JFrame - only a part transparent

Question

I want to make a part of a JFrame transparent. It should look similar like OneNote Screen Clipper. I basically have a fullscreen overlay of a partially transparent JFrame and then inside this JFrame I want to make some rectangles by dragging the mouse and make those rectangles fully transparent, like so:

---------------------------------------
|        partially transparent         |
|                                      |
|      -----------                     |
|      | fully   |                     |
|      | transp. |                     |
|      -----------                     |
----------------------------------------

How would I do that? Currently I have this:

public class Overlay extends JFrame implements MouseMotionListener, MouseListener {

    private Rectangle2D.Double selection;
        private Point start;
        private Point end;

    public Overlay() {
        addMouseMotionListener(this);
        addMouseListener(this);
        setSize(Toolkit.getDefaultToolkit().getScreenSize());
        setAlwaysOnTop(true);
        setUndecorated(true);
        setOpacity(0.2f);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    @Override
    public void paint(Graphics g) {
        Graphics2D g2d = (Graphics2D) g;
        g2d.clearRect(0, 0, getWidth(), getHeight());

        if (selection != null) {

            Area outside = new Area(new Rectangle2D.Double(0, 0, getWidth(), getHeight()));
            outside.subtract(new Area(selection));


            g2d.setClip(outside);               
            g2d.setColor(new Color(0.5f,0.5f,0.5f, 0.5f));
            g2d.fillRect(0, 0, getWidth(), getHeight());
        }

    @Override
    public void mousePressed(MouseEvent e) {

        start = e.getLocationOnScreen();
    }

    @Override
    public void mouseDragged(MouseEvent e) {
        end = e.getLocationOnScreen();
        selection =  new Rectangle2D.Double(start.x, start.y, end.x - start.x, end.y - start.y);
        repaint();
    }
}

But this doesn't work correctly as it repaints the background many times, and therefore it becomes darker/less transparent and also it is slow as hell.... the repaints take a lot of time and are very visible.

Answer 1

I figured it out myself finally:

public class Overlay extends JFrame {
    private Point start = new Point();
    private Point end = new Point();

    public Overlay() {
        OverlayMouseAdapter listener = new OverlayMouseAdapter();
        addMouseMotionListener(listener);
        addMouseListener(listener);
        setSize(Toolkit.getDefaultToolkit().getScreenSize());
        setAlwaysOnTop(true);
        setUndecorated(true);
        setBackground(new Color(255, 255, 255, 180));
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    @Override
    public void paint(Graphics g) {
        super.paint(g);
        Graphics2D g2d = (Graphics2D) g;


        g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.CLEAR,
                    0.1f));
        g2d.setColor(new Color(0, 0, 0, 255));
        g2d.fillRect(start.x, start.y, end.x -start.x, end.y-start.y);

        g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC));
        g2d.setColor(Color.BLACK);
        g2d.drawRect(start.x-1, start.y-1, end.x -start.x + 1, end.y-start.y+1);
    }

    private class OverlayMouseAdapter extends MouseAdapter {
        @Override
        public void mousePressed(MouseEvent e) {
            start = e.getLocationOnScreen();
        }

        @Override
        public void mouseDragged(MouseEvent e) {
            end = e.getLocationOnScreen();
            repaint();
        }
    }
}

We can just use te AlphaComposite.CLEAR which will remove the opacity from the background (at least that is what it is doing here).

Answer 2

As a quickfix, call

repaint((int) selection.x,(int) selection.y,(int) selection.width,(int) selection.height);

instead of repaint().

Still, this is not the best way of doing it.