Part of window translucent

Like the Calculator app that comes with OS-X, the window is partially translucent. In this app it’s mainly the top part.
How can I do the same thing?
Do I have to somehow grab a screenshot of the stuff that’s under my window, blur it, and paint it on my window? Or is there a better way of dealing with it. Since the method I just mentioned seems very CPU expensive.

Never mind… I just found this FORUM post, and HERE too