Re: Has working with raster graphics always been this complicated?

Knute Johnson <>
Tue, 26 Feb 2008 14:37:26 -0800
<47c45d2c$0$5828$> wrote:

OK, I'm getting close, but I seem to be missing something. I've
included my code so far. I know it's a bit messy, but I'm still
learning to tighten up java code. This is a proof of concept for me.
It achieves something similar to what I wanted to do originally. I
currently have the program draw an oval (circle) where the mouse
clicks. That's enough to let me see what happens real-time.
Eventually, I'll have the reveal "expand" from the point where the
user clicks.

Anyway, drawing an expanding oval or box isn't the problem. This
example takes one image file (revealArea) and is supposed to overlay
it on another image file (baseImage). I expected to be able to click
in the window and wherever my mouse is, an oval will "paint a hole"
into the revealArea image, exposing part of the baseImage below.

I currently have the code of the revealArea image commented out. In
lieu of that image, I simply draw a white filled rectangle to
substitute. The rectangle overlays the base image fine (as witnessed
from the flicker). Drawing the oval does something unexpected. I set
the paint color to blue with an alpha of 30. I tried an alpha of zero,
expecting the hole to appear. When I click in the window, I get a
translucent blue circle blended with the white rectangle and not the
"hole" I expected to be formed. Is there a way to "paint" the hole?
Also, what techniques can I use to avoid the flicker of the images
drawing over one another?


import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import java.lang.Math;
import javax.swing.*;
import javax.imageio.*;

public class Reveal extends Applet
   implements MouseMotionListener {

   int width, height;
   int mx, my; // the mouse coordinates
   Image baseImage;
   Image revealArea;
   Image backbuffer;
   BufferedImage revealBuffer;
   Graphics revealOverlay;

   public void init() {
      width = getSize().width;
      height = getSize().height;
      setBackground( );

      mx = width/2;
      my = height/2;

      baseImage = getImage( getDocumentBase(), "baseimage.gif" );
      revealArea = getImage( getDocumentBase(),"overlay.gif");

      revealBuffer = new BufferedImage(100, 100,
      revealOverlay = revealBuffer.getGraphics();
      revealOverlay.fillRect(10, 10, 80, 80);
      //revealOverlay.drawImage(scratchArea, 0, 0, 100, 100, null);


      addMouseMotionListener( this );

   public void mouseMoved( MouseEvent e ) {

   public void mouseDragged( MouseEvent e ) {
      int x = e.getX();
      int y = e.getY();

      revealOverlay.setColor(new Color(0, 0, 255, 30));
      revealOverlay.fillOval(x-5, y-5, 20, 20);


   public void paint( Graphics g ) {

      g.drawImage(baseImage, 0, 0, null);
      g.drawImage(revealBuffer, 0, 0, null);

This can get really tricky. The problem usually turns out for me anyway
is that you can't draw on an image and reduce its alpha. You have to
use an AlphaComposite to set the pixel values to something less than
opaque. Then you can draw that image over another image and the image
behind will be visible through the alpha hole. If you draw translucent
pixels over opaque pixels the image is still opaque. See the example below.

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

public class test extends JPanel {
     BufferedImage bi = new
     Graphics2D imageG;

     public test() {
         setPreferredSize(new Dimension(400,300));

         imageG = bi.createGraphics();

         // the green overlay image will not be green until the first
         // time you press the mouse button
         addMouseListener(new MouseAdapter() {
             public void mousePressed(MouseEvent me) {
                 // paint the whole image green
                 // save the composite
                 Composite c = imageG.getComposite();
                 // clear a hole in the green image
                 // adjust the alpha to keep a percentage of green
                 // you could use AlphaComposite.CLEAR will be the same
as using
                 // an alpha of 0f
                 // restore the composite

     public void paintComponent(Graphics g2D) {
         Graphics2D g = (Graphics2D)g2D;

         // erase everything with white
         // draw blue and red X
         BasicStroke bs = new BasicStroke(20.0f);

         // draw image with translucent hole

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


Knute Johnson
email s/nospam/knute/

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