Questions about the XDGA extension

[Sebastian Gerhardt]

Hello x.org developers,

I would like to have an overlay displayed on the
normal X Window desktop. That is, a graphic should
be displayed like a translucent window from a composing
window manager, but also should not react to mouse events.
Nevertheless, the windows/controls under that overlay 
shall react to mouse events as if the overlay wasn't there.

I think this can't be achieved with a normal/composite WM,
so I tried to use the DGA extension to blit the pixels 
directly in the video ram. But as soon as I call XDGASetMode
to obtain the VRAM pointer, the xserver stops drawing to
the screen until XDGA is shut down again. 

So it seems to me it is either X or direct access but they
can't work alongside. Is this correct or did I make a mistake?

The man-page mentions a pixmap:
  XDGAPixmap 
     Indicates that a Pixmap will be returned when the mode is 
     initialized. This means that rendering with Xlib is possible
     for this mode.

But what exactly is the purpose of that pixmap, if the xserver is
not operable during this time?

Are there maybe other ways to achieve such an overlay?


Thanks for any help,
Sebastian