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

Knute Johnson <>
Wed, 13 Aug 2008 20:57:55 -0700
Kenneth P. Turvey wrote:

I've got a simple problem that I'm sure someone in this group can help me
with. Let me say what I'm currently doing first.

I have an image that has some transparent portions and I want to cut a
circle out of the image. So first I trim it down to a square subimage
and then I go through it pixel by pixel and set the alpha channel to
clear for those pixels outside a circle with the sub-image's diameter.

This works great. I get close to exactly what I want. The customer has
added a new requirement that I need to deal with now. The sub-image
should have an anti-aliased border. That is, the pixels on the outside
of the circle shouldn't simply be on or off, but they should have an
alpha channel value that would most look the sub-image like a circle.

This is what you get when you use drawOval().

My thought is to use drawOval() to create a mask for the image, but I'm
not sure how to combine the mask with the image. I can't simply set the
alpha channel of the sub image to be the same as the alpha channel of my
mask since the image may have some transparency in it too. I'm quite
sure that Java has a way to combine these images. So how is it done?


I just answered this post in comp.lang.gui and now I see the back and
forth. If all you want is an anti-aliased mask use the code below.
Don't mess with your image, it's just too much work. If you want a
fancier matte around the image see the other post.

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

public class test extends JPanel {
     BufferedImage image,mask;

     public test() {
         try {
             // load the image
             image = File("kittens.jpg"));
             // setPreferredSize to size of image
             setPreferredSize(new Dimension(
             // create mask image
             mask = new BufferedImage(image.getWidth(),image.getHeight(),
             Graphics2D g = mask.createGraphics();
             // turn on anti-aliasing
             // fill with background color
             // set composite to clear
             // punch alpha whole in mask
         } catch (IOException ioe) {

     public void paintComponent(Graphics g) {

     public static void main(String[] args) {
         EventQueue.invokeLater(new Runnable() {
             public void run() {
                 JFrame f = new JFrame();
                 test fh = new test();


Knute Johnson
email s/nospam/knute2008/

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