Re: Anti-aliasing in image clipping to non-rectangular sub-images

Knute Johnson <>
Fri, 15 Aug 2008 14:14:07 -0700
Daniele Futtorovic wrote:

On 15/08/2008 02:03, Knute Johnson allegedly wrote:

Daniele Futtorovic wrote:

On 14/08/2008 17:20, Knute Johnson allegedly wrote:

Well there's a truth that it took me a while to learn and that is
that you can't draw alpha into an image.

Must be jolly interesting if it's something that took you a while to
learn. So... what does that sentence mean?

If you have an image, you basically can't draw on it (with the usual
Graphics(2D) methods) and reduce the alpha value of the pixels in the
image. Say you have a pixel that is white with an alpha of 255.
There is no way to draw on the image to change that pixel's alpha to
another value. Unless of course you use an AlphaComposite with a
value of Clear. That will change it to a black pixel with an alpha of 0.

Hmmm. Having played around, and more importantly thought about it a bit,
I'll have to concur with your assessment.

For the record, here's the last state of my experiments. It achieves the
desired effect, but it's cheating (drawing twice and clipping in-between):

package scratch;

import java.awt.*;
import java.awt.geom.*;
import java.awt.image.*;
import javax.swing.*;
import javax.imageio.*;

 * @author da.futt
public class XORTest
    public static void main(String[] ss) {
        new Loader() {
            public BufferedImage loadImage() throws Exception {

    private static abstract class Loader
    implements Runnable
        public abstract BufferedImage loadImage()
        throws Exception;

        private Image prepareImage()
        throws Exception
            BufferedImage i = loadImage();

            BufferedImage bi = new BufferedImage(i.getWidth(),
            Graphics2D g = bi.createGraphics();


            int d = Math.min(i.getWidth(), i.getHeight()) >> 1;

            Shape ell = new Ellipse2D.Float((i.getWidth() - d) >> 1,
            (i.getHeight() - d) >> 1, d, d);

            Shape oldclip = g.getClip();


g.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC, .3f));
            g.drawImage(i, 0, 0, null);


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


            return bi;

        public void run() {
            try {
            catch (Exception x) {

        private void run0()
        throws Exception
            JFrame f = new JFrame("XOR paint test");
            f.setContentPane(new BackgroundPanel(new BorderLayout()));

            f.getContentPane().setPreferredSize(new Dimension(300, 200));

            f.getContentPane().add(new JLabel(new ImageIcon(
            prepareImage())), BorderLayout.CENTER);



    private static class BackgroundPanel
    extends JPanel
        private static final int MARMOR_SIDE_PX = 20;

        public BackgroundPanel() {

        public BackgroundPanel(LayoutManager layout) {

        public void paintComponent(Graphics g) {
            Color white = new Color(0xf0f0f0),
            black = new Color(0x0f0f0f);

            int x = 0, y = 0, line = 0, col = 0;

            for (final int h = getHeight(), w = getWidth(); y < h;
                    y += MARMOR_SIDE_PX, line++, col = 0, x = 0)
                for (; x < w; x += MARMOR_SIDE_PX, col++) {
                    g.setColor((col + line) % 2 == 0 ? black : white);
                    g.fillRect(x, y, MARMOR_SIDE_PX, MARMOR_SIDE_PX);

I like that! And the checkerboard pattern is a great idea too. I think
I'll steal that one for the future :-).


Knute Johnson
email s/nospam/knute2008/

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