Make image transparent outside of selected area.

Question

I have some code that paints a BufferedImage to screen with ability to draw a single rectangle in the image at any location.
I want the part of the image that is not in the rectangle to have some transparency.
How can I accomplish this?
my Current code is:
    public void paint(Graphics g) {
         Graphics2D g2 = (Graphics2D) g;
         g2.clearRect(0, 0, this.getWidth(), this.getHeight());
         g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
               RenderingHints.VALUE_ANTIALIAS_ON);
         if (snapshot != null) {
            g2.drawImage(snapshot, 0, 0, this);
         g2.setStroke(dashed);
         g2.setColor(Color.WHITE);
         rectX = start.getX();
         rectY = start.getY();
         rectWidth = end.getX() - start.getX();
         rectHeight = end.getY() - start.getY();
         if (rectWidth < 0) {
            rectX += rectWidth;
            rectWidth *= -1;
         if (rectHeight < 0) {
            rectY += rectHeight;
            rectHeight *= -1;
         g2.draw(new Rectangle2D.Double(rectX, rectY, rectWidth, rectHeight));
      }start and end are of type Point.
snapshot is a BufferedImage

Answer 1

You can draw portions of an image with the other graphics#drawImage methods. Most notably this one
drawImage(Image img, int dx1, int dy1, int dx2, int dy2,
                     int sx1, int sy1, int sx2, int sy2,
                     ImageObserver observer)So you can draw the four regions of the image not included in the rectangle with a translucent AlphaComposite using the above method (4 times). And then you can use it one more time to draw the rectangular region with an opaque AlphaComposite.
Also, if start is of type Point then your rectangular coordinates are integers. This implies you can use g2.drawRect instead of creating a new Rectangle2D.Double object each paint call. It's just a side not though. It dosen't really matter much.
Edited by: Maxideon on Apr 6, 2009 11:25 PM
Too late, seems you found a way.
Edited by: Maxideon on Apr 6, 2009 11:26 PM
Ah, foolish me. I was thinking of the hard way. You can just draw the whole image at a certain transparency. And then draw the rectangular region as opaque.