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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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  • cyrusfox - Wednesday, December 15, 2010 - link

    You should totally be able to do a 4X1 display, 2 DP and 2 DVI, as long as one of those DP dells also has a DVI input. That would get rid of the need for your usb-vga adapter. Reply
  • gimmeagdlaugh - Wednesday, December 15, 2010 - link

    Not sure why AMD 6970 has green bar,
    while NV 580 has red bar...?
    Reply
  • medi01 - Wednesday, December 15, 2010 - link

    Also wondering. Did nVidia marketing guys called again? Reply
  • Ryan Smith - Wednesday, December 15, 2010 - link

    I normally use green for new products. That's all there is to it. Reply
  • JimmiG - Wednesday, December 15, 2010 - link

    Still don't like the idea of Powertune. Games with a high power load are the ones that fully utilize many parts of the GPU at the same time, while less power hungry games only utilize parts of it. So technically, the specifications are *wrong* as printed in the table on page one.

    The 6970 does *not* have 1536 stream processors at 880 MHz. Sure, it may have 1536 stream processors, and it may run at up to 880 MHz.. But not at the same time!

    So if you fully utilize all 1536 processors, maybe it's a 700 MHz GPU.. or to put it another way, if you want the GPU to run at 880 MHz, you may only utilize, say 1200 stream processors.
    Reply
  • cyrusfox - Wednesday, December 15, 2010 - link

    I think Anand did a pretty good job of explaining at how it reasonably power throttles the card. Also as 3rd party board vendors will probably make work-arounds for people who abhor getting anything but the best performance(even at the cost of efficiency). I really don't think this is much of an issue, but a good development that is probably being driven by Fusion for Ontario, Zacate, and llano. Also only Metro 2033 triggered any reduction(850Mhz from 880Mhz). So your statement of a crippled GPU only holds for Furmark, nothing got handicapped to 700Mhz. Games are trying to efficiently use all the GPU has to offer, so I don't believe we will see many games at all trigger the use of powertune throttling. Reply
  • JimmiG - Wednesday, December 15, 2010 - link

    Perhaps, but there's no telling what kind of load future DX11 games, combined with faster CPUs will put on the GPU. Programs like Furmark don't do anything unusual, they don't increase GPU clocks or voltages or anything like that - they just tell the GPU - "Draw this on the screen as fast as you can".

    It's the same dilemma overclockers face - Do I keep this higher overclock that causes the system to crash with stress tests but works fine with games and benchmarks? Or do I back down a few steps to guarantee 100% stability. IMO, no overclock is valid unless the system can last through the most rigorous stress tests without crashes, errors or thermal protection kicking in.

    Also, having a card that throttles with games available today tells me that it's running way to close to the thermal limit. Overclocking in this case would have to be defined as simply disabling the protection to make the GPU always work at the advertised speed.
    It's a lazy solution, what they should have done is go back to the drawing board until the GPU hits the desired performance target while staying within the thermal envelope. Prescott showed that you can't just keep adding stuff without any considerations for thermals or power usage.
    Reply
  • AnnihilatorX - Wednesday, December 15, 2010 - link

    Didn't you see you can increase the throttle threshold by 20% in Catalyst Control Centre. This means 300W until it throttles, which in a sense disables the PowerTune. Reply
  • Mr Perfect - Thursday, December 16, 2010 - link

    On page eight Ryan mentions that Metro 2033 DID get throttled to 700MHz. The 850MHz number was reached by averaging the amount of time Metro was at 880MHz with the time it ran at 700MHz.

    Which is a prime example of why I hate averages in reviews. If you have a significantly better "best case", you can get away with a particularly bad "worst case" and end up smelling like roses.
    Reply
  • fausto412 - Wednesday, December 15, 2010 - link

    CPU's have been doing this for a while...and you are allowed to turn the feature off. AMD is giving you a range to go over.

    It will cut down on RMA's, Extend Reliability.
    Reply

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