The numbers
Now, we come to the performance considerations of GPU-accelerated PhysX. We tested three different settings for each benchmark, although the most relevant numbers are with PhysX either enabled or disabled altogether. Just out of curiosity, I included numbers for PhysX without hardware acceleration, which essentially leaves the CPU to handle all the physics calculations in software. It wasn't pretty, as you'll soon find out.

Mirror's Edge caps maximum frame rates by default, so we disabled that cap for these exercises. All in-game detail levels were set to their highest values, except for antialiasing, which was disabled.

EA DICE included a timedemo with the retail version of Mirror's Edge (accessible by adding "-FlybyFlight" to the shortcut's path), so it's as good a place as any to start our benchmarking. The demo pans the camera through segments of the Flight chapter from the single-player campaign, specifically between checkpoints C and D. The demo begins with a helicopter shooting out a few windows while several enemies move through the corridor. We then move outside, with the camera zipping over a few rooftops and providing large-scale shots of the city.

On paper, GPU-accelerated PhysX looks like it induces a pretty nasty performance hit. Drops of 30% for minimum frame rates and 34% for average frame rates are northing to scoff at, but it's important to note that the game remains playable. Even when the action reaches its most heated points, minimum frame rates stay above 30 FPS, and averages push toward 60. The median low of 3 FPS for PhysX running on the CPU is no anomaly—the game runs just fine until a pane of glass breaks or a bullet ricochets. Afterward, Mirror's Edge becomes an unplayable mess. Do yourself a favor and don't turn PhysX on without hardware acceleration to back it up.

Unfortunately, after viewing the timedemo a few times, I became disappointed in its effectiveness as a benchmark representative of the entire game. There's just not a whole lot going on. The first 15 seconds are packed with PhysX effects (shattered glass and window shades being blasted) but the remaining 45 seconds are rather dull, and frame rates seem to reflect that. Performance results from the timedemo were a bit better than what I experienced while playing through other segments of the game, too, so I set out to find a better place to test the performance impact of PhysX effects.

Thankfully, there's no shortage of frantic action in Mirror's Edge, typically beginning with you looking down the barrel of a gun. I found that the Heat chapter provides possibly the best opportunity for PhysX to truly shine (and to punish a system). Beginning with checkpoint B, you find yourself in the lobby of an office building with four stories to work through and dozens of panes of glass between you and enemy fire. A number of banners hanging from the ceiling occasionally get caught in the crossfire, ensuring that breaking glass isn't the only PhysX simulation taking place.

We ran this test by playing through the opening section of Heat B, where Faith works her way up the lobby floors and eventually out of the building, before escaping along the roof. Since manual playtests are less precisely repeatable than timedemos, we played through this sequence five times for each setting and compiled the results.

Now that's more like it! These numbers reflect my experience with the rest of the single-player campaign much more closely. There's a far more noticeable 48% drop in the median low frame rate, accompanied by a 40% decrease in the overall average, when GPU-accelerated PhysX effects are enabled. The most intense moments managed to pull the frame rate under 30 FPS, but not by much. Let's not forget, though, that we're running a GeForce 8800 GTX that's over two years old at a high resolution with in-game detail settings cranked. You have plenty of room to tone down some of the eye candy if 26 FPS is too choppy for you. It wasn't an issue for me—I'm happy as long as I can pull off twitch wall-jumps.

Once again, enabling PhysX without any form of hardware acceleration results in an unplayable slide show as soon as the bullets start flying.