Anti-aliasing - Temporal AA
I discussed Temporal AA on page 4 (GPU Technologies - SmoothVision HD) but here are the sample patterns, with Colourless's FSAA tester to the rescue yet again.2X2T - 2X AA with 2X Temporal Multiplier - 4X AA Effective
4X2T - 4X AA with 2X Temporal Multiplier - 8X AA Effective
6X2T - 6X AA with 2X Temporal Multiplier - 12X AA Effective
2X3T - 2X AA with 3X Temporal Multiplier - 6X AA Effective
4X3T - 4X AA with 3X Temporal Multiplier - 12X AA Effective
6X3T - 6X AA with 3X Temporal Multiplier - 18X AA Effective
Summary
Given that it's hard to show the effect of temporal AA in motion in a real game title, I won't attempt it here. However, there are caveats in getting temporal AA to work that are worth considering before you consider its use, especially on R3x0 class hardware.Temporal AA needs Vsync to be enabled for the effect to work. This has two knock-on effects. Firstly, the effect is disabled if the framerate of your game drops below that of your monitor's refresh rate. The higher the temporal multiplier, the more you're going to notice it being switched off. For example, 4X3T (12X effective) suddenly becomes plain 4X if the framerate threshold is broken.
This also means that to maintain the effect of temporal AA well, your system and graphics card have to combine to keep game framerates above the refresh rate of your monitor. That'll mean minimum framerates of at least 60fps, for users of LCDs or CRT monitors. While that's certainly possible at the resolutions you'll like to use temporal AA with (less than 1024x768 for the most part), it might not work too well on the latest games at large resolutions.
The actual visible temporal AA effect, when the sample pattern switches per frame, can also manifest itself as slight edge shimmering or creeping on geometry edges. While I can't display it, enabling 6X2T AA on the R420 while playing Painkiller, a title the card can accelerate to the constant > 85fps I needed at 85Hz refresh rate at 1280x1024, would cause some geometry creep on building edges while creeping along dark alleys backlit by fire.
The performance hit from using temporal AA is non-existant and within margins of error. Swapping the sample pattern per frame is something the hardware can do easily.
It's not completely free AA by any means, those caveats combining to rule that out, but it is an effective use of the programmable sample patterns on R420 and R3x0 hardware, to provide free, really high quality AA whenever conditions allow it to happen. The interface for switching it on and off really needs to be doable on a per game basis, as easily as possible.
ATI's 3D control panel isn't the greatest piece of interface design you'll ever see, so it'll be interesting to see how ATI implement the user interface for temporal AA and to see if it can be enabled per game or per resolution at the very least.
