Nvidia's PhysX Particle Fluid Demo
Many of us love Unreal Tournament 3, but what kind of physics eye-candy can we expect to see in next-gen games? Nvidia has whipped up a couple of demos to showcase just that. One of those is the PhysX Particle Fluid Demo, which pretty much does what you'd expect: take a gazillion particles, make them look water-y, set them loose in a sample map, and have the GPU simulate their interactions. In theory, this technique should let game developers achieve the nirvana of fully interactive volumetric water. In practice, it looked more like tapioca soup.

Yes, the water flows sort-of-realistically and fills little pools like it's supposed to. But the liquid has a strange, almost jelly-like quality, and you can see circular "water" particles fly around every now and then. Perhaps a greater number of particles would make the effect more believable, or perhaps better-looking shader effects would do the trick. Either solution probably wouldn't improve the demo's already-low frame rates, though:

Volumetric, particle-based liquids may work great when everyone has GeForce GTX 200-class hardware (or better), but I'd be surprised if many developers were implementing this effect in their games today—especially when titles like 2K Games' BioShock manage to fake volumetric liquids quite believably.

As a side note, the software PhysX implementation in this test only seemed to use one processor core. CPU utilization was paradoxically higher in the hardware physics mode, even though the GPU shouldered the simulation work.

The Great Kulu
In this demo, Nvidia shows off soft-body physics through Kulu, a giant tentacled slug-caterpillar that chases you down corridors by hideously distorting itself like a trash bag full of Jell-O. I've had to sleep with the light on ever since testing this.

Gross.

Nvidia may have written this demo with its GeForce GTX 200 graphics cards in mind, but we had no trouble playing it at 1680x1050 on our lowly GeForce 8800 GT. We didn't benchmark this particular test, because we somehow couldn't run it with PhysX acceleration disabled. You probably get the idea by now, though—PhysX-heavy games and demos tend to run like slideshows without hardware acceleration.

The Great Kulu gives us an interesting glimpse at how games could feature more "organic" objects that bend and squeeze depending on what they collide with. I can't be the only one tired of seeing rag-doll character corpses that behave like they're made of cast titanium. Nvidia's demo goes a little over the top with completely Jell-O-like objects, but the effect remains cool nonetheless.

Update 08/19: The Great Kulu demo seems to only supports GPU-accelerated PhysX on GeForce GTX 200-series cards, so physics simulations ran on the CPU in my testing with the GeForce 8800 GT. Because frame rates felt (mostly) playable, I incorrectly assumed physics acceleration was forced on when it was actually forced off. Nvidia says the following about running the demo with software PhysX:

This demo is available for free and can be installed and played without a PhysX acceleration enabled. However, the minimum system requirements anticipate PhysX being accelerated and it is likely that non-PhysX accelerated systems will experience severe performance degradation at times of high physics load (the ending room). This degradation will not be present at all moments, but should be clearly evident during standard play.

This is more or less consistent with my experience, although I attributed the slowdowns to the GPU choking under the load instead of the CPU.