Examples of Physics: Stepping Iterations

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dop2000

- mumu64
- - Joined 8 Feb, 2009
  - 245 topics • 877 posts
- 1
- 2 Apr, 2023
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From the manual:

"Set stepping iterations. Set the number of velocity iterations and position iterations used in the physics engine. The default is 8 and 3 respectively. Lower values run faster but are less accurate, and higher values can reduce performance but provide a more realistic simulation."

Would you please describe me examples of when you used this option "set stepping iterations"?

Examples are easier for me to understand it.

Thank you in advance.
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- dop2000
- - Joined 26 May, 2016
  - 134 topics • 12,108 posts
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- 2 Apr, 2023
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When something is not working correctly with Physics objects (for example a joint breaks, or objects not colliding properly), you can try fixing it by changing these stepping parameters. Sometimes it helps, but most of the times not.

Here is a little more info:

Box2D also uses a larger bit of code called a constraint solver. The constraint solver solves all the constraints in the simulation, one at a time. A single constraint can be solved perfectly. However, when we solve one constraint, we slightly disrupt other constraints. To get a good solution, we need to iterate over all constraints a number of times.

There are two phases in the constraint solver: a velocity phase and a position phase. In the velocity phase the solver computes the impulses necessary for the bodies to move correctly. In the position phase the solver adjusts the positions of the bodies to reduce overlap and joint detachment. Each phase has its own iteration count. In addition, the position phase may exit iterations early if the errors are small.

The suggested iteration count for Box2D is 8 for velocity and 3 for position. You can tune this number to your liking, just keep in mind that this has a trade-off between performance and accuracy. Using fewer iterations increases performance but accuracy suffers. Likewise, using more iterations decreases performance but improves the quality of your simulation.