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Fetch4 Shadow Map Acceleration

Creating soft shadows is an area that taxes texturing units and pixel shaders. A shadow map is a texture that is rendered from the point of view of the light source. If there is no geometry that occludes the light source, the area is lit, if there is geometry, the area behind it is marked as shadowed. Without sampling the shadow map, there will be hard edges as the light falls off immediately where the shadow map boundaries fall. Soft shadowing is generally an expensive technique because it requires a lot of sampling around the boundary to create a more gradual boundary between light and dark. Sampling requires texture accesses and pixel shading to do the blending.

As with 3Dc texture compression, ATI is taking advantage of the properties of shadow maps to increase performance with a texture sampling feature they have dubbed Fetch4. Traditional textures are made of 4 components, a red, green, blue and an alpha channel, which is opacity of the texture. Since a shadow map is simply a single channel texture, ATI can instead access 4 adjacent single addresses instead of accessing a single address of 4 channels which means 4 times the number of samples can be taken per texture access.

Power Draw, Die Size and Other Notes

• Power draw on the Radeon X1900 XTX is very high. While it shares the same cooling solution as the X1800, it now draws a whopping 150W, giving it the dubious title of being the highest power draw component. With CPUs that draw near 120W and a video card that draws 150W, prospective owners should also take a look at their power supply and see if it is up for the task as these two components alone will put a strain of over 22 amps on the 12V rail. With CrossFire power draw will start edging towards the 35A mark at full CPU and GPU usage. ATI recommends a minimum 550W PSU and even then you'd want to get a very solid quality make.
• The transistor count has increased 20% from 320 million on the Radeon X1800 to 384 million transistors. Most of the extra transistor budget logically would go to the extra pixel shaders. If there are no other major underlying GPU changes, it would seem that each pixel shader is roughly only 2 million transistors. Since there is the option of either going wider or faster, it would seem logical that the next Radeon iteration can once again make leaps in pixel processing power with only a slight percentage increase in the number of transistors.
• There is no Radeon CrossFire X1900 XTX card but ATI asserts that the X1900 XTX will not clock down to match a CrossFire X1900XT Card. Note that this does not necessarily mean that the CrossFire card will automatically overclock. Both ATI and NVIDIA seem to have worked something out with regards to their dual card solutions that allow for asynchronous clockspeeds on the two different cards (NVIDIA enabled this a few driver revisions ago). The X1900 CrossFire will allow for resolutions up to 2560x1600 @ 72Hz.

A shot of the Radeon X1900 CrossFire Master Card

• The Radeon X1900s are HDCP ready but "not implemented." Again, I do not have clarification before going to press and hopefully this is something that can be toggled through a driver as this will be important for high def content on the PC in the future.
• Antialiasing can be performed at the same time as HDR. Anisotropic filtering is invariant. In some implementations, depending on the viewing angle, performance enhancing filtering techniques break.

Matt Horne will take it from here and go through the benchmark analysis and post his overall impressions of the X1900