screen annotation

[Joel Dice]

Hi all,

I'm interested in writing an application which allows a user to draw on 
their desktop (e.g. highlight, circle, or underline an area of interest). 
The closest thing I've found that already exists is 
http://www.home.unix-ag.org/simon/gromit/, but that only provides opaque 
annotations, whereas I'd like to allow semi-transparent (i.e. 
alpha-blended) annotations which can be "clicked through" such that input 
events in highlighted areas are passed through to the windows below as if 
the highlights weren't there.

I thought of a couple of ways to implement this:

  1. Create a full-screen window with an alpha channel that's completely 
transparent by default and draw the annotations on that.  The tricky parts 
are keeping the window in the foreground, hiding it from the window 
manager, and passing mouse and keyboard events through to the appropriate 
windows.

  2. Implement a compositing manager which draws the annotations on top of 
everything else.  I'm not sure how this would work if there's already a 
compositing manager running.

Has anyone tried doing something like this already?  Any advice on the 
above strategies or some other approach?

Thanks.