Overlay for 3rd party application

Question

Hi,

I'm looking for a way to display an overlay (hud) for a third party application on MacOSX. It should look like this.

My working steps would be:

1. find window by title
2. get resolution of window
3. create own invisible window
4. check for moving events of target window

I'm not very familiar with MacOS programming with Carbon and Cocoa and my prefered way would be to do it with C++, because I want to port my app to win and maybe linux later.

But I think I need to write a Cocoa or Carbon module for my app so any suggestions are welcome.

Thx, Richard