Drawing transparent pixels into BufferedImage?

Question

Hi,
I'm trying to do this:
1) Create a BufferedImage
2) Paint its background to black
3) Paint a rectangle using a fully-transparent color
4) Output this BufferedImage in the form of a PNG file using ImageIO
My expectation is for step 3 to paint in "transparent" but what seems to be happening is that Graphics interprets my paint command as "draw nothing, it won't change the image anyway". Or maybe the PNG writer in JDK 1.5 does not actually output transparent pixels? Either way, I'm not getting the results I want. Any ideas?
Thanks,
Gili

Answer 1

one should use Graphics.clearRect() for clearing a regionI'm not so sure about that. The API for clearRect states:
...the background color of offscreen images may be system dependent. Applications should use setColor
followed by fillRect to ensure that an offscreen image is cleared to a specific color.
So I tried to do as this suggests:
import java.awt.*;
import java.awt.image.*;
import java.io.*;
import javax.imageio.*;
public class Test0 {
    public static void main(String[] args) throws IOException {
        int W = 100, H = 100;
        BufferedImage bi = new BufferedImage(W, H, BufferedImage.TYPE_INT_ARGB);
        Graphics2D g = bi.createGraphics();
        g.setColor(Color.BLACK);
        g.fillRect(0, 0, W, H);
        g.setColor(new Color(0,0,0,0));
        g.fillRect(W/4, H/4, W/2, H/2);
        g.dispose();
        ImageIO.write(bi, "png", new File("temp.png"));
}...and it doesn't work?! I'm thinking this is a bug. What to do? Can't get enough of that Porter-Duff:
import java.awt.*;
import java.awt.image.*;
import java.io.*;
import javax.imageio.*;
import javax.swing.*;
public class Test {
    public static void main(String[] args) throws IOException {
        int W = 100, H = 100;
        BufferedImage bi = new BufferedImage(W, H, BufferedImage.TYPE_INT_ARGB);
        Graphics2D g = bi.createGraphics();
        g.setColor(Color.BLACK);
        g.fillRect(0, 0, W, H);
        g.setComposite(AlphaComposite.Clear); //<<---- magic pixie dust
        g.fillRect(W/4, H/4, W/2, H/2);
        g.dispose();
        ImageIO.write(bi, "png", new File("temp.png"));
        JLabel lbl = new JLabel(new ImageIcon(bi));
        lbl.setOpaque(true);
        lbl.setBackground(Color.GREEN);
        JFrame f = new JFrame("Test");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(lbl);
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);
}