GDC Notes: The Physics of PhysX

A funny thing happened after my meeting at GDC yesterday with folks from Ageia, who make the PhysX physics processor chip. I walked away from the demo kind of stunned. I went into the meeting never having seen a demo of the physics processor's abilities. I came out with my jaw dragging along the floor.

The PhysX permits developers to build out the full physics of many more objects in a game. The demo showed off a level of a next generation first-person shooter game, called Cell Factor, as enhanced by the developers to take advantage of the PhysX's prowess at, well, physics.

The question I left the demo asking myself was 'What is it that makes an action game look like a game, and an action movie look like a movie?' I realized, it's not just the main characters, or the main action sequences, but the fine background details that make the real world real. In many games, that there is any dynamic detail in the background is remarkable.

At the time Half-Life 2 came out, the ability to pick up a few small trash objects, and throw them, seemed worthy of note.



This is so much bigger than that.

In many games, smoke simply slides along laterally, not filling spaces or otherwise behaving like we know smoke does in real life.



PhysX adds details even to the way smoke billows out from an explosion---in this case, twisting and curving like a liquid, filling volumetric space as it flows. Wherever an object disrupts its flow path, the smoke flows around it in every direction, even up walls. And with the PhysX, dust and smoke particles can apply some force to objects; In the demo, when the avatar fired a gun at a wall near where several objects were arrayed, bits of exploding concrete pillar pushed one of the objects, a cylinder, away from the site of the blast.

In one scene during the demo, a truck which had been hit with some sort of rocket weapon was blown into (approximately) fourteen kajillion pieces. According to Ageia, each fragment of every destructible object remains persistently as a discrete object for the remainder of the level. A destroyed object doesn't disappear or fade away; All its constituent parts each become separate objects that the graphics board needs to keep track of, with rules of physics to follow. You could see why the test system's dual-SLi graphics board system was even having trouble keeping up a decent framerate with the sheer number of objects it needed to render in one outdoor scene. That part of the game was object rich.

The PhysX demo also showed off a new twist on an old weapon:



It's a flamethrower which sprayed burning viscous liquid, possibly napalm. Its ammunition resembles flaming maple syrup.

The avatar, in the demo, took advantage of the physics of this weapon to shoot flame at an otherwise hidden enemy through a pile of neatly stacked lumber. The flames passed through the gaps between boards in the lumber stack, setting the enemy player ablaze. Later, the avatar walked up to, and directly beneath, an enemy guard tower, and fired almost straight up. The enemy within was neatly dispatched by a shot of fire gel aimed inside the ceiling of the tower; The flames bounced off the ceiling, then gravity took over, and the burning gel came raining down over an enemy who was out of sight.

The Ageia folks also showed off how they model the behavior and destructability of cloth:



Before (left) and after the attack.

The building in this level was decorated with a large, draping flag, designed to look as if it were made of a rough-hewn fabric. As my avatar fired his rifle at this flag, bullets tore jagged holes in the fabric---discrete holes, one for each gun blast. Then, the character threw a larger object at the flag, a metal canister, which tore a wide vertical stripe of fabric from the top of the flag downwards. It also tore away adjoining bits of cloth that had been weakened by gunshots. The flag was left in tatters.

While the demonstration was impressive, there's a high price to join the PhysX club. Not only will you need the card, but you need a virtually top end system. Laptops are out (for the moment). The Ageia demo guy also had to kick the graphics processing setting for the game down a few notches. The PhysX can create thousands of objects in a scene, each of which will behave independently (according to the laws of PhysX?) when players interact with them. Run it at 100%, even on the dual SLi system they had set up, and the framerate will be unplayable; When the card has to render too many objects at once, it slows everything down.

Also, games will have to be customized for use with the PhysX. Roughly 100 titles currently have PhysX support. But some game developers will have to make difficult choices about how much to rely on a physics processor that won't be in every customer's PC. The problem isn't likely to go away quickly: At $300 (MSRP), the PhysX costs as much as a new main CPU. Not everyone will be able to afford one.

The objects that PhysX enables can cause additional damage, if the developer chooses to do so. Think shrapnel. But should the PhysX-enhanced player be permitted to inflict greater damage on the enemy than someone who doesn't have the card? Conversely, should it be easier to kill the PhysX player just because they might see a much larger explosion when the bad guy lobs a grenade their way? And online multiplayer games make this calculus even more difficult: How does the game treat damage if there's a mix of players with and without PhysX cards on the same server?



There's no question that PhysX is going to radically change the look and feel of games, make them more Jerry Bruckheimer-cinematic. But, at least initially, it will also separate gamers into physics haves and have-nots. Should the player with better physics processing always have a better chance at slaying the dragon than the player without? What do you think?