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Old 06-26-05, 02:47 AM   #1
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Join Date: Mar 2003
Location: Tulsa
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Arrow GeForce 7800 GTX antialiasing investigation

Geforce 7800GTX Anti Aliasing Investigation

Introduction: Last year at the release of the Nv4x hardware. Nvidia implemented Rotated Grid Multisampling for its Nv4x hardware greatly improving quality of their 4x AA and beyond modes. I took it upon myself to write up an Nv4x AA Investigation where I covered the various sample patterns and hidden modes. Since the actual sampling positions for the old modes hasnt changed any. I decided it would be redundant to rehash the same material. Instead, I have decided to focus on the new modes available to the G70 owners. Namely, Transparent AA and Gamma Corrected Anti Aliasing.

Gamma Corrected: Essentially provides superior color reproduction. It can improve your subjective opinion of the image. It can have noticable effect on thin surfaces such power lines often found in games such as Call of Duty or Half Life 2. It is easily toggled off and on through the advanced options in the Nvidia control panel.

Transparency Anti Aliasing: Transparent AA is a technique that can provide edge aliasing to transparent edges. Transparent edges such as alpha textures for instance have been a sore spot for many years since the introduction of multisampling. The idea is for the GPU to detect transparent textures such as leaves, grass, metallic fences (Half Life2) and properly provide anti aliasing to them. Nvidia offers both multisampling and supersampling aproaches to these new techniques which can have varied effects on the object being rendered. You can choose between multisampling or supersampling transparency or have it toggled from the control panel within the advanced settings tab.

Far Cry Comparison Shots

No AA Reference Image

Thoughts: This was a very difficult comparison to make. The hovering leaves and bobbing effect made taking an exact screenshot a bit difficult. Please bear with any minor inconsistencies you might see. While the results are not perfect I am very satisfied with what they represent.

2x AA Comparisons


2xAA/16xAF (Gamma Corrected)

2xAA/16xAF (Gamma Corrected, Transparent Multisampling)

2xAA/16xAF (Gamma Corrected, Transparent Supersampling)

Thoughts: With Far Cry I decided to focus on the leaf textures of the bush. As can be seen here. 2xAA does a reasonable job of cleaning up some of the aliasing along the beams. The textures of the bush however are not anti aliased at all. Once you enabled transparent multisampling, the aliasing is blurred out along the leaves. Under this circumstance it appears multisampling is more effective than super sampling.

4x AA Comparison


4xAA/16xAF (Gamma Corrected)

4xAA/16xAF (Gamma Corrected, Transparent Multisample)

4xAA/16xAF (Gamma Corrected, Transparent Supersample)

Thoughts: Seeing the usual step up from 2x to 4x shouldnt come as any surprise to anyone. However the impressive leap in quality from transparent multisampling should readily noticable on the leaf textures. Once again it seems transparent multisampling is doing a better job of removing edge aliasing from the bush.

8x AA Comparison


8xAA/16xAF (Gamma Corrected)

8xAA/16xAF (Gamma Corrected, Transparent Multisample)

8xAA/16xAF (Gamma Correct, Transparent Supersample)

Thoughts: The nature of supersampling automatically gives a large quality enhancements to the bush leaves. It can still however cause a notable performance hit. Enabling transparent multisampling and gamma correction reduce aliasing to unseen levels in Far Cry.

Far Cry Multi AA Comparison Screenshot

Note: Special thanks to Clay for helping me with the multiple comparison.


Performance Thoughts: The first thing you'll notice is that a Geforce 7800GTX SLI setup is CPU limited up to 4xAA at 1600x1200 resolution. Meaning image quality enhancements are practically free. The 4xAA Gamma correction tests seem to be the oddball here. Showing a drop in performance thats not consistent with 2x or 8x. I imagine this is a bug as it only happens with Far Cry. The other thing you might notice is how there is a difference in performance drops with transparency AA based off the two map graphs. This is because of whats actually being rendered. Indoor Far Cry scenes such as archive do not generally have much foliage or transparent textures. And in this case there are none in the archive maps. The river map however is filled with trees, grass, and a great deal of transparent foliage. The hardware definately seems capable of detecting transparent textures and only taking additional rendering performance when neccasary.

Note: 8xAA has some troubles scaling with Far Cry. People with single cards should see a more consistent framerate drop when turning on 8xAA.

Minor Update: Gamma Correction performance at 4xAA is a known bug and will be fixed in later drivers. Gamma correction now occurs on the Nvidia DACS with no performance hit. Also just as with Half Life 2 there is also a known bug that causes textures to be missing with transparent supersampling. It will be fixed in future drivers.
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Last edited by ChrisRay; 06-26-05 at 06:29 PM.
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