Feature Articles

NVIDIA GeForce 8800 GTX / GTS (G80) - The World's First DX10 GPU

By Vijay Anand - 9 Nov 2006

Lumenex Engine (AF, HDR), ROP and Memory Subsystem

Lumenex Engine (continued)

Angle-Independent Anisotropic Filtering

Besides notching up on the anti-aliasing standards, the Lumenex engine also updated the GPU's capability to process angle-independent anisotropic filtering - which finally puts them on par with ATI's Radeon X1K on anisotropic filtering (AF) image quality. NVIDIA graphics cards prior to the GeForce 8800 were unfortunately not capable of this (to save processing cycles) and were stuck with angle-dependent AF such as straight surfaces that completely adjacent to the ground or parallel to the ground. This caused certain textures to 'shimmer'. It's not something that would make or break an average user's upgrade/buying decision as it's not readily noticeable. However, ardent gamers and enthusiasts have a very keen eye and for them, this wasn't favorable. Thankfully, the GeForce 8800 series finally rectifies this shortcoming.

High Dynamic Range Lighting

High dynamic range (HDR) is all the rage in the latest of games as it offers some really nice visual effects portraying very bright light sources and very dark regions accurately as in real life without suppressing them. Current game engines embrace 64-bit HDR (assigning 16-bit per color component, including the alpha channel). While it suffices today, NVIDIA's taking no chances and has upgraded this to support 128-bits for an even higher-precision and more accurate representation of HDR (which NVIDIA likes to call it True HDR). So that's now 32-bits floating-point precision for each color component. We don't expect this to be of significance anytime soon as it would be sometime before game engines embrace 128-bit HDR. So for the moment, we can't quite relate how much better the already swell looking 64-bit HDR would feel like.

Here's another big equalizer that NVIDIA scored with the GeForce 8800 series - HDR with anti-aliasing is finally available! Note that we said "equalizer" as ATI's Radeon X1K series could handle this since they were announced last year. While we admit that besides the high-end Radeon X1950 series, the rest of them don't posses the raw processing power to let anyone enjoy fluent gaming with both HDR and AA combined. So while the entire Radeon X1K series had the option, they just weren't capable of it anyway. For the GeForce 8800 series which currently has the GTX and GTS versions, these are plenty fast and it's only right they were capable of handling this.

Early Z Technology & ROP

Other notable qualities of the Lumenex engine is a full 10-bit display pipeline, which again brings it on par with ATI's Radeon X1K series offering up to 1billion unique colors to be displayed - if and when we get 10-bit displays in the mainstream at all. Last but not least, Early Z technology is present on the GeForce 8800 series for improved rendering efficiency. As you know, Z-buffer (or also otherwise known as the depth buffer) stores information pertaining to which pixels in a scene are visible and which aren't. Z-Cull is a method that used the Z-buffer information to remove the non-visible pixels at a high-rate but this happens rather late in the rendering pipeline at the rasterization stage. So all this while we have the GPU having to process useless data that may never be rendered to screen. To save on this workload that consequently improves overall performance or conserves power, Early Z technology is employed to test individual pixel's Z value even before they enter the pixel shading stage and removes unnecessary pixels.

Speaking of Z processing which occurs in the Raster Operators (ROP), the GeForce 8800 GTX has six such ROP partitions with each capable of processing 4 pixels (or 16 subsamples). That's a total of 24 pixels/clock output for color and Z processing. The GeForce 8800 GTS has one less, five ROP partitions, which has a peak output of 20 pixlels/clock for color and Z processing.

Memory Subsystem

Once the pixel data has been completely processed at the ROP stage, they are then passed on to the frame buffer for scan out to the display device. To communicate with the frame buffer, the GPU has memory controllers of course, and the GeForce 8800 series has the weirdest count and total memory bus width of any GPU in recent history.

The GeForce 8800 GTX has six standard 64-bit memory controllers (which supports DDR1, DDR2, DDR3 and DDR4 memory) and that equates to a total of 384-bit memory bus width. Not your usual power of two values such as 128-bit and 256-bit bus widths. NVIDIA has evaluated the cost to performance aspects of various memory bus widths and at the moment, the 384-bit memory bus is fair strike between the two scales, which is a logical take in our view. With an odd memory controller size, the GeForce 8800 GTX is also associated with an unusual frame buffer size of 768MB. However if you do the math, six memory controllers interfacing a total of twelve 512Mbit x32 memory devices, you get a grand total of 768MB of GDDR3 memory (clocked at 1.8GHz DDR). The toned down GeForce 8800 GTS counterpart, has five memory controllers which gives the card a graphics memory bus width of 320 bits and total frame buffer size of 640MB (clocked at 1.6GHz DDR). Though unusual numbers, these specs are definitely better than any other solo GPU graphics card out in the market at the moment.

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