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Forceware 77.76 Preview |
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No Bridge, New AA |
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by Josh Walrath |
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New Antialiasing modes I am an antialiasing enthusiast. I admit it. I was sold on the idea when 3dfx introduced it to the gaming market with the T-Buffer and the Voodoo 5. Everyone else followed suit, but from that point on I refused to play anything without antialiasing enabled. In the past I have been fairly vocal in criticizing NVIDIA for not upgrading and updating their antialiasing unit significantly since the introduction of the GeForce 3. While improvements have been made here and there, we haven’t seen a significant change in the basic functionality and capabilities. It still features a maximum of four samples for each pixel, and it does not have programmable sample patterns like ATI does. I believe that I am not alone in this criticism. Apparently NVIDIA has taken this to heart and are introducing two new antialiasing modes that can only be used in SLI setups. These two modes can be used with any SLI enabled cards (even the 6600 LE). Best results would probably be taken from a pair of 7800 GTX’s due to the transparency antialiasing. Still, this driver improvement has opened up a nice, new feature for those with SLI setups. This feature can only be enabled by using the CoolBits registry hack though, but in future driver releases NVIDIA promises to make this functionality standard without resorting to CoolBits.
There are only two SLI Antialiasing modes, and they can only be used alone. This means that people expecting SFR or AFR type performance with this mode enabled are in for a disappointment. The way this rendering mode is set up basically causes each GPU to render the same frame. Once each card renders the frame, the information is sent to the master card. There in the master card’s framebuffer the output is then blended and output to the monitor. I was not aware that the framebuffer and antialiasing units of the 6x00 and 7x00 series of cards could handle this functionality, so I asked Brian Burke how exactly it works. Brain kindly replied:
While not the most illuminating answer, it basically tells us that the hardware can handle this functionality, and it is not necessarily a mere driver hack. ATI offers this type of functionality with their Crossfire series, but apparently it only works for Direct3D applications. Crossfire also has a special built compositing chip on the master card which handles the frame information integration, and so with its super AA modes it could be faster than the NVIDIA SLI-AA implementation. This remains to be seen as Crossfire reviews have not been released as yet.
We can see how the two 4X frames come out to be essentially one 8XS frame. The first SLI-AA mode is 8X. This mode has each GPU render a frame using 4X multisampling AA. This mode features four AA samples and one texture sample. This is probably the most commonly used AA method for NVIDIA and ATI video cards. Needless to say, all current cards can handle this type of antialiasing without a major performance hit. Once each card finishes the frame, it is sent to the master card’s framebuffer and the two frames are then blended, but the sample pattern for each frame is offset. This gives a final output that essentially appears to be 8x multisampling combined with 2x supersampling. The output should be very similar to the single card 8XS antialiasing mode, but the performance should be a whole lot better.
The resulting sample pattern for this mode is quite complex, and visually is about as perfect as one could get without going to a true stochastic pattern. The second mode is 16X. Each GPU renders the frame using 8XS antialiasing. This pattern is 4 multisamples and 2 supersamples done on the same pixel. This mixed mode pattern provides excellent edge antialiasing and texture filtering, but is very costly in terms of performance. In many CPU bound games, it can often be used with a minimal performance hit. In games that are GPU bound, it is a performance killer. The 16X mode takes these frames and the single frame output features 16 AA samples and 4 texture samples. This is not to say that this jump in AA quality is free though. It is non-trivial to take both frames from the different cards and do the offset and resolve pass. This takes time and hardware muscle. So, while the performance hit with SLI-8X will not be as extreme as 8XS on a single card, it will not be as fast as a single card’s 4X setting. The SLI-AA 8X user still gets the same image quality of 8XS at a much faster speed. One feature that is incredibly important is that SLI-AA works with nearly every game out there. No matter if it is Direct3D or OpenGL, or if it has problems with SLI AFR or SFR modes, the application will benefit from these antialiasing modes. NVIDIA is promising a 100% compatibility with all 3D games with the SLI AA modes. If the game can run on NVIDIA hardware, it can run with SLI-AA. Except of course games that do not support antialiasing in any way (like the first two Splinter Cell games) or applications utilizing FP16 HDR rendering (Splinter Cell 3 and Far Cry). It is nice to see NVIDIA giving the user another performance and quality option to work with. Running games such as Half Life 2 and Morrowind at high resolutions and AA settings is a very rewarding experience. The ability to further tweak the performance and image quality of products somebody already owns is a great example of “getting your money’s worth.” NVIDIA has added further value to current products by releasing this free driver. Wrapping it Up In this latest release NVIDIA is giving the users a lot of good stuff (well, at least the SLI users). Along with bug fixes, the addition of the 6600/6600 LE SLI and the new SLI-AA modes, NVIDIA is helping to deliver a more robust product to the end users. With the threat of ATI’s very interesting Crossfire platform, NVIDIA obviously felt the need to keep their product on par in terms of features. NVIDIA has also increased the number of SLI and game profiles to over 100. CoolBits can still be used to create and manage SLI profiles for games that are not officially supported. Future driver revisions will of course increase this number. It cannot be denied that NVIDIA is providing a lot of extra value for users with upgrades such as this. While we have yet to see if ATI’s Crossfire will match or exceed NVIDIA’s SLI in terms of performance, features, and functionality, NVIDIA is making it as tough as possible to trounce their solution. I also firmly believe that NVIDIA’s latest focus on drivers is a direct consequence of ATI’s Catalyst program. When each company innovates, the other is sure to follow. This of course benefits the customer, which is the most important benefit of all!
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Copyright 1999-2005 PenStar Systems, LLC. |
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