Will Perone


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I ran into the dreaded TimeOfImpact (TOI) infinite loop recently using Box2D (the Box2DAS3 port). Digging into it there are a few places that iterate indefinitely in the box2d TOI code: b2World::SolveTOI and b2TimeOfImpact::TimeOfImpact. TimeOfImpact does an iterative root finding algorithm that attempts to converge to a solution but there is a situation where the solution is something like 1/infinity so it just gets smaller and smaller and smaller so you have numbers like 1e-18 and so on. Another problem has to do with b2WeldJoint which appears to not work correctly when using a static body. This is because with a static body the m_sweep position is not set and there is no density. What is the solution?
  1. Add an iteration limit on b2TimeOfImpact::TimeOfImpact, limiting the loop to 50 iterations has been working for me
  2. Inside the alpha root finding algorithm in TimeOfImpact, add a check if (alpha < 0.000001) break; this will eliminate the 1/infinity solution problem.
  3. Add an iteration limit on b2World::SolveTOI. I have also been using 50 here. This will eliminate situations where more and more contacts keep being found and it never finishes resolving all of them.
  4. Don't use b2WeldJoint, especially if you are attaching to a static body. Either combine the fixtures into one b2Body or use b2RevoluteJoint with limits set on it.

Hope that helps and I am currently on the market for contracting/consulting so feel free to reach out :)

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