Thread: XOR'ing BufferedImages onto a Graphics2D object

i tried to make it short but it still ended up relatively long.. here's the code:

import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import javax.swing.*;
import java.io.File;
import javax.imageio.ImageIO;
 
 
public class Example extends JFrame{
	MyPanel panel;
	BufferedImage bg, sprite;
 
	public static void main(String[] args){
		new Example();
	}
 
	public Example(){
		mainLoop();
	}
 
	public void mainLoop(){
		try{
		bg = 	ImageIO.read(new File("")); // your file
		sprite =  ImageIO.read(new File("")); // your file
		}
		catch(java.io.IOException e){
			e.printStackTrace();
		}
		JFrame frame = new JFrame();
		frame.setSize(200,200);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		panel = new MyPanel(bg, sprite);
		frame.getContentPane().add(panel);
		panel.getInputMap().put(KeyStroke.getKeyStroke("RIGHT"),"moveRight");
		panel.getActionMap().put("moveRight",moveRight);
		frame.setVisible(true);
		while(true){
			panel.updateScreen();
		}
	}
 
	Action moveRight = new AbstractAction() {
    	public void actionPerformed(ActionEvent e) {
        panel.xPos += 3;
    	}
	};
}
 
class MyPanel extends JPanel{
	BufferedImage buffer = new BufferedImage(200,200,BufferedImage.TYPE_4BYTE_ABGR);
	Graphics2D g2d;
	BufferedImage sprite;
	int xPos = 0;
 
	public MyPanel(BufferedImage bg,BufferedImage sp){
		g2d = buffer.createGraphics();
		g2d.drawImage(bg,null,0,0);
		sprite = sp;
	}
       // issue lies here.
	public void updateScreen(){
		Graphics2D screen = (Graphics2D) this.getGraphics();
		g2d.setXORMode(Color.WHITE);		            			
		g2d.drawImage(sprite,null,xPos,30);
		screen.drawImage(buffer,null,0,0);
		g2d.drawImage(sprite,null,xPos,30);
	}
}

if you take out g2d.setXORMode(Color.WHITE); , the program runs as expected. the sprite bufferedimage moves across the screen if you hold down the right arrow key.

however, it leaves image debris. i know that i could have my method draw the background, and then draw the sprite every frame, but that is slow. what i am trying to do is use XORMode so that you draw the sprite to the buffer, display the buffer (so the user sees the sprite), and then XOR the sprite again, leaving the buffer as just the background.

edit: and i just noticed i had Example implement JFrame but made a JFrame object in the class anyway.. ignore that and the many other flaws. this was written at around 1 in the morning.

Originally Posted by nemo

why not?

Because you don't know what state the Graphics component is in, and accessing it from a different thread (eg the main thread as opposed the EDT thread) makes matters much worse. Things like window resizes, events, and/or layouts can change the graphics object. Meaning it can get wiped out, transformed, or worse set to null while you are trying to access it, resulting in unexpected behavior. And when I say unexpected I mean unexpected - it may work now but down the line you never know. Using paintComponent ensures you will paint to a Graphics object with the expected behavior.

i had it working using paintComponent, but it was at least 5 times slower than using a buffer. plus, if i need to, i can using a SwingTimer to control the painting of the screen manually.

If its that much slower that it is noticeable to a human, you should profile the code and fix the bottleneck. Things you are doing should be lightning fast, and if they are not there are always ways to fix it and speed it up.

basically, what's the advantage of an EDT over manually creating a buffer?

No unexpected behavior I mentioned above, Listeners (ActionListener, MousetListener, KeyListener, etc...) already managed for you, more assurance the code is portable..are just a few. And you aren't manually creating a buffer, you are retrieving the one created for you by Swing in the EDT.

and lastly, can anyone answer my original question, how to use XORMode when dealing with moving sprites?

I haven't played with this option for some time, so shouldn't lead you astray with my assumptions.