Java Graphics（2D）用于屏幕外渲染

Question

I'm getting into Java graphics at the moment and I'm reading the book Killer game programming in Java by Andrew Davison. He programs a simple window application that uses the Image class for off-screen rendering and then copying the image to the component. What I find confusing is that one can just create the image and then get the graphics context with:

dbg = dbImage.getGraphics();

I find it confusing, because then we only use the dbg Graphics object to draw stuff but later on we use the dbImage in the paintComponent method to display all the stuff we have drawn to the dbg Object:

g.drawImage(dbImage, 0, 0, null);

So how does the dbImage "know", that it contains all the graphics stuff we have drawn onto the dbg Graphics object?

Here is the whole code:

public class GamePanel extends JPanel implements Runnable{

    private static final int PWIDTH = 500;
    private static final int PHEIGHT = 400;

    private Thread animator;

    private volatile boolean running = false;

    private volatile boolean gameOver = false;

    private Graphics dbg;
    private Image dbImage = null;

    private Counter counter;

    public GamePanel(){
        setBackground(Color.white);
        setPreferredSize(new Dimension(PWIDTH, PHEIGHT));

        // create game components eg. counter.
        counter = new Counter(10);
        counter.start();
    }

    private void stopGame(){
        running = false;
    }

    @Override public void addNotify(){
        super.addNotify();
        startGame();
    }

    public void startGame(){
        if(animator == null || !running){
            animator = new Thread(this);
            animator.start();
        }
    }

    @Override
    public void run() {
        running = true;
        while(running){
            gameUpdate();
            gameRender();
            repaint();
            try{
                Thread.sleep(20);
            }
            catch(InterruptedException ex){
                // do something with the exception...
            }
        }
    }

    private void gameUpdate(){
        if(!gameOver){
            // update game state...
             if(counter.getCounter() == 0)
                 gameOver = true;

        }
    }

    private void gameRender(){
        if(dbImage == null){
            dbImage = createImage(PWIDTH, PHEIGHT);
            if(dbImage == null){
                System.out.println("dbImage is null");
                return;
            }       
            else
                dbg = dbImage.getGraphics(); // get graphics context for drawing to off-screen images.
        }

        // clear the background
        dbg.setColor(Color.white);
        dbg.fillRect(0, 0, PWIDTH, PHEIGHT);

        // draw game elements here...
        dbg.setColor(Color.black);
        dbg.drawString(Integer.toString(counter.getCounter()), 10, 10);

        if(gameOver)
            dbg.drawString("Game over...", 10, 20);
    }

    @Override public void paintComponent(Graphics g){
        super.paintComponent(g);
        if(dbImage != null)
            g.drawImage(dbImage, 0, 0, null);
    }
}

the important parts are only the paintComponent method and the gameRender() method though.

Answer 1

Basically, a Graphics object is not something you draw on . It's something you draw with .

When you call dbImage.getGraphics() , you are saying "Give me a toolbox that allows me to draw on this image". All of the draw methods in a Graphics object draw on the component for which it was created. They are not drawn on the Graphics object, they are drawn on the Image or the Component that belong to it.

Think of the Graphics object as the palette and brushes, and on the Image or Component as the canvas.

So when you are done running the operations with your dbg , it means you have an image full of colorful pixels that this Graphics object drew on it. Now you can take this image and copy it to another component - by using the drawImage "tool" in that component's Graphics - its "toolbox".