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Coverage Sampling Modes: Nvidia’s CSAA And AMD’s EQAA

Anti-Aliasing Analysis, Part 1: Settings And Surprises
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With the GeForce 8 series, Nvidia introduced a new technique it called coverage sampling anti-aliasing (CSAA). Essentially, CSAA is traditional MSAA with extra coverage samples added. Coverage samples test to see whether a polygon is present at the location of the sample, and these can be used to weight the calculation when determining the final color of the pixel. Because coverage samples are relatively easy for the hardware to gather, the result can be an increase in quality that requires a relatively small performance overhead compared to MSAA samples. Unfortunately, the value of coverage samples depends on the makeup of the individual pixel, so the resulting increase in quality ranges from some to none at all.

Starting with the Radeon HD 6900 series, AMD now offers enhanced quality anti-aliasing (EQAA), a mode that is essentially identical to Nvidia’s CSAA. Unfortunately, the ROPs in the rest of the company's Radeon portfolio (even including the Radeon HD 6800-series cards) are not able to handle EQAA, so most AMD graphics card owners won’t be able to take advantage of this feature. Nvidia owners have more flexibility in this respect, as the GeForce 8000, 9000, 200, 300, 400, and 500 series enable CSAA support.

Mode Confusion

Unfortunately, Nvidia has a somewhat inconsistent method of designating its anti-aliasing modes, with numbers sometimes indicating total MSAA samples, and at other times indicating a sum of MSAA and coverage samples. The suffix “Q” indicates pure MSAA in the case of Nvidia’s 8xQ setting, but the 16xQ setting has 8xMSAA plus eight coverage samples. This leads to confusing situations, as the 8x setting in the Nvidia driver does not represent 8x MSAA, but signifies 4x MSAA plus four coverage samples.

AMD deserves some kudos here for sticking to a naming convention that makes perfect sense. AMD’s level always indicates the number of MSAA samples, while the EQ suffix means the same number of coverage samples is added. For example, AMD’s 8x setting designates eight MSAA samples. But the 8xEQ setting designates eight MSAA samples plus eight additional coverage samples.

The following chart demonstrates how AMD and Nvidia anti-aliasing modes correspond to each other:

GeForce CSAA vs. Radeon EQAA
Anti-aliasing Levels
GeForce
Driver Mode
Combined Color/Coverage Samples
+ Extra Coverage Samples
Radeon
Driver Mode
2x
2+0
2x
n/a
2+2
2xEQ
4x
4+0
4x
8x
4+4
4xEQ
16x
4+12
n/a
8xQ
8+0
8x
16xQ
8+8
8xEQ
32x
8+24
n/a
n/a
16+0
16x


Coverage Sampling Image Comparisons

Coverage samples offer a relatively limited increase in overall anti-aliasing-based image quality on both Nvidia and AMD graphics hardware. Certainly, more MSAA samples result in a definite visual improvement, while more coverage samples can result in no quality increase at all. It is because of this that we’re not fans of Nvidia’s naming scheme, where 16x provides obviously lower quality compared to what 8xQ offers.

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