Bart/bullet3
1
0
Fork 0
Code Issues Pull Requests Releases Wiki Activity

use mult instead of max to combine friction properties

Browse Source 
use 0.5 friction for ground for a demo, and also a larger grid size
update description of deformable algorithm
disable SVD for now, has some issue with some compilers
This commit is contained in:
Erwin Coumans
2019-10-28 12:53:59 -07:00
parent 574c240fbc
commit 136607151e
5 changed files with 20 additions and 13 deletions
Download Patch File Download Diff File Expand all files Collapse all files

5
examples/DeformableDemo/DeformableContact.cpp
View File

@@ -105,7 +105,8 @@ void DeformableContact::initPhysics()
btVector3 gravity = btVector3(0, -10, 0);
m_dynamicsWorld->setGravity(gravity);
getDeformableDynamicsWorld()->getWorldInfo().m_gravity = gravity;
getDeformableDynamicsWorld()->getWorldInfo().m_sparsesdf.setDefaultVoxelsz(0.25);
getDeformableDynamicsWorld()->getWorldInfo().m_sparsesdf.Reset();
m_guiHelper->createPhysicsDebugDrawer(m_dynamicsWorld);
{
@@ -182,7 +183,7 @@ void DeformableContact::initPhysics()
psb2->setTotalMass(1);
psb2->m_cfg.kKHR = 1; // collision hardness with kinematic objects
psb2->m_cfg.kCHR = 1; // collision hardness with rigid body
psb2->m_cfg.kDF = 0;
psb2->m_cfg.kDF = 0.5;
psb2->m_cfg.collisions = btSoftBody::fCollision::SDF_RD;
psb2->m_cfg.collisions |= btSoftBody::fCollision::VF_DD;
psb->translate(btVector3(3.5,0,0));

BIN
src/.DS_Store vendored
View File

Binary file not shown.

17
src/BulletSoftBody/btDeformableMultiBodyDynamicsWorld.cpp
View File

@@ -17,14 +17,19 @@
/*
A single step of the deformable body simulation contains the following main components:
1. Update velocity to a temporary state v_{n+1}^* = v_n + explicit_force * dt / mass, where explicit forces include gravity and elastic forces.
2. Detect collisions between rigid and deformable bodies at position x_{n+1}^* = x_n + dt * v_{n+1}^*.
3. Then velocities of deformable bodies v_{n+1} are solved in
Call internalStepSimulation multiple times, to achieve 240Hz (4 steps of 60Hz).
1. Deformable maintaintenance of rest lengths and volume preservation. Forces only depend on position: Update velocity to a temporary state v_{n+1}^* = v_n + explicit_force * dt / mass, where explicit forces include gravity and elastic forces.
2. Detect discrete collisions between rigid and deformable bodies at position x_{n+1}^* = x_n + dt * v_{n+1}^*.
3a. Solve all constraints, including LCP. Contact, position correction due to numerical drift, friction, and anchors for deformable.
TODO: add option for positional drift correction (using vel_target += erp * pos_error/dt
3b. 5 Newton steps (multiple step). Conjugent Gradient solves linear system. Deformable Damping: Then velocities of deformable bodies v_{n+1} are solved in
M(v_{n+1} - v_{n+1}^*) = damping_force * dt / mass,
by a conjugate gradient solver, where the damping force is implicit and depends on v_{n+1}.
4. Contact constraints are solved as projections as in the paper by Baraff and Witkin https://www.cs.cmu.edu/~baraff/papers/sig98.pdf. Dynamic frictions are treated as a force and added to the rhs of the CG solve, whereas static frictions are treated as constraints similar to contact.
5. Position is updated via x_{n+1} = x_n + dt * v_{n+1}.
6. Apply position correction to prevent numerical drift.
Make sure contact constraints are not violated in step b by performing velocity projections as in the paper by Baraff and Witkin https://www.cs.cmu.edu/~baraff/papers/sig98.pdf. Dynamic frictions are treated as a force and added to the rhs of the CG solve, whereas static frictions are treated as constraints similar to contact.
4. Position is updated via x_{n+1} = x_n + dt * v_{n+1}.
The algorithm also closely resembles the one in http://physbam.stanford.edu/~fedkiw/papers/stanford2008-03.pdf
*/

3
src/BulletSoftBody/btDeformableNeoHookeanForce.h
View File

@@ -175,7 +175,7 @@ public:
btSoftBody::Tetra& tetra = psb->m_tetras[j];
btMatrix3x3 P;
firstPiola(psb->m_tetraScratches[j],P);
#if USE_SVD
btMatrix3x3 U, V;
btVector3 sigma;
singularValueDecomposition(P, U, sigma, V);
@@ -194,6 +194,7 @@ public:
Sigma[1][1] = sigma[1];
Sigma[2][2] = sigma[2];
P = U * Sigma * V.transpose();
#endif
// btVector3 force_on_node0 = P * (tetra.m_Dm_inverse.transpose()*grad_N_hat_1st_col);
btMatrix3x3 force_on_node123 = P * tetra.m_Dm_inverse.transpose();
btVector3 force_on_node0 = force_on_node123 * grad_N_hat_1st_col;

8
src/BulletSoftBody/btSoftBodyInternals.h
View File

@@ -1288,7 +1288,7 @@ struct btSoftColliders
c.m_node = node;
c.m_face = face;
c.m_weights = w;
c.m_friction = btMax(psb[0]->m_cfg.kDF, psb[1]->m_cfg.kDF);
c.m_friction = btMax (psb[0]->m_cfg.kDF, psb[1]->m_cfg.kDF);
c.m_cfm[0] = ma / ms * psb[0]->m_cfg.kSHR;
c.m_cfm[1] = mb / ms * psb[1]->m_cfg.kSHR;
psb[0]->m_scontacts.push_back(c);
@@ -1346,7 +1346,7 @@ struct btSoftColliders
c.m_bary = w;
// todo xuchenhan@: this is assuming mass of all vertices are the same. Need to modify if mass are different for distinct vertices
c.m_weights = btScalar(2)/(btScalar(1) + w.length2()) * w;
c.m_friction = btMax(psb[0]->m_cfg.kDF, psb[1]->m_cfg.kDF);
c.m_friction = psb[0]->m_cfg.kDF * psb[1]->m_cfg.kDF;
// the effective inverse mass of the face as in https://graphics.stanford.edu/papers/cloth-sig02/cloth.pdf
c.m_imf = c.m_bary[0]*c.m_weights[0] * n[0]->m_im + c.m_bary[1]*c.m_weights[1] * n[1]->m_im + c.m_bary[2]*c.m_weights[2] * n[2]->m_im;
c.m_c0 = btScalar(1)/(ma + c.m_imf);
@@ -1408,7 +1408,7 @@ struct btSoftColliders
c.m_bary = w;
// todo xuchenhan@: this is assuming mass of all vertices are the same. Need to modify if mass are different for distinct vertices
c.m_weights = btScalar(2)/(btScalar(1) + w.length2()) * w;
c.m_friction = btMax(psb[0]->m_cfg.kDF, psb[1]->m_cfg.kDF);
c.m_friction = psb[0]->m_cfg.kDF * psb[1]->m_cfg.kDF;
// the effective inverse mass of the face as in https://graphics.stanford.edu/papers/cloth-sig02/cloth.pdf
c.m_imf = c.m_bary[0]*c.m_weights[0] * n[0]->m_im + c.m_bary[1]*c.m_weights[1] * n[1]->m_im + c.m_bary[2]*c.m_weights[2] * n[2]->m_im;
c.m_c0 = btScalar(1)/(ma + c.m_imf);
@@ -1462,7 +1462,7 @@ struct btSoftColliders
c.m_bary = w;
// todo xuchenhan@: this is assuming mass of all vertices are the same. Need to modify if mass are different for distinct vertices
c.m_weights = btScalar(2)/(btScalar(1) + w.length2()) * w;
c.m_friction = btMax(psb[0]->m_cfg.kDF, psb[1]->m_cfg.kDF);
c.m_friction = psb[0]->m_cfg.kDF * psb[1]->m_cfg.kDF;
// the effective inverse mass of the face as in https://graphics.stanford.edu/papers/cloth-sig02/cloth.pdf
c.m_imf = c.m_bary[0]*c.m_weights[0] * n[0]->m_im + c.m_bary[1]*c.m_weights[1] * n[1]->m_im + c.m_bary[2]*c.m_weights[2] * n[2]->m_im;
c.m_c0 = btScalar(1)/(ma + c.m_imf);