Question

there. I’m using Java to build a desktop program which needs to frequently repaint. Just like below:

class myWindow extends JWindow {
    Image[] layers = new Image[8]  // Some png Image object filled here

    @Override
    public void paint(Graphics g) {
        int i;

        super.paint();
        for (i=0;i<=7;i++)
            g.paintImage(layers[i], /*other param*/);
    }
 }

As we can see, it costs time to paint, so I want to build a buffer for some of the layer. It should be an Image or Graphics object.
But then I found that I can take no method to merge the Image (just put them one on the other).
So my question is: How can I build a buffer as I mentioned above , or if I use paintImage() to draw on a alternative Graphics, How can I use this Graphics to draw my window
Thanks to all.

Answer 1

Here’s one way to merge the Image array.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.image.BufferedImage;

import javax.swing.JPanel;

class MyPanel extends JPanel {

    BufferedImage image;

    public MyPanel(Image[] layers) {
        this.image = createBufferedImage(layers);
    }

    protected BufferedImage createBufferedImage(Image[] layers) {
        Dimension maxSize = new Dimension();
        for (int i = 0; i < layers.length; i++) {
            Dimension d = new Dimension(layers[i].getWidth(this), 
                    layers[i].getHeight(this));
            maxSize = max(maxSize, d);
        }
        image = new BufferedImage(maxSize.width, maxSize.height, 
                BufferedImage.TYPE_INT_ARGB);
        Graphics g = image.getGraphics();
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, image.getWidth(), image.getHeight());

        for (int i = 0; i < layers.length; i++) {
            g.drawImage(layers[i], 0, 0, this);
        }
        g.dispose();
        return image;
    }

    protected Dimension max(Dimension a, Dimension b) {
        Dimension d = new Dimension();
        d.width = Math.max(a.width,  b.width);
        d.height = Math.max(a.height, b.height);
        return d;
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(image, 0, 0, this);
    }
 }