The render back-ends and antialiasing
The render back-ends in Cypress haven't escaped notice, either. Besides doubling up, the individual render back-end units have gained some new capabilities. A new read-back path lets the chip's texture units read from the compressed color buffers for antialiasing, which should improve performance with AMD's custom-filtered AA modes. Performance when using multiple render targets has purportedly improved, and comically, AMD has built in a provision for fast color clears because some software vendors were prone to clearing the screen many times, for whatever reason.

The larger L2 caches adjacent to the render back-ends should mean less of a performance hit when going from 4X to 8X multisampled antialiasing, according to AMD, although that seems a bit academic since the hit on the RV770 was really quite small.

The biggest news on the antialiasing front, though, is the return of supersampling. Yep, it's back! Most antialiasing methods in recent years have focused on object edges alone, especially the dominant form of AA, known as multisampling. Supersampling is more of a brute-force method in which every pixel onscreen is sampled multiple times, not just object edges. It's terribly inefficient, of course, except that it has the potential to improve image quality everywhere and is obviously the best choice, if you can afford to pay the performance cost. (Supersampling is de riguer among professional animators and the like.)

Because it touches every pixel on the screen, supersampling can address difficult cases that multisampled AA modes won't address—visible edges created by sharp color transitions or alpha transparencies in textures, shimmering in object interiors caused by pixel shaders without sufficient internal sampling rates, or any high-frequency noise your texture filtering algorithm has failed to eliminate. Using it in a game, you may simply find that objects onscreen appear to have more solidity to them.

AMD has gussied up its supersampling mode by toying with the sample patterns, too. I don't have an image of it, and conventional tools won't produce one, but the sampling pattern varies within a 2x2-pixel block, in an attempt to defeat our eyes' propensity to recognize regular patterns. Using four different patterns helps on this front.

The Radeon HD 5870 supports 2X, 4X, and 8X supersampling via a simple switch in the Catalyst Control Center, and the traditional box filters can be combined with custom-filtered AA modes to ratchet up the effective sample count. I'd like to write more about it, but I've only had a week to spend with the 5870 so far, and I think these antialiasing methods deserve their own article, at some point, complete with a suite of comparative screenshots—not that screenshots can capture the full impact of supersampling on image quality.

I'm going to tip my hand on the 5870's gaming performance in order to give you a look at the performance hit caused by various antialiasing methods. Hang on...

Yeah, so looking at the orange bars that represent 4X multisampled AA, a single 5870 is indeed faster than a Radeon HD 4870 X2. Yikes.

And, back on task, the performance hit when going from 4X MSAA to 8X MSAA is indeed smaller on the 5870 than on the 4870 X2—although, like I said, both are pretty much academic at this point.

Notice how much larger the performance hit is for 8X MSAA on the GeForce GTX 285. Nvidia's ROPs or render back-ends just don't handle 8X multisampling as well as AMD's, for some reason. Notice, though, that Nvidia pretty much makes up for its poor 8X multisampling performance via its coverage-sampling AA modes, which store fewer color samples than conventional multisampling. Nvidia's 16X CSAA mode employs more coverage samples and fewer color samples than 8X multisampling and delivers arguably comparable image quality with essentially no performance hit versus 4X MSAA.

For this reason, I've limited the bulk of my performance testing to 4X multisampled AA, where the GPUs are on common ground.

Oh, and yes, the performance hit with supersampling is brutal, but at 4X, the 5870 still achieves a very playable 47 FPS average in Left 4 Dead at 2560x1600 with 16X anisotropic filtering. If you have the power, why not use it?