XFX 7800GT SLI Video Card Review :: The 7800GT Features

7800GT Features

• .11um manufacturing process
• 302 million transistors
• Shader Model 3.0
• DirectX 9.0c Pixel Shader 3.0
• DirectX 9.0c Vertex Shader 3.0
• HDR
• Transparency AntiAliasing
• SLI AA
• Render To Texture
• OpenGL 2.0

Microsoft Windows XP is the most popular operating system for personal computers in the world today. An integral part of the Operating System is Direct3D, which has features to enhance games and 3D visualizations. Every year or two Microsoft updates Direct3D to support the new features of video cards released or being released in the near future. 2002 saw the release of DirectX 9.0 along with ATI's 9700 Pro. Included in DirectX 9.0 were two Shader Models, Models 2.0 and 3.0.

Pixel Shader 2.0 had support for 96 shader instructions in a single pass. Games like HL2 released last year had shaders using 50 instructions or less a pass at the maximum. As games get more and more complex, cards like the 9700 Pro have begun slowing down to near unplayable levels in games that take advantage of lots of Pixel Shader effects.

Pixel Shader 3.0 has the following advantages over 2.0: nearly unlimited (65,536+) instruction shader programs, dynamic branching and looping of shader programs, render to texture, Multiple Render Targets, a Back-Face register, and more. Pixel Shader 3.0 allows for effects never before seen in video games. The graphics chip on the Playstation 3 has at its roots the same PS as the GEFORCE 7800GTX.

Vertex Shader 2.0 had the following innovations: dynamic looping and branching, 256 shader instructions in a single pass, vertex texturing and Geometry instancing. The 9700 Pro was the second card to fully support VS 2.0, the first card being the Matrox Parhelia. However, the Parhelia did not get driver support for VS2.0.

Vertex Shader 3.0 has the following improvements over 2.0: nearly unlimited length vertex shader programs (65,536+), dyanamic branching and looping support, vertex texturing and Geometry instancing. Render to texture or Vertex Texturing allows for effects like displacement mapping to be done in the vertex shader. Geometry instancing allows for multiple similar objects to be rendered in a scene with differing attributes. For example a cluster of asteroids in a space simulator might vary slightly in position, in color, in shape. Instancing allows one asteroid to be stored instead of thousands and vary attributes of the asteroid by per-instance vertex streams.

Aliasing occurs in games when there are lines thinner than a pixel wide. Jagged edges occur giving a staircase effect look in a game. Traditional Antialiasing cures much of the jagged look in an image, except when transparent textures are used in a game. Things like vegetation and the interior of a chain link fence are normally not antialiased with traditional Super and Multisample Antialiasing.

NVIDIA introduced Transparency Antialiasing with the GEFORCE 7800GTX, to alleviate this. In Supersample mode, the chain link fence is antialiased, with smooth transitions from one link to another. In Multisample mode, the fence is not antialiased. This is probably a bad situation for MSAA with Transparency. Look at the screenshots below. ATI also has a version of this technology called Adaptive Antialiasing. AAA applies supersampling to transparent textures and multisampling to every other texture. Comparing the two, NVIDIA's 8XS mode is superior, but ATI's 4X mode looks better to my eye. Here's a sample of screenshots from XFX 7800GT and an ATI X1800XL card we have in the test labs. Look closely at the fence.

The human eye can distinguish between millions of variations in light. The range from the lowest light to the brightest light is called a dynamic range. Traditionally, computer games have had a limitation of a range of 255:1 or 8 bits per color component. With the advent of floating point pixel processing, a higher range is possible. With the 6800 and 7800 series, NVIDIA's cards are capable of 16-bit per color component lighting or a range of 65,535:1 instead of the 255:1 traditional video cards are limited to. This effect can be seen in games like the HL2 Lost Coast add-on recently released by Valve. This demo shows HDR to great effect. On one of the screenshots below HDR is disabled. Look at the sky, the sun, and the rocks on the beach. Notice the dull look to the sky and beach. The second screenshot shows an example of Blooming. Blooming is the effect you get when you move from a dimly lit area into a bright sunny area. The third shot is of full HDR. In this shot the area of the sun is much brighter, the rocks have a shiny sheen to them and the lighting is much more realistic.

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