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
Page 17:"The dual SLI connectors are for future applications, such as daisy chaining three G80 based GPUs, much like ATI's latest CrossFire offerings."
Using a third GPU for physics processing is another possibility, once NVIDIA begins accelerating physics on their GPUs (something that has apparently been in the works for a year or so now).
Missing Ghost - Wednesday, November 8, 2006 - link
So it seems like by substracting the highest 8800gtx sli power usage result with the one for the 8800gtx single card we can conclude that the card can use as much as 205W. Does anybody knows if this number could increase when the card is used in DX10 mode?JarredWalton - Wednesday, November 8, 2006 - link
Without DX10 games and an OS, we can't test it yet. Sorry.JarredWalton - Wednesday, November 8, 2006 - link
Incidentally, I would expect the added power draw in SLI comes from more than just the GPU. The CPU, RAM, and other components are likely pushed to a higher demand with SLI/CF than when running a single card. Look at FEAR as an example, and here's the power differences for the various cards. (Oblivion doesn't have X1950 CF numbers, unfortunately.)X1950 XTX: 91.3W
7900 GTX: 102.7W
7950 GX2: 121.0W
8800 GTX: 164.8W
Notice how in this case, X1950 XTX appears to use less power than the other cards, but that's clearly not the case in single GPU configurations, as it requires more than everything besides the 8800 GTX. Here's the Prey results as well:
X1950 XTX: 111.4W
7900 GTX: 115.6W
7950 GX2: 70.9W
8800 GTX: 192.4W
So there, GX2 looks like it is more power efficient, mostly because QSLI isn't doing any good. Anyway, simple subtraction relative to dual GPUs isn't enough to determine the actual power draw of any card. That's why we presented the power data without a lot of commentary - we need to do further research before we come to any final conclusions.
IntelUser2000 - Wednesday, November 8, 2006 - link
It looks like putting SLI uses +170W more power. You can see how significant video card is in terms of power consumption. It blows the Pentium D away by couple of times.JoKeRr - Wednesday, November 8, 2006 - link
well, keep in mind the inefficiency of PSU, generally around 80%, so as overall power draw increases, the marginal loss of power increases a lot as well. If u actually multiply by 0.8, it gives about 136W. I suppose the power draw is from the wall.DerekWilson - Thursday, November 9, 2006 - link
max TDP of G80 is at most 185W -- NVIDIA revised this to something in the 170W range, but we know it won't get over 185 in any case.But games generally don't enable a card to draw max power ... 3dmark on the other hand ...
photoguy99 - Wednesday, November 8, 2006 - link
Isn't 1920x1440 a resolution that almost no one uses in real life?Wouldn't 1920x1200 apply many more people?
It seems almost all 23", 24", and many high end laptops have 1900x1200.
Yes we could interpolate benchmarks, but why when no one uses 1440 vertical?
Frallan - Saturday, November 11, 2006 - link
Well i have one more suggestion for a resolution. Full HD is 1920*1080 - that is sure to be found in a lot of homes in the future (after X-mas any1 ;0) ) on large LCDs - I believe it would be a good idea to throw that in there as well. Especially right now since loads of people will have to decide how to spend their money. The 37" Full HD is a given but on what system will I be gaming PS-3/X-Box/PC... Pls advice.JarredWalton - Wednesday, November 8, 2006 - link
This should be the last time we use that resolution. We're moving to LCD resolutions, but Derek still did a lot of testing (all the lower resolutions) on his trusty old CRT. LOL