WE'VE WATCHED OVER the course of the past six months as NVIDIA has reaped enormous publicity benefits by resurrecting the SLI multi-GPU graphics capability that it inherited from 3dfx, and we've wondered when ATI would join the fray. The folks at ATI have been talking about their own multi-card graphics capability for what seems like ages, and today, they're finally taking the wraps off of their solution: ATI CrossFire.
The CrossFire platform is similar to NVIDIA's SLI, but it's distinctive in some intriguing ways. CrossFire does allow for the teaming of a pair of PCI Express video cards to increase graphics rendering capacity, but it doesn't rely on a bridge between the two cards. Instead, CrossFire requires that one of the two cards in a system be a special, CrossFire-capable unit that has a custom composting chip onboard designed by ATI. This chip, onboard the master graphics card, takes the image from the local Radeon VPU and mixes it with the image from the slave graphics card, which it receives over a DVI connection. The CrossFire card then outputs the composite image via its own DVI output (or to VGA via a converter).
ATI's CrossFire platform
ATI's Radeon VPUs have had the ability to combine images since the debut of the R300 chip, but this will be the first consumer-level implementation of the technology. CrossFire graphics cards will be able to work together several ways in order to produce a final image with higher performance than a single card. Those rendering modes are:
Supertiling This is the tiling mode built into the original R300 VPU and used by ATI partner Evans & Sutherland in its high-end visualization systems. The screen is split up into 32x32 tiles and the workload is distributed according to a checkerboard-style pattern, with one card taking what would be the black squares and the other the squares that would be red. Splitting up the workload in such relatively small tiles should result in a distribution of the load that's very close to an even split, and it should allow two cards to produce markedly higher fill rates and pixel shader throughputs than a single card. The vertex processing load will be duplicated on each card, however, so that a CrossFire system with supertiling won't likely achieve any higher geometry throughput than a single card. Supertiling will be the default CrossFire mode for all Direct3D applications, but will not work in OpenGL.
Scissor mode This mode will be familiar from SLI, where NVIDIA calls it split-frame rendering. Scissor mode divides the screen horizontally, giving the top portion of the screen to card A and the bottom portion to card B. The exact proportion of the split is adjusted on the fly as workloads change. This will be CrossFire's default mode for OpenGL and will also work in Direct3D.
Alternate frame rendering This one, ATI reminds us, is the mode used by its previous multi-chip solutions, like the Rage Fury MAXX. Alternate frame rendering involves buffering a few frames ahead of what's being displayed onscreen, with the cards alternating drawing entire frames. AFR can be enabled on a CrossFire system via ATI's control panel. This mode distributes both fill rate and geometry loads evenly between the cards, allowing for better scaling. ATI says AFR should be an option for both OpenGL and Direct3D applications.
Super antialiasing Uniquely, CrossFire rigs may provide image quality benefits even in games where fill rate and geometry throughput isn't normally at a premium via its Super AA capability. Super AA comes courtesy of the CrossFire compositing chip, which can combine images with different sample patterns produced by the two cards. CrossFire AA offers several new antialiasing modes from 8X up to 14X. 8X and 12X AA modes double up on 4X and 6X multisampling, respectively. 10X and 14X AA modes, on the other hand, combine 2X supersampling with 8X and 12X multisampling, respectively.
After talking to ATI's Catalyst driver team, we learned that although CrossFire's Direct3D and OpenGL defaults allow it to accelerate any application, the drivers will also ship with application-specific profiles. These profiles generally use Alternate Frame Rendering, which apparently offers better performance than superTiling or scissor modes. However, AFR apparently can't be blindly enabled without the danger of causing display corruption or stability problems, so scissor and supertiling modes provide good fallback positions for games that lack AFR profiles.