[ODE] interpolating between physics frames to get the actual positions in the render time

[David Whittaker]

The standard way of solving this problem is to slow your render speed down
to match up with the physics, something like this:

unsigned long curTime, lastTime = 0;

void mainloop()
{
  curTime = time();  //whatever ms resolution timer you have access to
  if (lastTime + (stepsize*1000) - curTime > 0)
    sleep(lastTime + stepsize - curTime);
  lastTime = curTime;

  //draw the world

  //forces, joint motors, collision detection, etc.
  dWorldStep(...);
}

Something like that should make your graphics match your physics
perfectly, unless the physics take longer than the stepsize, then you have
a whole other problem.  It might be simpler, depending on the architecture
you are building your program in (i.e. MFC vs. Qt vs. drawstuff...) to use
an event-based timer to invalidate your window, draw the last frame at the
top of your draw function, and at the end, run the physics step.  Then
make your stepsize as small as you can and still have a little bit of
sleep time on most runs through the loop.  If you really insist on running
the graphics faster than the the physics, you'll have to do the
interpolation yourself.

The only time that will come out of sync now is when your system is so
large or complex that it takes longer than the timestep to run the
worldStep.  So what can be done about that?  Well, there are a variety of
speed hacks you can try.  Reducing the complexity of the system, either by
turning multiple bodies and joints into a single body "fake" (who needs to
physically simulate toes, for example, when the whole foot as a single
body is good enough).  Disabling bodies that are at rest.   Trying my soon
to be added to the CVS stepFast solver.  Using frictionless or infinite
friction contacts where possible.  There's more in the manual.  And when
all else fails, increase the stepsize.

David

> Using the fixed time step physics results in slight time difference
> between the actual
> render time and physic time which can cause choppiness to the movements
> of objects...the effect would be more noticeable with kind of large
> timesteps tho.
>
> Instead of using just the last physics step position/orientation one
> could interpolate
> between the last two physics steps and get the position/orientation
> exactly at the
> rendering time..
>
>
> Therefore my question is..
>
> Is there any way to do that with ODE?
>
> It's certainly possible to do that outside ODE but it's kinda
> hassle..and what do you think about lerping between the frames in
> general..
>
>
>
> cheers,
>
>
>
> juhnu
>
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> ps. oh my first posting to this list ever.. have to buy a cake or
> something
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