facebook rss twitter

Review: ATI Radeon X1800 Crossfire

by Ryszard Sommefeldt on 15 December 2005, 18:32

Tags: ATi Technologies (NYSE:AMD)

Quick Link: HEXUS.net/qaeaq

Add to My Vault: x

X1K Crossfire Details and Information

While resolution limits are gone and integration is a relative piece of cake, there are still things to note.

The 512MiB XT-spec master board shown on the previous page can also be paired with Radeon X1800 XL 512MiB and Radeon X1800 XL 256MiB. What happens when you pair it with a board that's clocked slower, and might have less on-board memory to boot? Pair it with an X1800 XL and both cards run at their own speeds, but since performance is likely limited to that of the rate the slower board can generate its frame contribution, there's little to no benefit to be had from running the XT at XT speeds in that setup.

With an XL 256MiB, the XT doesn't change its clocks, but it does force you to reboot so that driver can remove its access to half the memory attached to the GPU, making the master board a 256MiB product. Err. While it's possible, I doubt anyone will realistically ever give that a serious go. A reviewer's luxury really, given just one X1800 Crossfire Edition SKU.

Rendering Modes

The rendering modes are unchanged from what we saw back with X850 Crossfire Edition. Therefore their discussion here is still relevant. Should that change at any time, we'll update this page section with the correct information.

SuperAA Image Quality

We analysed SuperAA, the ability for Crossfire to combine depth and colour samples from both GPUs to increase image quality by offering higher levels of antialiasing (both multi and supersampling), here. The sample grids and IQ levels possible are the same with X1K Crossfire, with the addition of X1K's adaptive AA and high quality AF into the mix. We mentioned on the previous page that the compositing FPGA has new skills, and that's the ability to do antialiasing sample resolve. On X850 Crossfire, sample data was sent over the PCI Express bus, via the root, to the master board which did the resolve on the GPU. No more, which means the performance of old should be gone for the most part. I wondered out loud to a colleague earlier about that ability being retrofit to X850, since the FPGAs are the same.

"Interesting that it uses the same FPGA as the X850 XT Crossfire. Wonder if the BIOS or driver for an X850 XT Crossfire board has access to the flash ROM that programs the FPGA. If so, it could feasibly give the Xilinx the new composite code for programming the logic, for SuperAA resolve via that chip."

And it's likely that they could, but the impetus for doing so with Crossfire 1.0 is likely at an all-time low within ATI. Qualification on a product that was so poorly received isn't likely to be great.

SuperAA performance will be looked at in due course.

Crossfire and 'HDR' with multisampling

With ATI's Radeon X1000-series hardware comes the ability for its ROPs (or render back-ends as they call them) to do Z-test on FP16 surfaces. That allows the hardware to multisample an FP16 rendertarget, applying geometry antialiasing to floating point renders of whatever's being drawn.

The term HDR, or high dynamic range to give its full title, is completely misused in graphics journalism and the online graphics community, with many pinning the term to just 16-bit per component FP rendering, when in fact it can apply to lower bit depth integer formats just as well. Despite the misuse however, it is true that ATI's hardware has the ability to apply AA to FP surfaces, which allows the developer a simpler way to support AA and 'HDR', without resulting to non-orthogonal 'hacks' to product similar output.

Crossfire on X1K, since the ROPs are orthogonal in that sense, allows for SuperAA IQ enhancement on multisampled FP16 surfaces. However, it's impossible to freely show that happening, precisely because there are no games out there that support the two together as ATI need for their hardware to go to work. There's a patch for Far Cry that some press have access to, but it's not public and we're not comfortable with showing its output and performance because of that, and after that there's nothing else. Retrofitting the ability to existing games that don't use a HDR approximation for lighting, or that do but don't work in FP16 space, is difficult.

The best case to look at it is games that work in fully FP for their rendering in the first place, and have it work transparently. There are no available games titles that do so, however.

Look out for editorial on HEXUS.core in early 2006 that completely explains what's going on, how HDR works and what it actually means, and how ATI's hardware works to multisample FP surfaces, as part of our ongoing work on a technical explanation of their new hardware that's long overdue. It'll also touch on HDR rendering in current and planned console parts to boot, and have ATI's comments on the issue from their perspective. Keep an eye out.

So we'd love to show you AA and 'HDR' on ATI's terms today, in Crossfire mode, but it's simply not possible.