when you say brightness are you talking about opacity?
i never used compiz intensively but is this what you are asking?
No, I meant brightness as the brightness you get to set on your monitor. Compiz can do that on individual windows. Seems like KWin does not have it.
On compiz (opacity, brightness, saturation):http://imageshack.us/f/188/screenshot005ac.png/
I use it to decrease brightness of white background windows at nightime when reading a pdf say.