nVidia 400 Series Image Quality Analysis

TrAA Decoupled

The next change is the decoupling of MSAA from TrAA. On GeForce 200 (and earlier) hardware, the level of TrAA was initially tied to the base level of MSAA in operation, though this has recently changed. On the 400 series, the level of TrAA can be independent of the MSAA level; the only requirement is that the level of TrAA doesn’t exceed the MSAA level.

This is obviously a very flexible system because it means in games with heavy vegetation you can drop the level of TrAA to keep performance up, while still maintaining a high level of edge anti-aliasing. Obviously the allowable combinations for AA are now quite large.

To see what is happening, here are the sample patterns for TrAA when it’s decoupled from MSAA on the 400 series (there will be differences on the 200 series and earlier). First for 8xMSAA (8xQ):

And now for 4xMSAA:

Here the blue dots are geometry while the green dots are shader/texture samples. You can see when the TrAA level matches the MSAA level, the samples are in lock-step and behave like they did on the 200 series (and earlier). Meanwhile, reduced TrAA levels utilize only a subset of existing MSAA samples, though existing sample positions are still retained. In the case of 2xMSAA no decoupling is possible, as this would obviously amount to 0xTrAA.

Also note the actual sample positions as well as their counts. For example, 8xQ/2xTrAA will look worse on alpha textures than regular 2xTrAA because of less effective sample placement on the part of the former.

Interestingly, ATi also used to have decoupled adaptive anti-aliasing in the past called performance, which took half the AAA samples of the base MSAA level. But this seems to have vanished from the driver some time ago.