Multisampling Anti-Aliasing: A Closeup View
Conclusion: What do we want?
Generally multisampling anti-aliasing (with AF activated) is superior to supersampling, because better quality is achieved at the same performance, or, respectively, comparable quality at higher performance. At least as long as resolution can be increased. A higher resolution has smoother edges and more detailed textures. Thus our primary goal is to have smooth, high resolution edges. When anti-aliasing is switched off, even on 1600x1200 the jaggies are too annoying for true quality nuts.
anti-aliasing can generate more quality than performance is burnt, and therefore it will never be "unnecessary". But how is this gain created?
If the resolution of both axes is doubled, "twice" as smooth edges are achieved, but it requires four times the performance. Basically it's the same with oversampling: more performance is lost, than quality is generated. A 4x rotated grid supersampling also requires 4 times the performance, but on the other hand it also smoothes the edges 4 times better. The performance investment is in balance with the quality gain.
f the resolution of both axes is doubled, "twice" as smooth edges are achieved, but it requires four times the performance. Basically it's the same with oversampling: more performance is lost, than quality is generated. A 4x rotated grid supersampling also requires 4 times the performance, but on the other hand it also smoothes the edges 4 times better. The performance investment is in balance with the quality gain.
4x OG doubles sampling accuracy on both axes while 4x RG offers
4 times base accuracy. Hence the RG mask more effectively catches
slopes which results in smoother edges.
With 4x OG on the other hand 2 subpixels can be omitted without really being penalised.
With 2x RG the edge smoothing is a bit worse than with 4x OG. But not
significantly, since the internal increase in axis resolution is the same.
Since multisampling was introduced with GeForce3, there have been complaints about all successors including GeForce FX 5900, that the 4x MSAA mode doesn't improve quality a lot compared to 2x, because the 4x mode still uses the inefficient rectangular grid.
There are cases which are generally problematic with multisampling, whether it's on a Radeon or a GeForce. Maliciously this could be called a "cheat" or at least a "hack", simply because several things are only calculated once per pixel, but are applied to all subpixels.
In any case a subpixel mask with rotated grid should be preferred over a mask with ordered grid though, because with the same number of subpixels (hence the same amount of work) the smoothing result is significantly better. The fact that GeForce FX doesn't support multisampling modes higher than 4x, and not up to 6x like Radeon 9500 and higher do, isn't really a disadvantage. What has an effect on image quality though, is the fact that the 4x mode uses an ordered grid, and thus the smoothing quality is far behind what Radeon 9500 or higher achieve. So be careful when reading anti-aliasing benchmarks comparing "4x vs. 4x"; they usually are pointless, because very different subpixel masks are used.
To avoid the mentioned multisampling problems, it would be nice if rotated grid supersampling was offered as well. GeForce cards since GeForce3 at least have hybrid modes which mix multisampling with oversampling, which unfortunately is the worst supersampling method though. In theory the gained quality costs too much speed to be worthwile, but in practice there are games which aren't too demanding, and there the unused performance can still be invested into additional quality.
To a large extent multisampling is fillrate free, and if both colour and Z compression are offered, only little additional bandwith is used. Therefore such anti-aliasing nearly is "for free". It can be expected that future games will account for potential multisampling problems and that, as a result, supersampling gets less and less important.
This article wouldn't have been possible without the patient help of Xmas and especially Demirug. I am very grateful for this support.