[Bf-committers] Some Maths / ideas / solutions for Softbodies.

bjornmose at gmx.net bjornmose at gmx.net
Tue Nov 30 15:34:46 CET 2004


I knew this would happen,
I was talking about the softbody stuff the meeting minutes (and the wiki )
are about. There is a explicit euler running to have first ( and fast )
results , but i am well aware of the problems with stiff ODEs, though my
masters in physics was some years ago. If you look at softbodies.c you'll
see that in the framestep driver there is the spot prepared for semi
implicit euler. So i see the the code like giving a minimal running physics
engine to see how far we can get, keeping it clean an simple. (code wise)  

So don't mix this this up with instinctive blenders softbodies :)

> hello, thanks for the precisions. :)
> 
> But i have some objections ;)
> In every paper that i could found on softtbodies/cloth it was written 
> clearly
> "Do not use explicit Euler"
> "Use adaptive method or Implicit method"
> Instinctive softbodies use explicit euler, and if you take a too thin 
> mesh, it explodes.
> Mabye cloth is not the guideline but people will certainly use it, and 
> it will crash with forward euler.
> Users will think it's a bug, and not a mathematical limitation. It is 
> not controlable by the imput values since it depends on the mesh 
> resolution.
> Because users will want bendable but not elastic cloth, they will set a 
> very low elasticity that will lead to stiff problem. Stiff problem leads 
> to a really small step, and then lot of calculation.
> Even if it seems fast, it will quicly become a slow method with stiff 
> problems.
> As in everywhere in pixar's paper i see 'do not use forword euler' i 
> really doubt that maya does it.
> Cloth and softbodies are the same algorithms, so i think we should adapt 
> them to fit the most difficult case .
> that was my point view :)
> 
> -Fred
> 
> 
> bjornmose at gmx.net wrote:
> 
> >Hmm,  
> >
> >I'd  like to point to that fact, that i never talked about cloth
> simulation
> >to be the guiding idea behind this approach. 
> >
> >When you look at the Maya docs there are two features we first want to
> have.
> >1. Goals : The soft object is hanging elastic in its original static cage
> >2. General vertex to vertex (object to object) springs to modify elastic
> >properties of the object(s).
> >
> >If you look closer at the Maya docs and the parameters used for setting
> such
> >systems up, may lead you to the suspect that Maya also is integrating the
> >system of ODEs, most likely using brute force forward euler, since for 
> >reasonalbe well behaving systems the calculation of the jakobian and
> setting
> >up a sparse matrix solver would be numerical overkill for a little yelo
> >wobble.
> >
> >so much for that
> >/ole
> >
> >  
> >
> >>
> >>
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