[vos-d] Thought problem 2: physics 2

[chris]

On 2/2/07, Ken Taylor <taylork at alum.mit.edu> wrote:
> chris wrote:
> > Thought problem 2: physics 2
> >
> > Suppose I am going to do a rigid body simulation. I put one box (box1)
> > on a plane, at the origin and hold another box (box2) suspended a
> > meter above the plane nearby. I release box2 at time t=20 and it
> > bounces, perhaps collides with box1 then eventually comes to rest. I
> > snap an image of the rest state of the sim.
> >
> > Now I repeat the entire sim with viewpoint, boxes and plane in exactly the
> same
> > position as before. I drop box2 at time t=8000, let it bounce and snap an
> > image of the sim when it is at rest.
> >
> > Question: will the two images of the two experiments show box2 in the
> > same rest position relative to box1?
> >
>
> Well first I was going to say "of course not."
>
> But then I realized... time is probably floating-point as well, and has less
> precision when the number is bigger... oh snap!
>
that's right - the results are different again - see attached.

> Of course... why not use a big integer for time?

A big integer at a fixed precision has larger relative error than a
small one so the physics calc will still produce different results.
Depending on size of number and precision, an integer can be worse
than a float, or vice versa.

So, keeping an observation centered at the origin of both space and
time can provide high fidelity *and* consistency.

chris

>
> -Ken
>
>
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