Saturday, November 21, 2009

A Rant On Control Panels

NVidia and ATI provide control panels for Windows and Linux. These control panels let users configure aspects of 3-d acceleration, often in ways that bypass an application's request. This post is a rant about such control panels, but in order to understand the rant, you must understand how X-Plane communicates with a video card.

Stranger In A Strange Land

X-Plane is an OpenGL application. OpenGL (the "open graphics language") is a "language" by which X-Plane can tell any OpenGL graphics card that it wants to draw some triangles.

Think of X-Plane as an American tourist in a far away land. X-Plane doesn't speak the native language of ATI-land or NVidia-Land. But if the hotel says "we speak OpenGL", then we can come visit and ask for a nice room and a good meal.

Of course, if you have ever been an American tourist (or live in a country that is sometimes infested with American tourists :-) you know that sometimes American tourists who speak only English do not get to see the very best a country has to offer. Without local knowledge, the tourist can only make generic requests and hope the results turn out okay.

An example of where this becomes a problem is full-screen anti-aliasing (FSAA). OpenGL allows an application to ask for anti-aliasing. The only parameter an OpenGL application can ask for is: how much? 2x, 4x, 8x, 16x. But as it turns out FSAA is a lot more complicated. Do we want super sampling or multisampling? Coversage Sample Anti-Aliasing? Do we want a 2-tap or 5-tap filter? Do we want temporal anti-aliasing?

As it turns out, NVidia-land is a very strange country, with many flavors of FSAA. Some are very fast and some are quite slow. And when X-Plane comes in and says "we would like 16x" FSAA, there is really no guarantee that we will get fast FSAA (for example, 16x CSAA) or something much slower (like 16x super-sampling). X-Plane is not native to NVidia-land and cannot ask the right questions.

Control Panels

So where do the control panels come in? Well, if X-Plane can only ask for "16x FSAA", how can NVidia give users control of the many options the card can do? The answer is: control panels. The NVidia control panel is made by NVidia - it is native to NVidia-land and knows all of the local tricks: this card has high-speed 5-tap filtering, this card does CSAA, etc.

At this point I will pass on a tip from an X-Plane user: you may be able to get significantly faster FSAA performance with an NVidia card on Windows by picking FSAA in the control panel rather than using X-Plane's settings. This is because (for some reason) X-Plane's generic OpenGL request is translated into about the slowest FSAA you can find. With the control panel you can pick something faster.

Bear in mind that only some users report this phenomenon. My NVidia card works fine even with X-Plane setting FSAA. So you may have to experiment a bit.

It Gets Weirder

When it comes to full-screen anti-aliasing, I can see why NVidia wants to expose all of their options, rather than have X-Plane simply pick one. Still, which do you think is best for X-Plane:
  • Multisampling
  • Supersampling
  • Coverage Sample Anti-Aliasing
  • Some Mix of the Above?
Antialiasing has become very complex, and some of these modes will absolutely wreck your framerates.

And FSAA is one of the better understood options. How about these:
  • Adjust LOD Bias
  • Anisotropic Filtering
  • Anisotropic Filtering Optimization
(I know what these do, and you shouldn't be messing with them.)

How about these?
  • CPU Multi Core Support
  • Multi-Display/Mixed GPU Acceleration
I haven't found any description of what those do, but I have reports from users that setting them to the "wrong" value (whatever that is) absolutely destroys framerates.

Suffice it to say, as an applications developer, the situation is a support nightmare. Users can modify a series of settings at home that we cannot easily see, that are poorly documented, that cause performance to be very different from what we see in our labs, sometimes for the worse, sometimes for the better.


Edward Glowworm said...

Thanks Ben, interesting background information to help me in my quest for the optimum setup on my rig.


Anonymous said...

Thanks a lot Benjamin
This matches with what I was experiencing in Linux when I started fiddling around with the NVidia control panel vs. XP-settings a few weeks ago.