On Sat, 7 Sep 2002 10:23:49 +0200, "Tim Nikias" <tim### [at] gmxde>
wrote:

>When using the string-system described on
>Hugo Elias' pages, my string gets very instable, wiggling
>about at all kinds of sections...

That's a typical result of integration instability of stiff
differential equations. There are three ways to avoid this:

1. Loose the equations, i.e. use softer springs
2. Increase integration precision, i.e. decrease the integration step
3. Improve precision, i.e. use a higher-order integration technique
(Adams, Runge-Kutta etc. or even implicit Euler)

>Do I need to have that low a moving distance and
>that high iteration?

To put it shortly, yes. If you are ready to dig into some math and
physics, I can point you to some more complex answers :)

>And isn't mass-speed lost with more iterations?

If you mean linear momentum (the product of mass and velocity, as a
vector), well, yes of course it is lost in the process... after all,
there *are* external forces at work, non-conservative at that,
otherwise your system would oscillate for ever.

There's a catch related to damping and integration step. You will
notice that if you reduce the integration step 10 times, for example,
the damping seems to increase dramatically. That's a pretty obvious
reason to that - you do apply damping (and that's a multiplication)
ten times more often :) So you have to reduce damping accordingly to
get the same (more or less) results.

There's quite a lot to say on the subject. If you need more info, be
prepared for a lot of reading - but it's so much fun! :))


Peter Popov ICQ : 15002700
Personal e-mail : pet### [at] vipbg
TAG      e-mail : pet### [at] tagpovrayorg

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