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Gigabyte 6800 Ultra Review :: Features
The GEFORCE 6800 Ultra has 16 superscalar pixel pipelines. Each pipeline is capable of doing up to 8 pixel shader operations in a cycle. This allows for up to 128 pixel shader operations in a cycle. Each pixel pipeline has 2 shader units. The 6800 Ultra has a clock speed of 400 MHz for the core which gives a maximum fill rate of 6.4 Gigapixels per second. NVIDIA decided to move to Shader Model 3.0 with their new architecture. SM 3.0 includes Pixel Shader 3.0 and Vertex Shader 3.0 compliance, which brings the GEFORCE 6800 Ultra into full support for DirectX 9.0 features up to and including the newly released DirectX 9.0c. ATI decided to use an expanded SM 2.0 in their new architecture, the R420. So what does this mean anyway? Programmable Pixel Shaders are small programs that are performed on pixels to generate special effects. First introduced in 2001 with DirectX 8.0 and the GEFORCE 3 series of cards, pixel shaders are most often used to give realistic looking shiny water in games like Morrowind, Tiger Woods 2004, Far Cry and other modern games. The problem I have with the use of pixel shaders in games today is that most developers don't use them for anything except beautiful water. Beautiful water is great, but you can do so much more. Realistic per pixel lighting and reflective effects like shiny armor are possible with pixel shaders. I like the old Chameleon demo for showing some of the effects possible. The GEFORCE 6800 architecture is capable of doing near-infinite length pixel shaders. DirectX 9.0c PS 3.0 limits the pixel shader length to 65,536 instructions in a single pass; however, the 6800 Ultra is capable of infinite length shaders. In truth, developers aren't really using long pixel shaders at the moment anyway so it's not a big deal. NVIDIA added flow control into their 6xxx architecture, which allows for subroutines, loops and branches in the pixel shader, which was previously the domain of the vertex shader. It's yet to be seen how using flow control in a game with the 6xxx architecture will affect performance. NVIDIA also added support for Multiple Render Targets (MRTs) to their cards. This allows you to save per-pixel data in multiple buffers. These buffers can then be used as parameters for photo realistic shaders. This is also known as deferred shading. ATI introduced this in their architecture with the RADEON 9700 Pro. Vertex Shaders are mathematical functions that can be performed on vertices. Effects that can be done with vertex shaders include matrix palette skinning which allows for realistic character animation, deformation of surfaces, for wave effects, and ripples, and vertex morphing which allows for emotions in characters like Nalu and Dawn and the emotions they show in their demos. Vertex Shader 3.0 offers some new effects and adds new extensions to the DirectX core. First is vertex texturing. This allows for special effects like displacement mapping using the vertex shader, render to vertex texture, particle systems, and complex character animation. Second is Geometry instancing. Geometry Instancing can reduce the amount of vertices that are required by reusing the vertices for all iterations of an object in a scene. Full Scene Anti-Aliasing can dramatically improve a game's image quality. The GEFORCE 6800 Ultra uses a method called Rotated Grid Multi Sample Anti Aliasing. The 6800 Ultra is capable of 4 sample AA. In the previous GEFORCE FX series the maximum was 2x in rotated mode. AA gets rid of jaggies in games which can be caused by lines or surfaces being smaller than one pixel thick. The GEFORCE 6800 Ultra is the first video card from NVIDIA that supports 16X anisotropic filtering, which matches the competition from ATI. Anisotropic filtering can sharpen blurry textures and the combination of FSAA and AF can dramatically improve image quality. Ultra Shadow is NVIDIA's patented shadowing algorithm. Games like DOOM 3 which is coming out this week, often use stenciled shadow volumes. Stenciled shadow volumes do not require texturing or color updates. When a game can use Ultra Shadow II the 6800 Ultra operates in a 32x0 mode instead of the 16x1 mode. Early benchmarks from HardOCP show that even the least powerful 6800 card available, the 6800 non-Ultra is at the moment faster than ATI's best offerings in Doom 3. This is at least partially due to the Ultra Shadow 2 and the fact that ATI's OpenGL driver has always been one of it's weak points.
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