Bullet operates in time (seconds) and has a fixed rate at which it takes simulation steps. The Bullet solver does not use frames as a time unit. The fixed rate determines the resolution of the simulation and is specified by the solver's Internal Fixed Frame Rate attribute, which is set to 60 steps per second by default.
For example, if Maya is set to playback at 30 fps, there would be 2 Bullet sub-steps per frame. This design and implementation better supports real-time gaming applications where drawing and computation time per-frame must remain near constant. Allowing the solver to vary the number of simulation steps taken between any two drawing frames would undermine the need to maintain a constant drawing speed.
The solver also uses an iterative solution algorithm to solve the dynamic systems equations instead of an explicit algorithm as another way the Bullet solver maintains constant time to step the simulation. Iterative solutions first approximate a solution, and then send these values back into the equations to arrive at more accurate solutions. The time the solver spends approximating the solution is constant and is only dependent on the number of equations.
When the solution for a single iteration varies by less than some epsilon, the system is considered to be solved. Depending on the simulation, the solution may require additional iterations before it converges on a solution. The solver’s Max Num Iterations attribute sets a maximum number of times the system of equations is iterated on before the solver stops calculating and uses the last approximate solution as the final solve. Therefore, the total time for a simulation step can be clamped. The drawback to this method is that the results may not be as accurate as more explicit solvers. For most applications in games and movies, the solver's degree of accuracy is sufficient.
See Bake Bullet Rigid Body Simulations for information on how to bake a simulation.
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