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CPU and GPU, meet the PPU

A company called Ageia just announced a new "physics processing unit," the …

Now this (spotted here) is a fascinating idea: a dedicated physics processing unit (PPU) to handle physics computation, just like the GPU handles graphics. I think pretty much anyone who's a gamer can get excited about something like this:

By performing advanced physics simulations in real time, the PPU can respond to gamer actions as well as environments contributing to pervasive interactive reality. By introducing dramatic amounts of physics, games can now react uniquely to each input, adding a tremendous variety of game play. Physics will offer a host of advanced features including universal collision detection, rigid-body dynamics, soft-body dynamics, fluid dynamics, smart particle systems, clothing simulation, soft-body deformation with tearing, and brittle fracturing for destruction of objects in gaming environments.

The first PhysX PPUs would come in the form of a PCI and/or PCIe add-in board, to which the CPU would offload much of the work of physics simulation. The company that designed this product, Ageia, has already gotten a number of developers on-board to support this in their upcoming games.

The press release linked above has some decent information in it, but if you want more details you have to hit this interview at GamersDepot. Unfortunately, the interview, the press release, and the company's website are extremely thin on real technical information. I have no idea what kind of architecture this is. All I know is that is has 125 million transistors, which is about the same number as the P4 Prescott. (NVIDIA's NV40, on the other hand, weighs in at 222 million transistors, which is more on par with IBM's forthcoming CELL processor.) It consumes only 25 Watts of power, which means that it's running fairly slow, which is about what one would expect for an add-on card like this. At any rate, hopefully the company will release a real whitepaper at some point that contains some actual information on the PPU's microarchitecture.

I think one of the best parts of the Gamers Depot interview linked above is the last page, where they ask some leading game developers their opinion on the PPU. In particular, the comments by John Williamson and Kevin Stephenson are spot-on. There's no doubt that the PPU is a great idea from a technical standpoint and will probably do wonders for game realism, but the big challenge will be in selling it.

In the interview and in the press release, Ageia goes to some lengths to frame the PPU as "the next GPU" and to propagate the impression that the rise of the PPU as a product category will mirror the rise of the GPU. In other words, they take this historical narrative that we're all fairly familiar with, and they then encourage the reader to think with that narrative about their own product. There's nothing wrong with this, but the developer comments do point out a few details of that historical narrative that are glossed over by Ageia in order to make the GPU = PPU fit more exact.

First, people were already buying video cards. Sure, most consumers weren't shelling out $200 for them, but they were buying them. Of course, the first 3D video card, the 3Dfx Voodoo 1, was a dedicated add-in board that you had to use alongside a regular video card via a crossover cable, so it does sort of fit the analogy of the dedicated PPU add-in board. This add-in issue was a problem for only the first generation of cards, though, as subsequent generations featured both standard 2D and 3D acceleration.

For the PPU to really take off, it's eventually going to have to piggyback on something else—possibly the graphics card. It's going to be very hard to convince gamers to shell out another few hundred dollars for another add-in board, unless there's a killer app that makes the board a must-have.

This brings me to my second point, and it's one that I haven't seen discussed yet: GLQuake was the killer app that launched the desktop 3D revolution. I bought a Voodoo 1 board, and I bought it to play Quake. I think this was pretty well true of the vast majority of 3Dfx users. Quake 1 laid the groundwork and created a market for 3D acceleration, and 3Dfx was in the right place at the right time with the right product to take advantage of Quake mania. The PPU will need a rough equivalent to GLQuake to create demand for it. This could come in the form of either a single must-have title, or a collection of really popular titles that require the PPU to take the game to a whole new level.

Fortunately for 3Dfx, the rendering engine was something that could be easily replaced to take advantage of 3D acceleration without affecting basic gameplay in a fundamental way. In other words, you could still play a fully functional version of Quake with a software renderer—it just didn't look as good. I'm not so sure that this is true of in-game physics. Physics affects basic gameplay in a way that extremely high polygon counts and particle effects don't, so it's difficult to envision a game that is essentially the same but comes in two different versions: one that has a standard software physics engine, and one that has a hardware-accelerated physics engine that's such a quantum leap in gaming experience that you absolutely have to run out and buy an add-in card. I'm not saying that such a game would be impossible to make, but I think it might be difficult.

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