NVIDIA's GeForce 8800 (G80): GPUs Re-architected for DirectX 10
by Anand Lal Shimpi & Derek Wilson on November 8, 2006 6:01 PM EST- Posted in
- GPUs
What's Gamma Correct AA?
Gamma correction is a technique used to map linearly increasing brightness data to a display device in a way that conveys linearly increasing intensity. As displays are nonlinear devices, gamma correction requires a nonlinear adjustment to be made to brightness values before being sent to the display. Ideally, gamma corrected linear steps in the brightness of a pixel will result in linear steps in perceived intensity. The application in antialiasing is that high contrast edges can appear under aliased if the brightness of a pixel isn't adjusted high enough for humans to perceive an increase in intensity after being displayed by the monitor.
Unfortunately, gamma correcting AA isn't always desirable. Different CRT, LCD, and TVs have different gamma characteristics that make choosing one gamma correction scheme more or less effective per device. It can also result in brighter colored sub-samples having a heavier influence on the color of a pixel than darker sub-samples. This causes problems for thing like thin lines.
To illustrate the difference, we'll look at images of Half-Life taken on G80 with and without gamma correction enabled.
16XQ No Gamma 16XQ Gamma
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16XQ No Gamma 16XQ Gamma
Hold mouse over links to see Image Quality
We can see the antenna decrease in clarity due to the fact that each of the brighter subsamples has a disproportionately higher weight than the darker subsamples. As far as the roof line is concerned, our options are to see the roof blurring out into the sky, or watching the sky cut into the roof.
Really, edge AA with and without gamma correction is six of one and half a dozen of the other. Combine this with the fact that the effect is different depending on the monitor being used and the degraded visibility of thin lines and we feel that gamma correct AA isn't a feature that improves image quality as much as it just changes it.
While we are happy that NVIDIA has given us the choice to enable or disable gamma correct AA as we see fit, with G80 the default state has changed to enabled. While this doesn't have an impact on performance, we prefer rendering without gamma correct AA enabled and will do so in our performance tests. We hope that ATI will add a feature to disable gamma correct AA in the future as well. For now, let's take a look at R580 and G80 compared with gamma correction enabled.
G80 4X Gamma ATI 4X Gamma
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G80 4X Gamma ATI 4X Gamma
Hold mouse over links to see Image Quality
At 4xAA with gamma correction enabled, it looks like ATI is able to produce a better quality image. Some of the wires and antenna on NVIDIA hardware area a little more ragged looking while ATI's images are smoothed better.
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aweigh - Friday, November 10, 2006 - link
You can just use the program DX Tweaker to enable Triple Buffering in any D3D game and use your VSYNC with negligable performance impact. So you can play with your VSYNC, a high-res and AA as well. :)aweigh - Friday, November 10, 2006 - link
I'm gonna buy an 88 specifically to use 4x4 SuperSampling in games. Why bother with MSAA with a card like that?DerekWilson - Friday, November 10, 2006 - link
Supersampling can make textures blurry -- especially very detailed textures.And the impact will be much greater with the use of longer more detailed pixel shaders (as the shaders must be evaluated at every sub-pixel in supersample).
I think transparency / adaptive AA are enough.
On your previous comment, I don't think we're to the point where we can hit triple buffering, vsync, high levels of AA AND high resolution (2560x1600) without some input lag (triple buffering plus vsync with framerates less than your refresh rate can cause problems).
If you're talking about enabling all these options on a lower resolution lcd panel, then I can definitely see that as a good use of the hardware. And it might be interesting to look at more numbers with these type of options enabled.
Thanks for the suggestion.
aweigh - Saturday, November 11, 2006 - link
I never knew that about SuperSampling. Is it something similar to Quincux blurring? And would using a negative LOD via RivaTuner/nHancer counteract the effect?How about NVIDIA's Digital Sharpness setting in Color Correction? I've found a smidge of sharpening can do wonders to improve overall clarity.
By the way, when you said Adaptive AA, were you referring to ATI cards?
Unam - Friday, November 10, 2006 - link
Derek,Saw your comment regarding the rationale for the test resolution, while I understand your reasoning now, it still begs the question how many of your readers have 30" LCD flat panels?
DerekWilson - Friday, November 10, 2006 - link
There might not be many out there right now, but it's still the right test platform for G80. We did test down to 1600x1200, so people do have information if they need it.But it speaks to who should own an 8800 GTX right now. It doesn't make sense to spend that much money on a part if you aren't going to get anything out of it with your 1280x1024 panel.
Owners of a 2560x1600 panel will want an 8800 GTX. Owners of an 8800 GTX will want a 2560x1600 panel. Smooth framerates with the ability to enable 4xAA in every game that allowed it is reason enough. People without a 2560x1600 panel should probably wait until prices come down on the 8800 GTX or until games that are able to push the 8800 GTX harder to buy the card.
Unam - Tuesday, November 14, 2006 - link
Derek,A follow up to testing resolutions, the FPS numbers we see in your articles, are they maximum, minimum or average?
Unam - Friday, November 10, 2006 - link
Who the heck runs 2560x1600? At 4XAA? Come on guys, real world benchmarks please!DerekWilson - Friday, November 10, 2006 - link
we did:1600x1200, 1920x1440, and even 1280x1024 in Oblivion
dragonsqrrl - Thursday, August 25, 2011 - link
....lol, owned.