
                            int red   = (rgb >> 16) & 0xFF;
                            int green = (rgb >>  8) & 0xFF;
                            int blue  = (rgb      ) & 0xFF;
                            int grey  = ((red+green+blue)/3);
                            g.setColor(new Color(grey, grey, grey));
                        }
                        // paint pixel 
                        // (@toDo: check if method for painting single pixel exists)
                        g.fillRect(x, y, 1, 1);		
                    }
                }
            }
        };
        w.setAlwaysOnTop(true);
        w.setBounds(w.getGraphicsConfiguration().getBounds());
        w.setBackground(new Color(0, true));
        w.setVisible(true);
    }
}