Date:
Sunday , July 20, 2008
Author:
Brent Justice
Editor:
Kyle Bennett
Google +1

AMD's ATI Radeon HD 4800 Series Custom Filtering AA

AMD’s new ATI Radeon HD 4800 series provides shader powered antialiasing features! We examine in-game IQ and AA scaling. We’ve got all the information on using the Edge-detect Custom Filtering AA and what it will do for you in-game. If you are not sure how to best leverage your new 4870 or 4850 video cards, we show you how.

Introduction

The goal of this article is to specifically focus on AMD’s new Edge-detect Custom Filtering AntiAliasing (CFAA) image quality and the performance you can expect when using a new ATI Radeon HD 4870 video card. While this is not an AMD vs. NVIDIA article, we do use a GeForce GT 260 for IQ comparisons. For those that did not notice, we did feature AMD's new AA modes in both our Radeon HD 4800 series launch day evaluation as well as our HD 4870 X2 Preview, but we simply did not want to lose focus on overall performance and have chosen to showcase CFAA here, all on its own.

Prices for the 4870 span from $255 after MIR to $295. Prices on the GTX 260 span from $269 after MIR to $329 for 260 cards without custom coolers. As you can see, in the last couple of weeks, the 4870’s performance has forced the GTX 260 pricing into much more competitive territory. So when it comes to AA, IQ, and smooth gameplay, is Edge-detect Custom Filtering AA anything to get excited about and does it provide a better AA gaming experience than what NVIDIA has to offer?

The Radeon HD 4800 series (Radeon HD 4850 and 4870 video cards) launched with a new image quality feature to improve antialiasing using an Edge-detect filter that does not tax memory bandwidth. This new Edge-detect filter flies under the banner of the “Custom Filtering.”

If you recall, Custom Filtering was announced with the Radeon HD 2900 XT back in May of 2007. Up until now, there were two filters supported under Custom Filtering; “Narrow-Tent” and “Wide-Tent.” We looked at the image quality of those two filters here in the 2900 XT evaluation and actually found them to negatively impact overall image quality. We disliked the two methods so much we published a “Don’t Use Narrow Tent and Wide Tent AA” section. Narrow and Wide Tent modes not only smoothed edges, but “smoothed out” texture detail as well.

The new Radeon HD 4800 series has a new filter, called “Edge-detect.” Thankfully, Edge-detect is nothing like Narrow or Wide-Tent, and actually improves image quality.

CFAA – Edge Detect

Article Image Article Image

The new Edge-detect CFAA filter is unique because it uses the stream processors to carry out its workload, and does not consume memory bandwidth or memory space. The filter uses an edge detect algorithm that delivers 12X and 24X CFAA modes with the same memory footprint as 4X AA and 8X AA respectively. This new filter does not cause blurring like Narrow-Tent and Wide-Tent do. This new filter also fully works when Adaptive AA (the transparency AA mode of the 4800 series) is enabled. First let us walk you through how enabling it works, and what happens, and then we will tell you some important things you need to know concerning Edge-detect CFAA.

Enabling CFAA

Article Image

When you launch Catalyst Control Center (CCC) you will see that the default setting is “Use Application Setting” and “Box” filter selected under the AA slider options. In this default configuration you can set AA through your games in-game settings, and because the filter is on “Box” you will only receive the default 2X AA, 4X AA and 8X AA (MSAA) modes. The party begins when you click the filter drop down box.

When you click on the filter drop down box you will see that there are four filters there, Box, Narrow-tent, Wide-tent, and Edge-detect. (Ignore Narrow-tent and Wide-tent! Both should be removed from the menu.) As we stated above, the Box filter is the default 2, 4, and 8 MSAA modes. Click on Edge-detect, and click Apply; you have now enabled Edge-detect AA.

How it Works

Article Image Article Image

Edge-detect AA only works in combination with 4X and 8X AA, it doesn’t do anything at 2X AA. You can confirm this by un-checking “Use application settings” and manually slide the slider to 4X AA. Now, click on Edge-detect in the filter box and click Apply. You will see that the “Samples” rate has changed to reflect 12X. Now, if you slide the slider back to 2X AA you will notice that the filter box automatically changes back to Box filter. Continuing the other way, if you set the slider to 8X AA with Edge-detect enabled it will show 24X under “Samples.” This should help break it down a bit easier:

4X Box Filter AA = 4X MSAA

4X Edge-detect AA = 12X CFAA

8X Box Filter AA = 8X MSAA

8X Edge-detect AA = 24X CFAA

The next question you are going to ask is can you use Edge-detect CFAA when selecting AA from within the game? The answer is most certainly “YES.” You simply leave the check mark on “Use application settings” and then click the filter box and select Edge-detect. Now, when you set 4X AA and 8X AA in the game you will really get 12X AA and 24X AA respectively. To go back to regular 4X AA and 8X AA simply select Box filter in the control panel.

Adaptive AA

Article Image

The Radeon HD 4800 series also supports Adaptive AA, which reduces aliasing on alpha textures. In the current driver (Catalyst 8.7 beta here) when you enable ADAA you can select either Super-sampling or Multi-sampling, and then Performance or Quality. These selections are going to be simplified in a future driver. When we were testing the Radeon HD 4870 X2, with its beta driver, this option was reduced to only having the option for Performance or Quality; there was no drop down box for Super-sampling or Multi-sampling. We asked AMD what this means now and they responded:

"When using adaptive AA the driver analyses the current rendering and decides whether supersampling or multisampling is needed in order to improve the quality of the current transparent object. We found that the old controls in the panel were confusing for some of our users as the presence of the drop-down box implied that they could select between supersampling and multisampling, whereas actually the box simply controlled whether the tuning controls for the supersampling or multisampling portion of the adaptive AA mode were being displayed in the panel. The driver behaviour has not changed, and the control panel interface has been altered to be simpler and less confusing."

Along with Adaptive AA you can also use Edge-detect CFAA modes and this is where it gets complicated. The sample rate of ADAA will follow the sample rate of the CFAA mode on objects where Super-sampling is applied, but on objects where Multi-sampling is applied it will follow the MSAA sample rates. For example, alpha texture AA image quality will actually look better using 8X MSAA + ADAA rather than 4X MSAA + CFAA + ADAA.

Let’s break it down. 4X AA plus CFAA will give us 12X samples, but when we turn on Adaptive AA for alpha textures, there may be some that only receive 4X AA because that is the sample rate of MSAA. But, if we enable 8X AA plus Adaptive AA, it will be taking 8X samples on those alpha textures. So, 8X MSAA is higher than 4X MSAA, which is better on alpha textures. But, on polygon edges 12X CFAA is going to be better than 4X and 8X MSAA. As you can see, there are a lot of combinations to deal with.

Something to Be Aware Of

We asked AMD about CFAA support in games, if it is transparent to the game, and if there are any games where it does not work. This is their response in its entirety, loaded with good information regarding CFAA:

"The CFAA filters are transparent to the application so they will work in essentially all cases. At the moment CFAA is supported in the DX9 and OpenGL drivers, and does not function in DX10 applications, but full DX10 support is coming very soon. In the case of DX10 applications if the developer writes their own custom AA filter (which is one of the new capabilities that developers have in DX10) then our custom filter will not be applied in order to guarantee that the developer gets the result that they wanted. An example of a DX10 game where the developer has written their own custom AA filter to improve image quality is Call of Juarez - in this title CFAA will not operate."

So take note, right now CFAA does not work in DX10 right now. Also, some games where developers will write their own custom filters, AMD’s CFAA filters will not operate.