"Write Combining" and other G400tweak Qs

[Smiff]

Hi, I've been meaning to ask this for ages...

Could someone please tell me what the "Write Combining" option in G400tweak does? And why does my PC sometimes crash if I turn it on?

Does all that stuff in G400tweak still work with Powerdesk 6?

While i'm here, what do "Sub Pixel Accuracy" and "Anisotropic Filtering" do exactly? And should I have triple buffering on?

Thanks alot for answering my many Qs
Smiff

[Smiff]

ahh poo I just realised this should be in "Matrox Hardware" really, could a nice moderator type person please move it over there?

Thanks, sorry
Smiff

[Tronic]

I have no idea about those, except for triple buffering. Most people want to keep it on.

I need to tell you how it works, and before I can do that I must tell you how single/double buffering works.

Single buffering is not used with 3D-cards, except for some rare situations. It is quite commonly used it old dos-2D-games (and in almost all windows 2D-games). In this mode, everything that is drawn to the screen, is drawn directly to the screen buffer (1st buffer), and will be visible when next screen refresh happends (and because that happends 60-100+ times in second where 3D-screen can be drawn ~30 times/s, it looks really ugly, you see unfinished 3D-screens).

Double buffering helps here. Picture is drawn into 2nd buffer, that is not visible on screen, while screenbuffer (1st buffer) contains last frame drawn. When picture is ready, 3D-card waits until screen refresh happends (if vsync is on. if it isn't, card doesn't wait) and quickly copies 2nd buffer into 1st one.

This sounds perfect, doesn't it? So, where is 3rd buffer needed, I hear you asking..

As you see, when picture is ready, 3D-card just sits waiting for vsync to happen. After that it copies the screen and draws another one. After that it waits again and ...

So, when triple buffering is used, image is drawn into 3rd buffer, and when it is ready, it gets copied into 2nd buffer (where it waits for vsync) and while it still sits waiting for vsync (in 2nd buffer), next image is being drawn into 3rd buffer.

Triplebuffer can make framerate a bit higher, but there's a downside with it; it uses memory.

In 1024x768x32bpp every buffer uses 3MB of gfx memory.

With triple buffer, 9MB of your 32MB is "wasted" for 2D-buffers, rest 23MB is used by Z-buffer (that has nothing to do with these 3 buffers), textures, bumpmaps and other things.

If you have problems with games loading new textures all the time, or you have only 64MB of RAM (so it loads from HD!), use double buffering.

How to know that a game is loading new textures? It doesn't do that on every frame, so if it stops for a "long" time (means 1/10s or more), but not on every frame, it loads new textures.

Hey, Matrox gurus , I'd like to know more about those advanced options too, please let me know...

Even more things that I'd like to know:
-Does PLL clock (or whatever that base frequency is called) affect anything directly?
-What does WARP Clock do?

[Smiff]

Wow thanks alot Tronic you really explained that Got a bit of an essay going there eh? I thought 'triple' sounded good and I'll keep it on then

There must be someone around here who knows what all those other G400tweak things do. I'm surprised Matrox doesn't include them in their own "options" driver panel, strange that yes?

I was wondering what a WARP clock was too, maybe soemone at Matrox watches too much Star Trek?

so we're wondering about:

• "Write Combining"
• "Sub Pixel Accuracy"
• "Anisotropic Filtering"
• "WARP clock"

Thanks if you know what they are

[Nuno]

Now, let´s see...

Write combining: Not 100% sure, but I think it´s a feature of AMD cpu´s, and using it should speed up things.

Sub pixel acuracy: the name is pretty self explanatory. All 3d cars have it (since eaarly voodoo days). It is a feature that makes geometry calculations more precise, preventing the shimmering effect textures when moving around the 3d scene. People of the early G200 days surely remember how it looked horrible in Tomb raider and forsaken

Anisotropic filtering: It´s a rendering algorythm superior (quality wise) to bilinear and trilinear filtering.
Bilinear filtering is done in one pixel by taking the average color of the 4 pixels surrounding it. Trilinear is just bilinear with the mipmap transitions smoothed. Anisotropic filtering averages far more pixels (8/16...)and not equally (here anisotropic - not regular (?). G400 doesn´t really support it (just partially) and no current consumer video card does, just because true anisotropic filtering requires a HUGE computing power, that no graphic chip has to perform it at a decent framerate.

WARP: the WARP is a part of the Gx00 chips that are highly flexible doing both integer and single precision fp calculations. Current drivers use it to the triangle setup part of the rendering process.


[This message has been edited by Nuno (edited 09 July 2000).]

[Stil]

Write combining is a feature of the Pentium Pro and later CPUs.

Typically, when a program (a device driver in this case) writes data to physical RAM, the writes occur in the order in which the code executed. This is very slow, and does not make efficient use of the cache.

PPro's and higher allow a device driver writer to specify certain regions of memory and say "cache these writes". This is pretty safe for graphics drawing and DMA writes, but not direct hardware access (PIO).

Write combining can result in around a 10x speed improvement to memory writes so it should always be on. The driver will know when it can and can't actually use it.

[Smiff]

Thanks guys, you cleared that up Except I don't get why my poota crashes with Write Combining on? A P3 would support it ok yes? odd. unless it's an AMD thing like you said Nuno. Some conflicting advice here

[Nuno]

Nope, I said I wasn´t sure enough, Stil should be right...