From: David Masover <ninja@...>
Date: 2009-03-10T19:52:58+09:00
Subject: Re: Ruby OpenGL Gears example that uses shiny and velvet GLSL shaders

Albert Schlef wrote:
> David Masover wrote:
>   
>> For those with KDE4, the default keystroke to toggle
>> desktop effects is [snip]
>>     
>
> I understand nothing in these things but I wonder: why do desktop 
> effects affect your OpenGL performance?

Because desktop effects -- at least the shiny, new ones -- use OpenGL.

> I mean, the animation is run in 
> a window of its own, so why has KDE anything to do with it?
>   

Here, do a quick demo and find out: Run KDE4 or Compiz, with wobbly 
windows on. Launch glxgears, or this gears demo, or anything else 
OpenGL. Now drag it around.

See how that "window of its own" wobbles?

I bet if you do the fancy raindrop effect in Compiz, that'll send 
ripples all over those spinning gears.

That's because with compositing, every window on the screen is going to 
be fed through some KDE effects, and through OpenGL, at least once. And 
it's not just the wobbliness -- how else would you apply a smooth drop 
shadow to a window, for example? You can do it in software, but it will 
probably be slower.

Of course, it's important to keep in mind that glxgears is not a 
benchmark (or at least, not a good one), but I could show you similar 
results even when trying to play a fullscreen game -- KDE apparently 
isn't smart enough to figure out that the game is fullscreen, so it 
doesn't automatically disable compositing.

So, KDE4 has a keystroke that toggles compositing. Compiz, I believe, 
will detect fullscreen windows and disable compositing, at least on that 
window.