[CarPhysics] Sort of survey ;)

Hi, I want to implement constraints in my rigid body system. Before starting i would like to ask some question to you, physics guuruu ;o) What system for "Rigid Body with constraint" did you use ? and why ? in your opinion why didn't you choose another system ? here the papers i read : David Baraf "Fast Contact Force Computation for Nonpenetrating Rigid Bodies" Matthias Buck , Elmar Schomer "Interactive Rigid Body simulation with obstacle constraint" Devendra Kalra "A general formulation of rigid body assemblies for computer modeling" Richard Chaney "Simulating Single Rigid Bodies" Michle Clene "Rigid Body Simulation with Contact and Constraints" thanks for your time, cheers, Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-44.24 Monster:-124.44 LFS GTI Online 1:31:02 LFS GTI Offline (HLVC) 1:31:38

Hi Sebastien! I'm not sure who that physics guru is that you're talking about, but I'll try to answer as well ;). I myself started the whole business by this extremely nicely written and instructive piece http://www-2.cs.cmu.edu/afs/cs/user/baraff/www/pbm/constraints.pdf as linked from http://www-2.cs.cmu.edu/afs/cs/user/baraff/www/pbm/pbm.html by Andrew Witkin (An Introduction to Physically Based Modelling: Constrained Dynamics). This in itself should be enough to get you started I believe (and more, in fact). From here on, basically I adapted/polished the stuff on my own. All the sytems basically use the same core principles, it's just the little details which help them be adapted to special applications that mostly distinguishes them. I hope this was of some use, -Gregor On Mon, 24 Feb 2003, Sebastien Tixier wrote:

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> Hi, > > I want to implement constraints in my rigid body system. Before > starting i would like to ask some question to you, physics guuruu ;o) > > What system for "Rigid Body with constraint" did you use ? and why ? > in your opinion why didn't you choose another system ? > > here the papers i read : > > David Baraf > "Fast Contact Force Computation for Nonpenetrating Rigid Bodies" > > Matthias Buck , Elmar Schomer > "Interactive Rigid Body simulation with obstacle constraint" > > Devendra Kalra > "A general formulation of rigid body assemblies for computer modeling" > > Richard Chaney > "Simulating Single Rigid Bodies" > > Michle Clene > "Rigid Body Simulation with Contact and Constraints" > > thanks for your time, > > cheers, > > Sebastien TIXIER - Game Developer > Dynamics and Car Physics > http://www.eden-studios.fr > GPLRank Normal:-44.24 Monster:-124.44 > LFS GTI Online 1:31:02 > LFS GTI Offline (HLVC) 1:31:38 > >

: I hope this was of some use, : : -Gregor Thats freakey - I was about to post "You should get in Touch with the Good Dr Veble " Doug :D

Hehe, but are you sure about the "Good" part? What about my Evil Dr plans about taking over the world... muhahaha... ahem, sorry, did I say that out loud ;)? On Mon, 24 Feb 2003, Doug Ellison wrote:

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> > : I hope this was of some use, > : > : -Gregor > > Thats freakey - I was about to post "You should get in Touch with the Good > Dr Veble " > > Doug > > :D >

"Gregor Veble" <[email hidden]> wrote in message news:[email hidden]... : Hehe, but are you sure about the "Good" part? What about my Evil Dr : plans about taking over the world... muhahaha... ahem, sorry, did I say : that out loud ;)? I dont think anyone heard you. :D Dr Gregor "Evil" Veble is it then . Muhaaa Doug

On Mon, 24 Feb 2003 11:56:36 +0100, Gregor Veble <[email hidden]> wrote:

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>I myself started the whole business by this extremely nicely written and >instructive piece > >http://www-2.cs.cmu.edu/afs/cs/user/baraff/www/pbm/constraints.pdf > >as linked from > >http://www-2.cs.cmu.edu/afs/cs/user/baraff/www/pbm/pbm.html

Hi Gregor! Nice to see you're still reading this NG. :) Didn't your implementation end up being a bit of overkill though on the CPU side? I think I recall you saying at one time that it was funny that after the constraint system was in, it behaved mostly like the old simpler system. :) (my mind may have cunningly construed this though tough ;-) ). But ofcourse you've found a brilliant way to speed up all this, and we'll see it in action in RL, no doubt. :) Cheers, Ruud PS I don't use constraints myself btw, so no survey results from this side. Ruud van Gaal Free car sim: http://www.racer.nl/ Pencil art : http://www.marketgraph.nl/gallery/

Hi Ruud! Yup, that's what I did say; the basic handling isn't all that much different, which is a good thing (means I didn't make any stupid errors in both versions, hopefully ;) ). If you stay at the same level of modelling things you could call it a sort of overkill, yes. But it's not even funny how many further possibilities towards various effects such a framework opens. Since it ran more than fine on my old machine (the PIII450) I don't think performance is that much of an issue. Besides (and I'm fully biased here) what are the processors for if not physics :D -Gregor On Mon, 24 Feb 2003, Ruud van Gaal wrote:

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> > Hi Gregor! > Nice to see you're still reading this NG. :) > > Didn't your implementation end up being a bit of overkill though on > the CPU side? I think I recall you saying at one time that it was > funny that after the constraint system was in, it behaved mostly like > the old simpler system. :) (my mind may have cunningly construed this > though tough ;-) ). > > But ofcourse you've found a brilliant way to speed up all this, and > we'll see it in action in RL, no doubt. :) > > Cheers, > Ruud > PS I don't use constraints myself btw, so no survey results from this > side. > > Ruud van Gaal > Free car sim: http://www.racer.nl/ > Pencil art : http://www.marketgraph.nl/gallery/ >

On Mon, 24 Feb 2003 11:56:36 +0100, Gregor Veble <[email hidden]> wrote:

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>Hi Sebastien! > >I'm not sure who that physics guru is that you're talking about, but I'll >try to answer as well ;). > >I myself started the whole business by this extremely nicely written and >instructive piece > >http://www-2.cs.cmu.edu/afs/cs/user/baraff/www/pbm/constraints.pdf

nice paper, Here's another thing i still have some problems. How do you deal with multiple dynamic contacts, contacts with not null normal velocity at contact ? Exemple. If a block is colliding with a triangle soups, and got for exemple, 10 contacts in front and 1 contact in rear. In real word the block should not get any torque, but in my model, front contacts gives more impulse than the rear one, and the block dont bounce straight :( how to add dynamic contacts on the constrained rigid body system ? cheers, Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-44.24 Monster:-124.44 LFS GTI Online 1:30:62 LFS GTI Offline (HLVC) 1:31:14

Hi Sebastien! I don't think there exists a simple answer to your question. The problem you are experiencing has to do with over-constraining, as I'm sure you know. For a body with 6DOF, more than six contact constraints require a special approach as there does not exist a unique solution to this problem if the constraints are really rigid. I can see two ways out; the first one is 'softening' the contacts slightly, by making the contacts slightly 'springy'. This should bring your linear system (where you calculate the constraint 'forces') that you need to solve away from singularity. This is also one of the suggestions that is given in ODE (it's called constaint force mixing). Another solution is using methods such as singular value decomposition which find one of the solutions (out of many possible); this, however, can prove costly in terms of performance. Another question you have is how to deal with non-zero velocities upon contact creation. This is handled very elegantly in the constraint formalism; you must have some equations that limit your accelerations due to constraints. In such a system, thos equations should also have the posibility to correct any velocity/position mismatches, such that the constraint accelerations tend towads eliminating those in the next timestep. Then the nonzero velocity upon contact creation can be handled in a similar fashion, namely it's just another 'error' in velocity that you need to handle properly. Greetings, -Gregor On Tue, 25 Feb 2003, Sebastien Tixier wrote:

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> > Here's another thing i still have some problems. > How do you deal with multiple dynamic contacts, contacts with not null > normal velocity at contact ? > > Exemple. If a block is colliding with a triangle soups, and got for > exemple, 10 contacts in front and 1 contact in rear. In real word the > block should not get any torque, but in my model, front contacts gives > more impulse than the rear one, and the block dont bounce straight :( > > how to add dynamic contacts on the constrained rigid body system ? > > cheers, > > Sebastien TIXIER - Game Developer > Dynamics and Car Physics > http://www.eden-studios.fr > GPLRank Normal:-44.24 Monster:-124.44 > LFS GTI Online 1:30:62 > LFS GTI Offline (HLVC) 1:31:14 > >

dont konw if you know this paper => www.cs.ubc.ca/grads/resources/thesis/Nov02/Michael_Cline.pdf 111 pages of great value :) Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-44.24 Monster:-124.44 LFS GTI Online 1:30:62 LFS GTI Offline (HLVC) 1:31:14