I'm trying to understand how to make a fade in/fade out of a window with
the Xrender and Xcomposite extension. From all that i had been able to
read over the internet, to do a fade in, i should render the window
(that will appear) off-screen with XComposite, then draw it on the
screen while playing with the alpha channel brought by the XRender
extension ?

As you can see, it's really unclear in my mind, and i do not really
success neither to find any good docs on those two extensions, nor to
understand the few code examples that I can find over the internet (they
are full of obscure functions like "XRenderFillRectangle" ?!).

So, can anyone give me a good link on how to do things like this, or
explain me how to use XRender/XComposite extension please ?

Thanks a lot.