Java: Filling a BufferedImage with transparent pixels

After you clear the background with the CLEAR composite, you need to set it back to SRC_OVER to draw normally again. ex:

//clear
g2.setComposite(AlphaComposite.getInstance(AlphaComposite.CLEAR));
g2.fillRect(0,0,256,256);

//reset composite
g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER));
//draw
g2.setPaint(Color.RED);
g2.fillOval(50,50,100,100);

For the sake of completeness, here is a working, testing, and fast function that is cross-platform compliant.

  static public BufferedImage createTransparentBufferedImage(int width, int height) {
     // BufferedImage is actually already transparent on my system, but that isn't
     // guaranteed across platforms.
     BufferedImage bufferedImage = new BufferedImage(width, height, 
                        BufferedImage.TYPE_INT_ARGB);
     Graphics2D graphics = bufferedImage.createGraphics();

     // To be sure, we use clearRect, which will (unlike fillRect) totally replace
     // the current pixels with the desired color, even if it's fully transparent.
     graphics.setBackground(new Color(0, true));
     graphics.clearRect(0, 0, width, height);
     graphics.dispose();

     return bufferedImage;
  }

You could get the underlying int[] array of your BufferedImage (make sure to use a compatible format: that is, one that is backed by an int[]).

Then fill the int[] with ints whose alpha value are 0 (0 will do ; )

A System.arraycopy will be very fast.

You have to know that directly writing in the int[] is a lot faster than using setRGB.

Now BufferedImage are a bit of a black art in Java: depending on what you're doing and on which platform/JVM you're doing it, you may lose hardware acceleration (which may never have been there in the first place anyway). In addition to that, you may very well not care at all about hardware acceleration anyway because you may not be working on, say, a game requiring 60+ FPS to be playable etc.

This is a very complicated topic and there's more than one way to skin the BufferedImage cat. As far as I'm concerned I work directly in the int[] when I've got to mess at the pixel level because I think it makes much more sense than trying to use higher-level drawing primitives and I do really don't care about the potential lost of hardware acceleration.

If you cast the Graphics object to a Graphics2D object, you can set a Composite object thru

AlphaComposite composite = AlphaComposite.getInstance(AlphaComposite.CLEAR, 0.0f);
Graphics2D g2d = (Graphics2D) image.getGraphics();
g2d.setComposite(composite);
g2d.setColor(new Color(0, 0, 0, 0));
g2d.fillRect(0, 0, 10, 10);