The technology

Like NVIDIA’s SLI, CrossFire supports multiple rendering modes to split the graphics load as evenly as possible among both RADEON graphics cards.

One of the oldest methods used to accomplish this is to split the screen in half, with one graphics card handling the top section while the second card is responsible for the lower segment. The frame buffer data is then combined and sent to the monitor. It’s important to note that while we say split in half, the screen isn’t split perfectly in the middle as in some scenes the bottom of the screen may be more complex than the top, or vice versa. The screen can also be split horizontally or vertically. The driver determines the most efficient way to split the workload on the fly as conditions change.

ATI has dubbed this mode “scissor mode”. This is CrossFire’s default mode for OpenGL and also works in Direct3D applications as well.

Pretty simple board layout

Not many capacitors near CPU socket

The second mode that’s probably familiar to many of you is alternate frame rendering (AFR). ATI made this mode famous with their Rage Fury MAXX graphics cards a few years back. With AFR, each card handles alternating frames. Graphics core one handles everything in frame 1, while graphics core two then handles all of frame number 2. Both chips will continue to repeat this process, with one card handling the even frames, while the second card is responsible for the odd-numbered frames. Each chip renders every other frame instead of alternating portions of the same frame, as the other methods do. The completed frames from both cards are then sent to the compositing engine on the CrossFire board, which then sends them to the display.

Alternate frame rendering is available for both OpenGL and Direct3D applications. It’s the method used for all DOOM 3/Quake 3 engine games, with scissor mode being used for older OpenGL applications.

The final rendering mode ATI CrossFire supports is supertiling. In supertiling, the scene is split into 32x32 squares known as “tiles”. Each card then handles alternating tiles, which are then output to the display. By splitting the workload into small tiles, it can be distributed very close to an even split

Supertiling is the preferred rendering mode for Direct3D applications, but isn’t available for OpenGL.

The key ingredient that makes all this happen is the compositing engine chip, which is located on the CrossFire graphics card. The compositing chip takes the data from its local RADEON VPU and combines it with the data from the slave graphics card, which it receives over a DVI connection. The chip is responsible for handling all communication between the two cards, as well as handling blending and arithmetic operations. From there the CrossFire card outputs the image via DVI adapter to the monitor, where it’s displayed on your screen.

Better Anti-aliasing

Besides performance, another key addition CrossFire supports is better AA. Both graphics cards use slightly different sampling patterns which are then blended together by the compositing engine to produce the final image, ultimately providing sharper visuals. CrossFire also provides new AA modes. In addition to the standard 2, 4, and 6 sample AA modes ATI has provided previously, CrossFire also supports 8x, 10, 12x, and 14x modes.

For CrossFire’s 8xAA mode, both cards are running in 4xAA mode, for an effective equivalent of 8xAA likewise at 12xAA, both cards are running in 6x mode. The 10x and 14x modes mix multisampling with supersampling for even better image quality. At the 10x setting for example, the cards mix 8x multisampling with 2x supersampling, while 14x combines 12x multisampling with 2x supersampling.