Inner-Transparent Selection Window in Java using GlassPane

Question

I am trying to achieve the following

http://www.qksnap.com/i/3hunq/4ld0v/screenshot.png

I am currently able to draw rectangles successfully on a semi-transparent glasspane background using the following code:

    protected void paintComponent(Graphics g) {
          Graphics2D g2 = (Graphics2D) g;
          g.setColor(Color.black); // black background
          g.fillRect(0, 0, frame.getWidth(), frame.getHeight());
          g2.setColor(Color.GREEN.darker());
          if (getRect() != null && isDrawing()) {
            g2.draw(getRect()); // draw our rectangle (simple Rectangle class)
          }
         g2.dispose();
}

Which works great, however, I would love to have the area within the rectangle be completely transparent while the outside was still darken much like the screenshot above.

Any ideas?

Answer 1

..have the area within the rectangle be completely transparent while the outside was still darken much like the screenshot above.

• Create a Rectangle (componentRect) that is the size of the component being painted.
• Create an Area (componentArea) of that shape (new Area(componentRect)).
• Create an Area (selectionArea) of the selectionRectangle.
• Call componentArea.subtract(selectionArea) to remove the selected part.
• Call Graphics.setClip(componentArea)
• Paint the semi-transparent color.
• (Clear the clipping area if more paint operations are required).

Answer 2

As Andrew has suggested (just beat me while I was finishing off my example)

protected void paintComponent(Graphics g) {
    super.paintComponent(g);

    Graphics2D g2 = (Graphics2D) g.create();
    g.setColor(Color.black); // black background

    Area area = new Area();
    // This is the area that will filled...
    area.add(new Area(new Rectangle2D.Float(0, 0, getWidth(), getHeight())));

    g2.setColor(Color.GREEN.darker());

    int width = getWidth() - 1;
    int height = getHeight() - 1;

    int openWidth = 200;
    int openHeight = 200;

    int x = (width - openWidth) / 2;
    int y = (height - openHeight) / 2;

    // This is the area that will be uneffected
    area.subtract(new Area(new Rectangle2D.Float(x, y, openWidth, openHeight)));

    // Set up a AlphaComposite
    g2.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.5f));
    g2.fill(area);

    g2.dispose();
}