build islands once and process islands arbitrary number of times in update constraints

src/BulletSoftBody/btDeformableBackwardEulerObjective.cpp

@@ -143,6 +143,8 @@ void btDeformableBackwardEulerObjective::setConstraints()
 {
     // build islands for multibody solve
     m_world->btMultiBodyDynamicsWorld::buildIslands();
+    // for repeated constraint solve, splitIslands has to be set to true
+    m_world->setSplitIslands(true);
     projection.setConstraints();
 }
 

src/BulletSoftBody/btDeformableBodySolver.cpp

@@ -19,7 +19,7 @@
 btDeformableBodySolver::btDeformableBodySolver()
 : m_numNodes(0)
 , m_cg(10)
-, m_contact_iterations(10)
+, m_contact_iterations(5)
 {
     m_objective = new btDeformableBackwardEulerObjective(m_softBodySet, m_backupVelocity);
 }

src/BulletSoftBody/btDeformableContactProjection.cpp

@@ -51,9 +51,11 @@ static btVector3 generateUnitOrthogonalVector(const btVector3& u)
 void btDeformableContactProjection::update()
 {
     ///solve rigid body constraints
-    m_world->getSolverInfo().m_numIterations = 1;
-    m_world->btMultiBodyDynamicsWorld::solveInternalConstraints(m_world->getSolverInfo());
-
+    {
+        m_world->getSolverInfo().m_numIterations = 1;
+        m_world->processIslands();
+        m_world->btMultiBodyDynamicsWorld::solveInternalConstraints(m_world->getSolverInfo()); // process constraints deferred in the previous step
+    }
     // loop through constraints to set constrained values
     for (int index = 0; index < m_constraints.size(); ++index)
     {