Evaluating Physics Engines For Games - PAL -

Physics engines are a high-tech aspect within a next-gen game. They are in charge of simulating all the physics as realistic as possible. Many times the accuracy of the physics is reduced to meet the real-time requirements of the game. Many companies prefer to buy the physics engine as a middleware and apply their effors in others parts of the game. Nowadays there well established companies offering this middleware, like for example Havok, Nvidia-AGEIA. There are of course new comers and open-source engines such as Pixelux, Bullet, ODE, Tokamak, etc.

The big question is: which physics engine is the best ?

The answer is straighforward: None of them is in overall the best and it is very case dependent. However, there is still a way to make some comparisons. So, I came across this paper:

Evaluation of Real Time Physics Simulation Systems,
A. Boeing, T. Braunl
Procc. Computer graphics and interactive techniques in Australasia and South East Asia, 2007

The work on the paper compares different physics engines: AGEIA-Nvidia, Bullet, JigLib (from my friend Danny Chapman), Newton Physics, ODE, Tokamak, True Axis. The authors implemented a physics abstraction layer (PAL) to carry out the comparison. You can even download PAL. I suppose that it has been an amazing work to wrap all the physics engine in only one application. Unfortunately, HAVOK is not compared nor PIXELUX.

The comparison was based on five tests:

1. Integrator performance (i.e. solution of ODE's)
   
2. Material properties (i.e. frictions, restitution)
   
3. Constraint stability
4. Collision detection
5. Stacking test

The authors had these results: AGEIA-Nvidia has the best integrator performance. True Axis gave the best results for restitutions followed by Bullet and AGEIA-Nvidia. Static friction was best simulated by Newton followed by AGEIA-Nvidia. With respecto to constraints, Tokamak provides the best results solving constraints in the least time, ODE provided the most accurate results but requires a lot of time to solve the constraints. In this aspect, among the worst you can find AGEIA-Nvidia. This is very surprising since I've been using AGEIA-Nvidia and it has been quite accurate when solving the constraints. Collision detection was best carried out by Tokamak and Bullet, however, it should be noted that other engines failed this test because the test was not properly adapted to the engine. For box stacking the author only provided a visual test claiming that all the engines pass the test with the best performance for Tokamak.

The main conclusion that the authors made is that Bullet engine provided the best results overall.

Although this result is the conclusion of a huge work, it should be taken with a lot of care, since many aspects were not included in the comparison and that the PAL system does not support correctly some engines (for example AGEIA-Nvidia for collision detection), Pixelux and Havok were not included in the comparison, support from the physics engine provider, other aspects such as fluid simulation, soft body simulation and some specific optimisations of the engines were not included. Which ones are optimised for PS3 or Xbox360 ? Results are evidently tested on a PC. Finally, some tests are not suitable in many cases, for instance the integrator performance may vary for each engine and for each object being simulated.

Still, PAL is a good starting tool to start testing physics engines.

Anyway, my favorites are AGEIA-Nvidia, Havok and Bullet and the one we are developping :)