ARGB off the off-screen

[sshachar at t2.technion.ac.il]

Hi,

To follow up on my previous questions,

Can I control the alpha values of the on-screen root window? 

That is, I want to be able to use the entire X visible screen as a source for a
final composite operation done externally in HW (for instance with a HW cursor)
and in order to get the desired effect I need to be able to draw various alpha
values to different on-screen objects (all windows have to be with nonzero
alpha, exept maybe a video overlay window for example)

What do I need to do in order to achieve this effect?