ATI's X8xx CrossFire Graphics Arrive
by Derek Wilson on September 26, 2005 1:00 PM EST- Posted in
- GPUs
Super AA and CrossFire
By rendering the same frame on both cards with different subpixel sample patterns, the images can be blended together to provide a smoother image at any given resolution than a single card is capable of rendering. At the same time, as resolution increases and pixel size decreases, antialiasing becomes less important. This is yet more support for believing that greater than 1600x1200 resolutions should be supported on a high end setup like this.For those who will be running CrossFire solutions at 1280x1024 or even 1600x1200, Super AA will be a welcome addition to image quality. ATI already leads the industry in AA quality because they use a programmable sample pattern when antialiasing a pixel while NVIDIA uses a fixed ordered grid approach. The difference really starts to add up when SLI AA and Super AA are compared.
From left to right, 4xAA, 8xSLI AA/10xSuper AA, 16xSLI AA/14xSuperAA. ATI is the top row.
The more even spread that ATI is able to maintain over a single pixel gives the CrossFire solution a better result. Even though NVIDIA's 16x SLI AA has 16 geometry sample points and 4 texture sample points compared to ATI's 12 geometry points and 2 texture points, the distribution of ATI's sample points provide more efficient coverage. Incidentally, if ATI had named their modes like NVIDIA's AA modes, they would have said 20xAA rather than 16xAA.
NVIDIA's 4 texture sample points (essentially super-sampling/SSAA sample points) could provide better interior, texture, and transparent surface antialiasing. Unfortunately, their arrangement limits their usefulness in this regard. Thus we have to declare ATI the clear winner in the AA department. Of course, Super AA mode does take quite a performance hit as we will find out later. But take a look at what it can do to Half-Life 2 at 800x600 (no AA, 4xAA, 6xAA, 10xAA, 14xAA from top to bottom):
While differences beyond 4xAA are harder to spot, take a look at the antennae on the bottom-left of the images. You can see how the barely-visible parts are rendered better, particularly with the 10xAA and 14xAA modes. If you have the GPU performance to handle such features, they're a nice addition.
The down side of Super AA (aside from the performance hit) is that it will only run in full screen applications. Windowed applications are still stuck with 2x 4x 6x and no AA. Even if a Super AA mode is selected, only half the subpixel samples are used. We are not sure if this is a hardware or software limitation, but those of us who play MMORPGs in the background will need to be aware of this issue.
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justly - Tuesday, September 27, 2005 - link
I’m still a little confused about this 60MHz limit at 1600x 1200.Can someone please explain what I am looking at (in the graphs) when every 1600x1200 resolution that is tested is above 60MHz. If crossfire really is limited to 1600x1200@60MHz then what do these numbers represent? Wouldn’t this also make the entire benchmarking section of this article pointless?
Pete - Wednesday, September 28, 2005 - link
Framerate is separate from the physical refresh (or redraw, in the case of LCDs) rate of your monitor. The cards can produce higher than 60fps at 16x12, but you'll only see at most 60 of those frames in a given second. It's the same principle as with single cards today. Your frame rate might be 120fps, but if you're monitor's refreshing at 85Hz, you aren't going to see all those frames. They're just--for lack of a better term--thrown away.justly - Wednesday, September 28, 2005 - link
Thanks Pete, but I do understand the difference between a cards frame rate and a monitors refresh rate.My confusion is in regards to this quote from the article “The problem is that ATI is currently fixing maximum CrossFire resolution to 1600x1200@60Hz”, and then we see every game test ran at 1600x1200 over 60Hz (BTW thanks for ignoring my typo of MHz instead of Hz in my earlier post).
If the numbers in the game tests are the actual frame rates then why limit the resolution? Furthermore can the cards driver limit the monitors refresh rate? And again looking at the scores why would they want to do this?
I feel as if I am missing a vital piece of the puzzle.
Fluppeteer - Thursday, September 29, 2005 - link
Sorry, now I think *I'm* missing something.Where does it say anything about the games being run at over 60Hz refresh?
The cards manage significantly higher frame rates than 60fps when rendering
the games, but I've not spotted the monitor setup being mentioned *at all*.
Maybe I'm going blind. The graphs just show frame rate - no mention of refresh.
How do you mean "why limit the resolution"? Who's limiting it to what (in this case)?
The driver is responsible for programming the video outputs on the card, and
for reporting the available modes to the Display Properties dialogue. If the
driver won't let you set a higher resolution, then you can't (without using
something like PowerStrip) even if the card could physically do it. So yes,
the driver could limit it. The modes presented are a the result of negotiation
between the monitor and the driver, unless they're manually overridden. ATi's
drivers have been known not to present all the modes the monitor/card can handle
unless persuaded by a registry hack, but I don't think that's what's going on here.
In case of further confusion, it's up to the card to tell the monitor how to refresh
- the card "pushes", the monitor doesn't "pull". Some monitors have an internal
buffer to allow their screens to be updated separately from the frame buffer, and
obviously the refresh rate affects TFTs less than CRTs (a TFT won't fade to black
a fraction of a second after you last updated it), but the rate at which the video
gets from the card's frame buffer to the monitor is determined by the driver's
decision of how to program the card.
As for why ATi(?) would want to do this, I'm sure they don't. In theory Crossfire
should support higher single-link resolutions (at lower refreshes) - 1920x1200 at
50Hz, for example - and I believe it actually does, with the right monitor attached.
However, the X800/X850 series start to run out of performance above 1600x1200, when
their hierarchical Z buffer doesn't fit any more (according to an article on higher
resolution rendering with 7800GTXs; I could be wrong), so 1600x1200 would be a
reasonable limit anyway; also few LCD monitors support more than that without being
widescreened, and many games don't like a wide screen configuration. What you can't
do is exceed the single link bandwidth, so the card isn't psysically capable of
sending 1600x1200 at 85Hz for a CRT. 1600x1200 on an LCD should be fine, because
60Hz won't flicker; it's only CRT owners and those (few) with dual link LCDs who
have a problem.
I would guess this test was performed with a 20" LCD, on the basis that a 60Hz
1600x1200 CRT would drive the reviewer nuts quickly, and they're more common than
1920x1200 panels; hence no test above this. Any specific details of refresh will
presumably be the result of the driver talking to the monitor, and may not correspond
to what you'd get with a *different* monitor. Almost all TFTs will be within the
limits, but if you've got a decent CRT then it's definitely a problem. Nice of ATi
to buy us all 2405FPWs, isn't it?
--
Fluppeteer
FreshPrince - Tuesday, September 27, 2005 - link
even though their overall sli performance is lower than nvidia, if you compare the percentage increase from ati single to sli with the percentage increase from nvidia single to sli...ati comes out on top.which basically tells us when ati comes out with a 7800gtx killer, they will have much higher sli numbers than nvidia.
the limitations, I'm sure they will square out in future implementations of crossfire.
I hope ati comes out with all-in-wonder editions of the sli cards...can you say 2 channels at the same time without any lag? :D
TheInvincibleMustard - Wednesday, September 28, 2005 - link
"ati comes out on top"... umm ... did you read/understand the tables at the bottom of page 6? Quoted directly from the review -- "the 4xAA modes show that SLI provides better scaling" ... meaning that SLI gets closer to the theoretical doubling of performance (ie, 100%) than CrossFire does. In fact, the only situation in which a move from single-card to dual-card scales better on the ATI side than the nVidia side is Doom3 at 16x12/noAA (41.3% vs 34.0%)...
This tells us nothing substantial about performance improvements when ATI comes out with R520, but in my opinion it would seem that nVidia has the stronger dual-card algorithms. If I had the choice of spending an additional $425 (cost of a master card, estimated) for an average performance improvement of ~37% with noAA or ~56% with 4xAA for ATI's CrossFire, versus spending an additional $450 (cost of another 7800GTX) for an average performance improvment of 48.5% with noAA or 79.8% with 4xAA for nVidia's SLI, I think I would definitely go with nVidia's solution, as that additional $500 for another 7800GTX offers more bang-for-buck than for one of ATI's master cards. Combine this with the fact that 7800GTX SLI offers better performance overall anyway, and it's a no-brainer. The situation's a little different if you've already got a 6800Ultra (but then why are you looking at CrossFire?) or an x850xt (but then why are you looking at SLI?), but those situations have no bearing. Simply put, nVidia has hands-down won this "round" of multi-card rendering, even ignoring current availability.
-TIM
PS -- I would be curious what would be possible with two AIW's hooked up in CrossFire ... that could certainly have some interesting situations open up ...
yacoub - Tuesday, September 27, 2005 - link
So that would denote that AA "costs" more on the ATI setup than it does on the NVidia setup, right?
DerekWilson - Tuesday, September 27, 2005 - link
Not necessarily ...The ATI could have performed better because of the way HL2's code runs on ATI hardware allows a higher maximum performance under similar CPU limitation. It could be a driver efficiency issue. It could be a number of things ...
It's too hard to make a statement about anything when we are bouncing hard off of CPU limited performance.
yacoub - Tuesday, September 27, 2005 - link
Please re-word this caption so it makes more sense given the image (six boxes):DerekWilson - Tuesday, September 27, 2005 - link
the commas seperate list items ... I describe 3 columns there and if ATI is the top row, NVIDIA must be the bottom ... 6 boxes ...I'm not sure how to be more clear. If I can get a good suggestion, I'll certainly change the caption.