Full screen and transparent OpenGL window in Xlib

I am trying to create a full screen and transparent OpenGL window in XLib for the purpose of applying shaders to the screen. Suppose, for example, applying a blur or pixel effect. At the moment I'm trying to do something really simple like rendering the current date in the middle, blurring the background and that.

What is the appropriate setting for this? Do I have to grab the desktop when my application launches and use it as a bitmap to manipulate in my shader? Or should I do some kind of transparency protocol?