Another New Anti-Aliasing Mode: Enhanced Quality AA

With the 6800 series AMD introduced Morphological Anti-Aliasing (MLAA), a low-complexity post-processing anti-aliasing filter. As a post-processing filter it worked with a wide variety of games and APIs, and in most cases the performance overhead was not very severe. However it’s not the only new anti-aliasing mode that AMD has been working on.

New with the 6900 series is a mode AMD is calling Enhanced Quality Anti-Aliasing. If you recall NVIDIA’s Coverage Sample Anti-Aliasing (CSAA) introduced with the GeForce 8800GTX, then all of this should sound quite familiar – in fact it’s basically the same thing.

Under traditional MSAA, for a pixel covered by 2 or more triangles/fragments, 2, 4, or 8 subpixel samples are taken to determine what the final pixel should be. In the process the color of the triangle and the Z/depth of the triangle are both sampled and stored, and at the end of the process the results are blended together to determine the final pixel value. This process works well for resolving aliasing along polygon edges at a fraction of the cost of true super sampling, but it’s still expensive. Collecting and storing the Z and color values requires extra memory to store the values and extra memory bandwidth to work with the values. Ultimately while we need enough samples to determine colors of the involved triangles, we do not always need a great deal of them. With a few color/Z samples we have all of the color data we need in most cases, however the “hard” part of anti-aliasing becomes what the proper blending of color values should be.


1 Pixel Covred by 2 Triangles/Fragments

Thus we have EQAA, a compromise on the idea. Color/Z samples are expensive, but just checking if a triangle covers part of a subpixel is very cheap. If we have enough color/Z samples to get the necessary color information, then just doing additional simple subpixel coverage checks would allow us better determine what percentage of a pixel is covered by a given polygon, which we can then use to blend colors in a more accurate fashion. For example with 4x MSAA we can only determine if a pixel is 0/25/50/75/100 percent covered by a triangle, but with 4x EQAA where we take 4 color samples and then 4 additional coverage-only samples, we can determine blending values down to 0/12/25/37/50/62/75/87/100 percent coverage, the same amount of accuracy as using 8x MSAA. Thus in the right situation we can have quality similar to 8x MSAA for only a little over 4x MSAA’s cost.


MSAA & EQAA Sample Patterns

In reality of course this doesn’t always work out as well. The best case scenario is that the additional coverage samples are almost as good as having additional color/Z samples, while the worst case scenario is that additional coverage samples are practically worthless. This depends on a game-by-game, if not pixel-by-pixel basis. In practice additional coverage samples are a way to slightly improve MSAA quality for a very, very low cost.

While NVIDIA has had the ability to take separate coverage samples since G80, AMD has not had this ability until now. With the 6900 hardware their ROPs finally gain this ability.

Beyond that, AMD and NVIDIA’s implementations are nearly identical except for the naming convention. Both can take a number of coverage samples independent of the color/Z samples based on the setting used; the only notable difference we’re aware of is that like AMD’s other AA modes, their EQAA mode can be programmed to use a custom sample pattern.

As is the case with NVIDIA’s CSAA, AMD’s EQAA mode is available to DirectX applications or can be forced through the drivers. DirectX applications can set it through the Multisample Quality attribute, which is usually abstracted to list the vendor’s name for the mode in a game’s UI. Otherwise it can be forced via the Catalyst Control Center, either by forcing an AA mode, or as is the case with NVIDIA, enhancing the AA mode by letting the game set the AA mode while the driver overrides the game and specifies different Multisample Quality attribute. Thus the “enhance application settings” AA mode is new to AMD with the 6900 series.

To be honest we’re a bit ruffled by the naming choice. True, NVIDIA did go and have to pick daft names for their CSAA modes (when is 8x not 8 sample MSAA?), but ultimately CSAA and EQAA are virtually identical. NVIDIA has a 4 year lead on AMD here, and we’d just as well use NVIDIA’s naming conventions for consistency. Instead we have the following.

Coverage Sampling Modes: CSAA vs EQAA
NVIDIA Mode
(Color + Coverage)
AMD
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

AMD ends up having 1 mode NVIDIA doesn’t, 2xEQ, which is 2x MSAA + 2x cover samples; meanwhile NVIDIA has 16x (4x MSAA + 12 cover samples) and 32x (8x MSAA + 24 cover samples). Finally, as we’ll see, just as is the case for NVIDIA additional coverage samples are equally cheap for AMD.

Tweaking PowerTune Meet the 6970 & 6950
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  • AnnihilatorX - Thursday, December 16, 2010 - link

    I disagree with you rarson

    This is what sets Anandtech apart, it has quality over quantity.
    Anandtech is the ONLY review site which offers me comprehensive information on the architecture, with helpful notes on the expected future gaming performance. It mention AMD intended the 69xx to run on 35nm, and made sacrifices. If you go to Guru3D''s review, the editor in the conclusion stated that he doesn't know why the performance lacks the wow factor. Anandtech answered that question with the process node.

    If you want to read reviews only, go onto google and search for 6850 review, or go to DailyTech's daily recent hardware review post, you can find over 15 plain reviews. Even easier, just use the Quick Navigation menu or the Table of Content in the freaking first page of article. This laziness does not entrice sypathy.
    Reply
  • Quidam67 - Thursday, December 16, 2010 - link

    Rarson's comments may have been a little condescending in their tone, but I think the critism was actually constructive in nature.

    You can argue the toss about whether the architecture should be in a separate article or not, but personally speaking, I actually would prefer it was broken out. I mean, for those who are interested, simply provide a hyper-link, that way everyone gets what they want.

    In my view, a review is a review and an analysis on architecture can compliment that review but should not actually a part of the review itself. A number of other sites follow this formula, and provide both, but don't merge them together as one super-article, and there are other benefits to this if you read on.

    The issue of spelling anf grammer is trivial, but in fact could be symptomatic of a more serious problem, such as the sheer volume of work Ryan has to perform in the time-frame provided, and the level of QA being squeesed in with it. Given the nature of NDA's, perhaps it might take the pressure off if the review did come first, and the architecture second, so the time-pressures weren't quite so restrictive.

    Lastly, employing a professional proof-reader is hardly an insult to the original author. It's no different than being a software engineer (which I am) and being backed up by a team of quality test analysts. It certainly makes you sleep better when stuff goes into production. Why should Ryan shoulder all the responsibility?
    Reply
  • silverblue - Thursday, December 16, 2010 - link

    I do hope you're joking. :) (can't tell at this early time) Reply
  • Arnulf - Thursday, December 16, 2010 - link

    "... unlike Turbo which is a positive feedback mechanism."

    Turbo is a negative feedback mechanism. If it was a positive feedback mechanism (= a consequence of an action resulting in further action in same direction) the CPU would probably burn up almost instantly after Turbo triggered as its clock would increase indefinitely, ever more following each increase, the higher the temperature, the higher the frequency. This is not how Turbo works.

    Negative feedback mechanism is a result of an action resulting in reaction (= action in the opposite direction). In the case of CPUs and Turbo it's this to temperature reaction that keeps CPU frequency under control. The higher the temperature, the lower the frequency. This is how Turbo and PowerTune work.

    The fact that Turbo starts at lower frequency and ramps it up and that PowerTune starts at higher frequency and brings it down has no bearing on whether the mechanism of control is called "positive" or "negative" feedback.

    Considering your fondness for Wikipedia (as displayed by the reference in the article) you might want to check out these:

    http://en.wikipedia.org/wiki/Negative_feedback
    http://en.wikipedia.org/wiki/Positive_feedback

    and more specifically:

    http://en.wikipedia.org/wiki/Negative_feedback#Con...
    Reply
  • Ryan Smith - Thursday, December 16, 2010 - link

    Hi Arnulf;

    Fundamentally you're right, so I won't knock you. I guess you could say I'm going for a very loose interpretation there. The point I'm trying to get across is that Turbo provides a performance floor, while PowerTune is a performance ceiling. People like getting extra performance for "free" more than they like "losing" performance. Hence one experience is positive and one is negative.

    I think in retrospect I should have used positive/negative reinforcement instead of feedback.
    Reply
  • Soda - Thursday, December 16, 2010 - link

    Anyone noticed that the edge missing og the boards 8-pin power connector ?

    Apparently the AMD made a mistake in the reference design of the board and didn't calculating the space needed by the cooler.

    If you look closely on the power connector in http://images.anandtech.com/doci/4061/6970Open.jpg you'll notice the missing edge.

    For a full story on the matter you can go to http://www.hardwareonline.dk/nyheder.aspx?nid=1060...
    For the english speaking people I suggest the googlish version here http://translate.google.com/translate?hl=da&sl...

    There are some pictures to backup the claim the mistake made AMD here.

    Though it haven't been confirmed by AMD if this is only a mistake on the review boards or all cards of the 69xx series.
    Reply
  • versesuvius - Thursday, December 16, 2010 - link

    I have a 3870, on a 17 inch monitor, and everything is fine as long as games go. The hard disk gets in the way sometimes, but that is just about it. All the games run fine. No problem at all. Oh, there's more: They run better on the lousy XBOX. Why the new GPU then? Giant monitors? Three of them? Six of them? (The most fun I had on Anandtech was looking at pictures of AT people trying to stabilize them on a wall). Oh, the "Compute GPU"? Wouldn't that fit on a small PCI card, and act like the old 486 coprecessor, for those who have some use for it? Or is it just a silly excuse for not doing much at all, or rather not giving much to the customers, and still charge the same? The "High End"! In an ideal world the prices of things go down, and more and more people can afford them. That lovely capitalist idea was turned on its head, sometime in the eighties of the last century, and instead the notion of value was reinvented. You get more value, for the same price. You still have to pay $400 for your graphic card, even though you do not need the "Compute GPU", and you do not need the aliased superduper antialiasing that nobody yet knows how to achieve in software. Can we have a cheap 4870? No that is discontinued. The 58 series? Discontinued. There are hundreds of thousands or to be sure, millions of people who will pay 50 dollars for one. All ATI or Nvidia need to do is to fine tune the drivers and reduce power consumption. Then again, that must be another "High End" story. In fact the only tale that is being told and retold is "High End"s and "Fool"s, (i.e. "We can do whatever we want with the money that you don't have".) Until better, saner times. For now, long live the console. I am going to buy one, instead of this stupid monstrosity and its equally stupid competitive monstrosity. Cheaper, and gets the job done in more than one way.

    End of Rant.
    God Bless.
    Reply
  • Necc - Thursday, December 16, 2010 - link

    So True. Reply
  • Ananke - Thursday, December 16, 2010 - link

    Agree. I have 5850 and it does work fine, and I got it on day one at huge discount, but still - it is kind of worthless. Our entertainment comes more exclusively from consoles, and I discrete high end card that commands above $100 price tag is worthless. It is nice touch, but I have no application for it in everyday life, and several months later is already outdated or discontinued.

    My guess, integrated in the CPU graphics will take over, and the mass market discrete cards will have the fate of the dinosaurs very soon.
    Reply
  • Quidam67 - Thursday, December 16, 2010 - link

    Wonderfully subversive commentary. Loved it.

    Still, the thing I like about the High end (I'll never buy it until my Mortgage is done with) is that it filters down to the middle/low end.

    Yes, lots of discontinued product lines but for example, I thought the HD5770 was a fantastic product. Gave ample performance for maintstream gamers in a small form-factor (you can even get it in single slot) with low heat and power requirements meaning it was a true drop-in upgrade to your existing rig, with a practical upgrade path to Crossfire X.

    As for the xbox, that hardware is so outdated now that even the magic of software optimisation (a seemingly lost art in the world of PC's) cannot disguise the fact that new games are not going to look any better, or run any faster, than those that came out at launch. Was watching GT5 in demo the other day and with all the hype about how realistic it looks (and plays) I really couldn't get past the massive amount of Jaggies on screen. Also, very limited damage modelling, and in my view that's a nod towards hardware limitations rather than a game-design consideration.
    Reply

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