New FSAA and Anisotropic Filtering Modes
Developing its GeForce 8800, Nvidia took care not only about its speed, but also about its image quality, something that the company’s earlier products used to be inferior to ATI’s in. There are changes and improvements in both full-screen antialiasing and anisotropic filtering. We’ll discuss the new FSAA algorithms first.
Before the GeForce 8800, the best-quality FSAA supported by Nvidia’s solutions was the hybrid 8xS mode that combined super- and multisampling. It provides a superb quality of antialiasing, but super-sampling led to a terrible performance hit, making this mode unpractical. Thus, the maximum practical FSAA mode available on the GeForce 7 was ordinary 4x multisampling whereas the ATI Radeon X1000 could offer the user 6x multisampling, not as nice-looking as Nvidia’s 8xS, but much more suitable for practical purposes.
The GeForce 8800 now solves Nvidia’s problems with FSAA on single-chip graphics cards. First, 8x multisampling has been added to the list of available modes (it is called 8xQ in the ForceWare settings). Second, the new GPU supports three new antialiasing modes: 8x, 16x and 16xQ that use the so-called Coverage Sampling Antialiasing method (CSAA).
Theoretically, 16x multisampling could be used earlier, too, but it’s only with CSAA that this high-quality antialiasing method can be normally used in practice with a performance hit similar to that with ordinary 4x multisampling.
The main difference of CSAA from ordinary multisampling is in minimizing the number of combined samples. With 16x multisampling antialiasing, there are 16 samples per each pixel on the screen. With CSAA, there are fewer samples and, accordingly, a smaller performance hit.
It should be noted that the CSAA method saves only on the number of color/Z samples, but not on the number of samples from the so-called coverage mesh. So, the percentage of samples fitting into the triangle referring to the original pixel is much higher than with the classic 4x MSAA. The economy on the number of color samples is justifiable because it is the color information that puts the biggest load on the graphics memory subsystem whereas the degradation in the antialiasing quality with 4 color/Z samples per pixel and at 16 coverage samples will be inconspicuous. It may be conspicuous if there’s a high contrast between the antialiased polygon and the background, because 4 samples may be insufficient for an accurate averaging of the color of the resulting pixel, and the antialiasing quality will be close to 4x MSAA.
The GeForce 8800 supports three CSAA modes in total: 8x, 16x and 16xQ. The first two modes operate with 8 and 16 coverage samples, respectively, at 4 color/Z samples. The 16xQ mode provides the highest quality and uses 8 color/depth samples, thus approaching the classic 16x MSAA in quality.
Nvidia’s new approach reduces memory usage and saves its bandwidth and thus provides a higher level of performance than with full-scale 16x multisampling. The resulting quality is close to the latter method and is much higher than what is provided by the notorious SLI AA 16x mode with its effect of blurred textures. According to Nvidia, the card’s performance is a mere 10-20% lower in 16x CSAA mode than in 4x MSAA mode. We’ll check out this claim later on.
As for anisotropic filtering, the GeForce 8800 features an algorithm whose filtering quality doesn’t depend on the angle of inclination of the texture plane. It is similar to the algorithm ATI’s cards use on your enabling the High Quality AF option, but Nvidia says it still provides a somewhat better filtering quality. We’ll check this out later, too.