[GRLUG] X overlay drawing app?

[Adam Tauno Williams]

On Mon, 2010-04-19 at 13:58 -0400, Michael Mol wrote:
> Tricky to do, particularly with compositing window managers. I can
> only think of three ways an app writer would do it:
> 1) Create a full screen window with backing store, draw the
> annotations on top of it, and find a way to get X to refresh the
> backing store periodically. (Will lead to either massive CPU overhead,
> or the appearance of a very laggy system) Avoid capturing any input.
> (I don't know enough to know how they'd do that part.)
> 2) Create a full screen window with an RGBA color space. (I don't even
> know if X even supports this. It would require some compositing tricks
> I'm not sure current compositing window managers do.) Avoid capturing
> input. (I again don't know how to do that part.)
> 3) Create a *huge* number of windows, for each apparent block blit to
> the screen; this would be incredibly hackish and wasteful, but *might*
> work. If you can't avoid capturing the input, it would have the odd
> side-effect of "erasing" underlying controls and buttons. Might make
> an interesting prank app, that way.
> If such an app does exist, I'd definitely be interested in poking at
> it and seeing how it works!

It would probably be easier to intercept an RFB stream from the X VNC
module and compose an overlay on that [the RFB protocol is really
simple, relatively speaking].  You'd probably have to anotate your
screen in a window displaying a scaled image of your screen.