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Valve Defuses NVidia Half-Life 2 Issues
suineg writes "Gabe Newell, Valve's main man, has commented on Half-Life 2 and anti-aliasing problems with DX9 cards over at HalfLife2.net: '[The current problems] ...will look like a bright or dark line on the edge of a polygon. This is not a new problem. Artifacts may show up more frequently in Half-Life 2 simply because we've eliminated lots of other artifacts, and because we have a lot of variation in scene lighting due to our art direction.'" As far as solutions go, Newell has some: "ATI has supported... [the centroid work-around] form of anti-aliasing for the 9000 series... [as for] NVIDIA's [hardware], that doesn't support centroid sampling... you trade off some pixel shader bandwidth to clamp the texture coordinates."
Does "Can't see the forest for the trees" apply here?
Centroid antialiasing takes the average of the sampled pixels and applies it to the entire sampled area (or, in most cases, just the edges of the area, as centroid antialiasing does not look good over a large surface, but can smooth out problem cases like the edges of polygons). This will make some situations look better, and some look worse (the example in the white paper I read mentions that as two objects move towards the camera the colour may 'snap' from one to the other, although it's still only in the edge cases that Valve is discussing).
Basically, it's an algorithm used to reduce the memory (and memory bandwidth) requirements of high-sample anti-aliasing techniques (for example 4x FSAA, which at say 1280x1024@32bpp would require ~160MB of RAM and/or slow rendering 16x). Algorithms that work for high accuracy without the slow-downs of full antialiasing will use centroid algorithms in combination with a number of other algorithmic shortcuts which, when combined, can produce nearly the same image, though with more artifacts and problem cases.
-PainKilleR-[CE]
I wonder if its a case of driver bugs being used as features.
It's actually just a combination of a few different things.
1) Most developers, if they absolutely must pack textures into larger textures, learn pretty quickly to put a border on their textures so that oversampling of any type will still pick up the right texture (or at least a similar texture), they even tell you to do this in some of the books aimed at teaching 3d game programming to new developers.
2) 'Centroid' antialiasing techniques is a shortcut to do antialiasing without the high overhead of 'true' antialiasing, but it looks so bad (because it averages the values) that it's normally only used on these edge cases (which tend to be problematic in many antialiasing algorithms), where people are unlikely to notice.
3) depending on how antialiasing algorithms are implemented by nVidia, the centroid algorithms may never be available. For instance, if the entire process is done in hardware, it's unlikely that they'd change the hardware unless they felt it would improve their algorithm. Even then, those with older hardware would still not gain the benefit.
For whatever reason, nVidia doesn't use centroid algorithms in their antialiasing, you could either look at this as a quality choice, or as just a simple matter of using a different set of algorithms. More than likely, not using this is part of the reason that nVidia's antialiasing has usually been slower than competitors' antialiasing (first 3dfx and now ATI). That being said, not using this may also result in more accurate antialiasing, even though the edge cases may have more obvious problems when developers don't compensate for it. Chances are that if nVidia utilized a centroid algorithm in a newer driver without specifying the games it should be used for or giving an option to enable/disable it, then those games that actually border their textures would look worse with the centroid algorithm when they previously had no problems.
-PainKilleR-[CE]
They cheat on the FSAA. What they do is they don't actually sample the entire screen, they basically just run AA on the textures when they are rendered, then anti-alias the edges as they are rendered. Although technically what you said is true, if you think about it, its fairly unfeasable to say, render it at double resolution (say 3200x2400), then render that down into 1 1600x1200 screen. The memory requirements alone are massive. 1 24 bit 1600x1200 screen is 5.76 megs of video memory as it is, so one at 3200x2400x24 bits is roughly 24 megs. That is a lot of video memory to be using just for anti-aliasing, not to mention, can you imagine what kind of memory bandwidth you'd need to move that much data around every frame?
Like I said, there're a number of ways you could look at it. In what way does ATI's AA look better? It's something that you can't easily measure, because it could look better simply because the algorithms are faster, or because the algorithms do less (or because the edge cases, as in the centroid algorithm's case, are harder to see flaws with).
nVidia's newest cards also do FSAA in a significantly different manner from their previous cards (ie FX cards vs. non-FX cards), utilizing buffers from other operations to perform the AA instead of rebuilding their buffers (whether or not ATI does this I don't know). For higher level AA (6x and 8x) they actually use the 2x and 4x AA methods on portions of the screen and then combine the results in the buffer, which may also lead to ATI's results looking better.
Of course, to me FSAA never looks better than rendering at a higher resolution, because none of the current cards do FSAA by simply rendering at the higher resolution and then scaling down to the displayed resolution (which would give better image quality, but causes bigger performance hits).
In the end, if one card (nVidia) shows artifacts on the polygon edges because they don't use centroid algorithms in their AA and another card (ATI) does use the centroid algorithms, many people will say that the card that uses centroid 'looks' better, especially in a game that causes a lot of problems in those edge cases. It doesn't mean that those edge cases are more accurate in terms of antialiasing algorithms, simply that the eye doesn't catch them as easily.
-PainKilleR-[CE]