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

Knute Johnson <>
Wed, 13 Aug 2008 20:52:48 -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'm assuming you want to draw an image inside of an anti-aliased ring of
background color. If that's not what you want ignore the code below.

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

public class FuzzyHole extends JPanel {
     BufferedImage image,mask;

     public FuzzyHole() {
         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
             // set composite to SrcOver
             // set stroke a little bigger to avoid rings
             g.setStroke(new BasicStroke(2f));
             // draw concentric circles with increasing alpha
             for (int i=0; i<23; i++) {
                 // background color with alpha
                 g.setColor(new Color(255,255,255,255-i*11));
         } 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();
                 FuzzyHole fh = new FuzzyHole();


Knute Johnson
email s/nospam/knute2008/

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