[ODE] ODE spatial inconsistency

STenyaK (Bruno González) stenyak at gmail.com
Thu Jun 21 11:27:17 MST 2007

On 6/21/07, Marcin Pilat <pilat at cpsc.ucalgary.ca> wrote:
> deterministic version of ODE (using dWorldStep) and when I run the
> sample at the same x,y position, the result is identical every run.
> However, the problem arises when I vary that x,y position.

That makes sense if you use a "lossy" format such as IEEE754, which is
the usual thing in PCs. Imagine you model a screw of 1mm in planet
earth and another one 10.000 astronomic units far away... you can't
pretend both to behave exactly the same, up to an infinite level of
precision, given current computers hardware. You probably can't even
model that without errors of *meters*... the furthest screw would
probably have a size of either 0m or... 1m (or whatever). That's an
extreme case, but should be valid for smaller numbers too.

If you want extreme precission, i'm sure there are math libraries that
provide classes with variable precision. For example, allowing to use
128bits for integers instead of 32bits when you need it (at the
expense of processing time); that might be what you're looking for.

All the above is AFAIK, someone correct me if i'm wrong.

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