Without compositing, an application must redraw its window contents when obsured by other windows
You're not telling me anything new, I've been writing software professionally for about 15 years now.
(those white blotches you get when eg. dragging a dialog across a window).
Nah... you only get those in edge situations:... the app is borked (message pump and/or main thread blocked), Windows is borked (multitude of possible reasons... ), no graphical drivers installed... all situations in which desktop compositing (let's call it what it is
) won't help much anyway.
With compositing, the window contents are drawn into an offscreen surface, and the window manager can repaint the exposed portions using the graphics hardware essentially for free. Because of hardware compositing, effects like window shadows and the Windows 7 translucent title bars also cost nothing.
I agree but I think it's a moot point in real life. Like I said before, my main (and only one that has Win partitions) machine spends very little time redrawing parts of windows that get shown by other windows moving away. With 3 full hd screens (1horiz + 2vert), I have very little need to move windows. Or to even overlap them too often. I remind you that my argument was subjective from the very beginning. I was talking about my situation.
As for window shadows... don need or want them. As far as desktop shadowing goes, I only want it under the mouse pointer and that's hw accelerated, DWM or not. About the window chrome translucency... it might be free (or at least very cheap) computing wise but it's ugly and wasteful in screen real estate. So again... I don't want it.
To summarize: the argument was subjective in that it applied to my situation and preferences and no... I don't think I'm making my system slower.