Re: transparency in PNG images

Knute Johnson <>
Mon, 10 Jan 2011 15:30:12 -0800
On 01/10/2011 01:59 PM, John B. Matthews wrote:

In article<C2HWo.10145$111.6619@newsfe12.iad>,
  Knute Johnson<> wrote:

On 01/08/2011 11:49 PM, John B. Matthews wrote:

In article<>,
   Roedy Green<> wrote:

Is there any way to get part of the image back to transparent? Or
do I have to carefully arrange things that I never paint any part
of the image that will be transparent?

You should be able to fill with Clear:

      g2d.fillRect(x, y, w, h);

Here's some examples:


Doesn't Clear set the alpha to 1.0f?

Yes, the static instance named AlphaComposite.Clear has its alpha set to
1.0, but that value does not change what the AlphaComposite.CLEAR rule
does. When the graphics context's composite is set to an instance of
Clear, no pixels change. Later, as fillRect() modifies the destination
pixels, the CLEAR rule is applied: "Both the color and the alpha of the
destination are cleared (Porter-Duff Clear rule). Neither the source nor
the destination is used as input." It's the one (only?) scenario in
which you can "paint with alpha," if I may borrow your phrase.

When the CLEAR rule has been chosen in the demo, getPixel() always
reports { 0, 0, 0, 0 }, irrespective of any other color or alpha
settings. The demo is slightly misleading, as it uses the symbol "Clear"
to represent the rule named "CLEAR" in the Java API and named "clear" in
the Porter-Duff article.

I just discovered why this has been causing me so much problem in the
past. Well one of the reasons anyway.

If you Clear an image that is OPAQUE the result is black with an alpha
of 1.0f. If you Clear a TRANSLUCENT image, the result is black with an
alpha of 0.0f. That is not well documented at least for the
semi-literate amongst us.

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

public class test extends JPanel {
     BufferedImage kittens;

     public test() throws Exception {
         URL url = new URL("");
         kittens =;
// comment out next line to see opaque image
         kittens = convertToCompatible(kittens);
         if (kittens.getTransparency() == Transparency.OPAQUE)
         if (kittens.getTransparency() == Transparency.TRANSLUCENT)
         int w = kittens.getWidth();
         int h = kittens.getHeight();
         setPreferredSize(new Dimension(w,h));
         Graphics2D g = kittens.createGraphics();

     public void paintComponent(Graphics g) {

     public static void main(String[] args) {
         EventQueue.invokeLater(new Runnable() {
             public void run() {
                 try {
                     JFrame f = new JFrame();
                     f.add(new test(),BorderLayout.CENTER);
                 } catch (Exception e) {

     static BufferedImage convertToCompatible(BufferedImage image) {
         GraphicsEnvironment ge =
         GraphicsDevice gd = ge.getDefaultScreenDevice();
         GraphicsConfiguration gc = gd.getDefaultConfiguration();

         BufferedImage compatible =

         if (compatible.getType() == image.getType())
             return image;

         ColorConvertOp op = new ColorConvertOp(

         return op.filter(image,compatible);


Knute Johnson

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