How to create an overlay window in Java?

Question

I'm trying to create a HUD style display for a foreign application.

To do this, I'd need to make a transparent overlay window, that would be placed on top of the window of the foreign application. The overlay window should allow me to place widgets and draw text on it. Events should get forwarded to the underlying window, if they happen on the transparent area (and otherwise allow the widgets to work as expected).

I'm doing this on OSX with Java. I'd hope to do this with pure Java with portability to other platforms, but if not possible, I'd be fine with solutions that would just allow me to do this on OSX through Cocoa (Rococoa) or Carbon.

Answer 1

The Java tutorials show one way this can be accomplished. Here's another approach that uses RGBA (red, green, blue, alpha) values for the background colour:

public class Overlay {

    public static void main(String[] args) {
        JFrame frame = new JFrame("Transparent Window");
        frame.setUndecorated(true);
        frame.setBackground(new Color(0, 0, 0, 0));
        frame.setAlwaysOnTop(true);
        // Without this, the window is draggable from any non transparent
        // point, including points  inside textboxes.
        frame.getRootPane().putClientProperty("apple.awt.draggableWindowBackground", false);

        frame.getContentPane().setLayout(new java.awt.BorderLayout());
        frame.getContentPane().add(new JTextField("text field north"), java.awt.BorderLayout.NORTH);
        frame.getContentPane().add(new JTextField("text field south"), java.awt.BorderLayout.SOUTH);
        frame.setVisible(true);
        frame.pack();
    }
}