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
Hold mouse over links to see Image Quality
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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JarredWalton - Wednesday, November 8, 2006 - link
They did the same thing with the original Halo, porting it (and slowing it down) to DX9. MS seems to think making Halo 2 Vista-only will get people to upgrade to the new OS. [:rolls eyes:]stmok - Wednesday, November 8, 2006 - link
How else are they gonna get gamers to upgrade to Vista? :)(by cornering them into adopting Vista, using DirectX 10.0)
Its sad and pathetic at the same time.
DirectX 10.0 should be a "transitional" solution...That is, it covers both XP and Vista. This allows people to gradually upgrade their hardware, and if they wish, to Vista. What MS is doing now, is throwing everyone (developers and consumers) into the deep end, and expecting them to pay for the changes. (I suspect some would be put off by this, while the majority will continue to accept it...Which is unfortunate).
Great article BTW. Interesting to see the high-end stuff...But I doubt I can afford it in this lifetime!
I have two questions!
(1) Any chance of looking at a triple video card setup?
(I saw a presentation slide which had 2 video cards in SLI, while a third showed something else on screen).
(2) Any idea when the GF8600-series comes?
(mainstream market solution).
yyrkoon - Thursday, November 9, 2006 - link
Great, links arent working ?http://www.gamedev.net/reference/programming/featu...">http://www.gamedev.net/reference/programming/featu...
yyrkoon - Thursday, November 9, 2006 - link
http://www.gamedev.net/reference/programming/featu...">This article was written by a friend of mine back in April after an interview with ATI. Perhaps this will clear some things up.
yyrkoon - Thursday, November 9, 2006 - link
When you break all hardware/software ties to something that has been around for 4-5 years? Its not that easy making it "transitional". From a software perspective, D3D10 is not compatable with XP in the least.I for one, think this is a step in the right direction.
JarredWalton - Thursday, November 9, 2006 - link
Supposedly all of the changes to the WDDM make porting DX10 back to Windows XP "impossible", although I'm more inclined to think the correct term would be "difficult" and you also have to add in "it doesn't fit with MS marketing protocol". WDDM is quite different in Vista however, so maybe there's some substance to the claims.cosmotic - Wednesday, November 8, 2006 - link
On page 9:--Briefly explain what a sub-pixel is in the sentence before--
JarredWalton - Wednesday, November 8, 2006 - link
Due to the size of this article and the amount of time it took to get ready, let me preempt any comments about the spelling and grammar. I am in the process of editing the final document as I read through it, and there are spelling/grammar errors. If they bother you too much, check back in an hour. If you read this an hour from now and you still find errors, then you can respond, though it would be useful to keep all responses in a single thread like this one.Thanks in advance,
Jarred Walton
Editor
AnandTech.com
xtknight - Thursday, November 16, 2006 - link
On p 12 (gamma corrected AA):"This causes problems for thing like thin lines."
acejj26 - Wednesday, November 8, 2006 - link
"If DirectX 10 sounds like a great boon to software developers, the fact that DX10 will only be supported in Windows XP is certain to curb enthusiasm. "I believe this should say "DX10 will only be supported in Windows Vista..."
Not to be rude, but shouldn't the article be edited BEFORE being published??