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### 4.13.2 Physics using ODE

The ``odedynam'` plugin implements physics for Crystal Space using the ODE library.

#### Doing the Timed Physics Simulation

To make sure the physics simulation proceeds correctly in time you must tell ODE how much time has elapsed since previous frame so that it can calculate the collisions and physical interactions that happened during that time. The `iDynamics->Step()` function is responsable for that. As a parameter it has a delta which is a floating point number that represents the number of seconds to calculate. You should not make this number too big because that will make the calculations less accurate. A good number for accurate calculation is around ``0.01'`. ``0.02'` is also good and makes calculations a bit faster. However, what should you do if the elapsed number of seconds is bigger then that number? In that case you should divide the elapsed time with the delta and perform that number of steps one by one. It is also very important to make sure that the delta you pass to the `Step()` function is constant. If this number is variable then ODE will not behave well and you get stability errors. Here below I present a possible code snipped that will ensure that the delta is constant and also makes sure that no time is lost so that the speed of physics simulation is constant even with variable framerate:

    csRef<iVirtualClock> vc = ...;
    csRef<iDynamics> dynamics = ...;
    float delta = 0.01f;
    float remaining_seconds = 0.0f;
    ...
    void ProcessPhysicsEveryFrame ()
    {
      csTicks elapsed_time = vc->GetElapsedTicks ();
      float seconds = float (elapsed_time) / 1000.0f;
      seconds += remaining_seconds;
      while (seconds >= delta)
      {
        ProcessForces (delta);
        dynamics->Step (delta);
        seconds -= delta;
      }
      remaining_seconds = seconds;
    }

This snippet of code will make sure that `Step` is only called with a constant delta. If there is a small time left after the loop (less then delta) then that will be remembered (in ``remaining_seconds'`) for the next frame.

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