Crossfire Software – A quick reminder…

By default CrossFire is disabled when you first install the system. To enable CrossFire mode you enter Catalyst Control Centre and select the CrossFire section (in the advanced section). Put a tick in the box and after a few seconds you receive the message that crossfire is enabled. No reboots required unless your cards have mismatched memory.

Now its up and running what is it actually doing?

When CrossFire is enabled the idea is that when you launch a 3d application the driver enters a preset mode which splits the graphics rendering between the two cards before combining the image on the master and outputting it to your display. There are three modes of rendering available on Crossfire here’s a brief outline of how each works:

SuperTiling: In this mode the screen is split into sections of 32x32 pixels and each board renders half of the alternate squares.

Alternate Frame Rendering: AFR mode uses each card to render alternate frame . So one card handles all the even frame where as the other handles the odd numbered frames.

Scissor Mode: The screen is split in half (either horizontally or vertically) and each card handles half of the screen. (If needed the split can actually be changed to other levels such as 60-40)

If you have Catalyst AI enabled in CCC it will automatically determine the mode of rendering which is used (Based on App Detection). According to the ATI documentation on CrossFire the modes used are generally Scissor or SuperTiling. Should you not have Catalyst AI enabled Direct3d applications will use SuperTiling and OpenGL will use Scissor.

Super AA
SuperAA uses a different pattern on each card to improve the image over normal AA modes. So every frame has twice the number of AA samples and therefore should provide much more defined images. The double samples are where the new selections come from –
8xAA is effectively twice 4xAA
12xAA is twice 6xAA.

Super Sample AA
SSAA is much more demanding on the hardware than MSAA as it renders the scene at a higher resolution than is required. The resulting image is then downsampled to the resolution chosen by the end user for their game. In CrossFire SSAA mode the extra pixels required are rendered on the additional card to reduce the overall impact of the AA mode. One other limitation of SSAA is that it results in an ordered grid sample pattern which doesn't efficiently AA jagged edges. To overcome this limitation 10x and 14x Super AntiAliasing on Crossfire actually combine SSAA and MSAA.

Using this method different multi-sample locations are used on each GPU as well as offsetting the pixel centres slightly (half a pixel).

So, in basic terms each card is rendering the image from a different angle.
The exact levels of SSAA and MSAA used are as follows:
Super AntiAliasing 10x = 2xSSAA +4xMSAA
Super AntiAliasing 14x= 2xSSAA +6xMSAA