Bullet Softbodies: PBD?

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raigan2
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Bullet Softbodies: PBD?

Post by raigan2 » Mon Sep 14, 2009 3:56 pm

hey,
Someone mentioned that the Bullet cloth/soft-body system uses the Position Based Dynamics solver; is this true? I can't find anything in the docs or source which mentions the algorithm (though it was only a quick glance).

If so, was any agreement made with NVidia (or whoever owns the Ageia patents) wrt licensing PBD?

thanks,
Raigan

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Erwin Coumans
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Re: Bullet Softbodies: PBD?

Post by Erwin Coumans » Mon Sep 14, 2009 5:39 pm

Bullet soft body implementation is based on Thomas Jacobsen method, as described in his GDC 2001 paper:
http://www.teknikus.dk/tj/gdc2001.htm

Hope this helps,
Erwin

(please keep patent discussion in this sub forum)

raigan2
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Joined: Sat Aug 19, 2006 11:52 pm

Re: Bullet Softbodies: PBD?

Post by raigan2 » Mon Sep 14, 2009 5:54 pm

(Sorry, thanks -- I forgot about this new sub-forum!)

Could you possibly point me towards the relevant source files for the softbody solver? I've looked through everything with "soft" in the filename but I can't seem to find the actual solving loop.

The main problem is that the PBD patent seems to cover Jakobsen's method of solving distance constraints, as silly/impossible as that may be :(

And as soon as you start to apply Jakobsen's method to any constraint other than distance constraints (i.e angle constraints, collision constraints, etc) you start to get into PBD territory. So I take it this issue is being ignored/sidestepped for the time being?

Basically I'd like to release some tutorials/source at some point, and referring to PBD would make it much easier to show how Jakobsen's method can be applied to other types of constraints. But I guess I'll just have to only refer to Jakobsen (and Gauss-Newton/Kalman/etc)?

thanks again,
Raigan

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Erwin Coumans
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Re: Bullet Softbodies: PBD?

Post by Erwin Coumans » Mon Sep 14, 2009 6:09 pm

Bullet soft body is an application of the relaxation method, see some links below. It mixes Thomas Jacobsen' position constraints with velocity-based constraints. Contacts are handled by velocity/impulse-based dynamics.

btSoftBody::solveConstraints()
http://code.google.com/p/bullet/source/ ... =1754#1578

If you plan to release any work, it is best to refer to patent-free publications, such as Thomas Jacobsen's work. According to Thomas, his work was influenced by Bart Barenbrug, see his PhD thesis and Dynamic library, using iterative constraints for various constraint types:
http://users.bart.nl/users/starcat/dynamo/
"All You Need is Force" http://users.bart.nl/users/starcat/dyna ... /aynif.pdf

The diversity of the iterative relaxation methods, applied to constraints has been pointed out by Thomas Jacobsen, but also in this 1992 paper by Kees van Overveld / Erik van Loon paper "Hanging cloths and dangling rods: A unified approach to constraints in computer animation"
http://www3.interscience.wiley.com/jour ... 2/abstract
Hanging cloths and dangling rods: A unified approach to constraints in computer animation wrote: diversity
Ail types of constraints, occurring in a given application, are dealt with in the same, unified, manner. The algorithms A, may be developed in isolation, one for every constraint function, which will generally be much easier than designing a constraint solving strategy for an entire problem.
Also check out the related paper from 1993, "30 Years after Sketchpad: Relaxation of Geometric Constraints Revisited" with PDF download
http://citeseerx.ist.psu.edu/viewdoc/su ... .1.1.32.24

Thanks,
Erwin

raigan2
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Re: Bullet Softbodies: PBD?

Post by raigan2 » Mon Sep 14, 2009 8:49 pm

Thanks for the links and info; I actually came across Sketchpad a month ago and was pretty shocked at how long the idea has been around!

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