Search found 78 matches

by Nathanael
Mon Jun 02, 2008 1:18 am
Forum: Links, Papers, Libraries, Demos, Movies, Comparisons
Topic: Deformables
Replies: 0
Views: 2548

Deformables

An interesting method to implement tetrahedral / bending constraint in a mass spring system (without using tetrahedral/bending constraint gradients).
http://physbam.stanford.edu/~fedkiw/pap ... 008-02.pdf

Nat.
by Nathanael
Wed May 21, 2008 1:18 pm
Forum: General Bullet Physics Support and Feedback
Topic: Softbody bug? Fit softbody rabbit mesh inside a tube mesh
Replies: 5
Views: 1819

Re: Softbody bug? Fit softbody rabbit mesh inside a tube mesh

I have been able to reproduce the problems. - Soft vs solid cylinder work as expected. - Soft vs Bvh/GImpact mesh is not yet implemented. - Soft vs Soft is at a very early stage of development, and is currently very unstable. Improvement of soft vs soft collisions is at the top of the priority list,...
by Nathanael
Sat May 17, 2008 9:19 am
Forum: Research and development discussion about Collision Detection and Physics Simulation
Topic: Collision detection for soft bodies
Replies: 2
Views: 2187

Re: Collision detection for soft bodies

Soft body versus soft body use discreet face/vertex collision only, it can handle self collision, but need some decomposition to avoid false positives not implemented yet (see http://gamma.cs.unc.edu/CDCD/ , and http://www.cs.unc.edu/~geom/RTRI/i3d08_RTRI.pdf ). The current implementation is very si...
by Nathanael
Sat May 17, 2008 8:30 am
Forum: Research and development discussion about Collision Detection and Physics Simulation
Topic: Decomposition of bodies intro tetrahedrons
Replies: 3
Views: 1561

Re: Decomposition of bodies intro tetrahedrons

For open source implementation there is http://tetgen.berlios.de/ . As for algorithms, Delaunay tetrahedralization (ex: http://citeseer.ist.psu.edu/frey96delaunay.html ) work well but is not as easy as triangulation to implement. In many cases, thin shell is enough to simulate deformable bodies, che...
by Nathanael
Wed May 07, 2008 4:26 pm
Forum: General Bullet Physics Support and Feedback
Topic: Bullet issues !?
Replies: 17
Views: 5266

Re: Bullet issues !?

For my comprehension, what you did is that you reduced the penetration length, am I that right ? Could that have any side effect somewhere else in the library ? Yes, you're right, basically, Bullet solve for velocities at contacts position such as velocityBodyA-velocityBodyB=-PenetrationDepth/Dt in...
by Nathanael
Mon May 05, 2008 3:54 pm
Forum: General Bullet Physics Support and Feedback
Topic: Bullet issues !?
Replies: 17
Views: 5266

Re: Bullet issues !?

As a quick fix, you can smooth out velocity drift correction by patching the method 'getDistance' in 'BulletCollision\NarrowPhaseCollision\btManifoldPoint.h' with that: btScalar getDistance() const { static const btScalar frames=16; const btScalar factor=btMin<btScalar>(1,(m_lifeTime/frames)); retur...
by Nathanael
Mon Apr 28, 2008 3:59 am
Forum: Research and development discussion about Collision Detection and Physics Simulation
Topic: EPA
Replies: 20
Views: 5888

Re: EPA

How robust is it on polytopes without a margin? Have you tested degenerate configurations of intersecting boxes? I think this is where things start to can fall apart when trying to use EPA as a replacement for SAT. To test the EPA we only need to compare it with brute-force SAT. No optimizations ar...
by Nathanael
Sun Apr 27, 2008 9:12 am
Forum: Research and development discussion about Collision Detection and Physics Simulation
Topic: EPA
Replies: 20
Views: 5888

Re: EPA

John, It will be nice to have a tests suite for gjk/epa in Bullet, something where we can add tests easily, compare accuracy (or failure) again SAT for polytopes of different implementations (epagjk,epagjk2,Solid,and yours), i don't have time to do that right now, but having a rock solid epa for Bul...
by Nathanael
Wed Apr 16, 2008 6:34 pm
Forum: General Bullet Physics Support and Feedback
Topic: Getting body velocity in local space
Replies: 5
Views: 3209

Re: Getting body velocity in local space

The function i gave you include angular velocity, call it with body->getCenterOfMassPosition() as worldposition if you only want linear velocity, or: static btVector3 getLinearVelocityInBodyFrame(btRigidBody* body) { return( body->getWorldTransform().getBasis().transpose() * body->getLinearVelocity(...
by Nathanael
Wed Apr 16, 2008 2:59 pm
Forum: General Bullet Physics Support and Feedback
Topic: Getting body velocity in local space
Replies: 5
Views: 3209

Re: Getting body velocity in local space

The following should work, btVector3 getVelocityAtWorldPosition(btRigidBody* body,const btVector3& worldposition,bool local) { /* That's the 'localpoint' */ const btVector3 relpos=worldposition-body->getCenterOfMassPosition(); /* That's velocity in world space */ const btVector3 wvel=body->getVeloci...
by Nathanael
Mon Apr 14, 2008 2:04 am
Forum: Release Announcements
Topic: Bullet 2.68 RC1: soft body preview (updated April 11)
Replies: 7
Views: 14467

Re: Bullet 2.68 RC1: soft body preview (updated April 11)

I've just seen that only convex shapes are supported for collision detection against soft bodies, is there any plan for the concave one, or should I stop using compound shape in my software :) ? Yes only convex shapes are supported for the moment, composites are work in progress, and trimeshs are p...
by Nathanael
Sun Mar 23, 2008 2:46 am
Forum: Research and development discussion about Collision Detection and Physics Simulation
Topic: Soft bodies early preview
Replies: 41
Views: 26279

Re: Soft bodies early preview

it happens but some methods seem numerically more robust and can recover gracefully I did test it, each application of gradient correction does in fact recover from sign inversion through (graceful:)) rotations, that could be a problem for one element, but usually more than one tetrahedron share a ...
by Nathanael
Sat Mar 22, 2008 2:15 pm
Forum: Research and development discussion about Collision Detection and Physics Simulation
Topic: Soft bodies early preview
Replies: 41
Views: 26279

Re: Soft bodies early preview

However if you use only one rigid frame and move the vertices out from the object mass center it would not work well for many important cases. make a long straight cylinder and bend it in a U shape, the mass center is in the middle and it looks like that the intermediate "pulled" shapes would look ...
by Nathanael
Sat Mar 22, 2008 1:32 pm
Forum: Research and development discussion about Collision Detection and Physics Simulation
Topic: Soft bodies early preview
Replies: 41
Views: 26279

Re: Soft bodies early preview

We have a tetrahedron having zero volume that would look like a triangle, given that you are trying to restore the volume by using a displacement relative to the mass center node a will never be able to escape b,c,d apparently? i called it a tetrahedron to give it more intuition but you can call it...
by Nathanael
Sat Mar 22, 2008 1:20 pm
Forum: Research and development discussion about Collision Detection and Physics Simulation
Topic: Soft bodies early preview
Replies: 41
Views: 26279

Re: Soft bodies early preview

When i switched between the 2 methods in a FEM solver i never noticed any artifacts and the QR code is just 4-5 lines of code without any iterations. However it can be argued that we are not talking about a FEM solver here so it's difficult to tell without trying it out. what method are you using f...