Bullet 学习笔记之实例 GraspDeformable
参照 GraspDeformable 实例，探究软体抓取的仿真流程。
1 仿真场景的构成
仿真场景类为 btDeformableMultiBodyDynamicsWorld ，派生关系为 btDiscreteDynamicsWorld -> btMultiBodyDynamicsWorld -> btDeformableMultiBodyDynamicsWorld
其中，从成员变量上看，btMultiBodyDynamicsWorld 类在前者（父类）的基础上增加了 multiBody 相关的项，如 m_multiBodies m_multiBodyConstraints m_multiBodyConstraintSolver m_solverMultiBodyIslandCallback 等，其定义部分如下：
///The btMultiBodyDynamicsWorld adds Featherstone multi body dynamics to Bullet
///This implementation is still preliminary/experimental.
class btMultiBodyDynamicsWorld : public btDiscreteDynamicsWorld
{
protected:
btAlignedObjectArray<btMultiBody*> m_multiBodies;
btAlignedObjectArray<btMultiBodyConstraint*> m_multiBodyConstraints;
btAlignedObjectArray<btMultiBodyConstraint*> m_sortedMultiBodyConstraints;
btMultiBodyConstraintSolver* m_multiBodyConstraintSolver;
MultiBodyInplaceSolverIslandCallback* m_solverMultiBodyIslandCallback;
...
}
而 btDeformableMultiBodyDynamicsWorld 又在其父类的基础上增加了 SoftBody 相关的项，比如 m_softBodies m_deformableBodySolver 等，其定义如下：
class btDeformableMultiBodyDynamicsWorld : public btMultiBodyDynamicsWorld
{
typedef btAlignedObjectArray<btVector3> TVStack;
///Solver classes that encapsulate multiple deformable bodies for solving
btDeformableBodySolver* m_deformableBodySolver;
btSoftBodyArray m_softBodies;
int m_drawFlags;
bool m_drawNodeTree;
bool m_drawFaceTree;
bool m_drawClusterTree;
btSoftBodyWorldInfo m_sbi;
btScalar m_internalTime;
int m_ccdIterations;
bool m_implicit;
bool m_lineSearch;
bool m_selfCollision;
DeformableBodyInplaceSolverIslandCallback* m_solverDeformableBodyIslandCallback;
...
}
从成员函数的角度看，所有的仿真步骤是在 btDeformableMultiBodyDynamicsWorld::stepSimulation(...) 中完成。
2 GraspDeformable 实例初始化
2.1 内部成员变量初始化
参照 GraspDeformable 实例，其中的初始化环节，分别给定了仿真场景中相关变量的初值机类型，具体来说，有：
m_collisionConfiguration = new btSoftBodyRigidBodyCollisionConfiguration();
m_dispatcher = new btCollisionDispatcher(m_collisionConfiguration);
m_broadphase = new btDbvtBroadphase();
btDeformableBodySolver* deformableBodySolver = new btDeformableBodySolver();
btDeformableMultiBodyConstraintSolver* sol = new btDeformableMultiBodyConstraintSolver();
sol->setDeformableSolver(deformableBodySolver);
m_solver = sol;
m_dynamicsWorld = new btDeformableMultiBodyDynamicsWorld(m_dispatcher, m_broadphase, sol, m_collisionConfiguration, deformableBodySolver);
以及
btDeformableMultiBodyDynamicsWorld::btDeformableMultiBodyDynamicsWorld(...)
{
...
m_solverDeformableBodyIslandCallback = new DeformableBodyInplaceSolverIslandCallback(constraintSolver, dispatcher);
}
btMultiBodyDynamicsWorld::btMultiBodyDynamicsWorld(btDispatcher* dispatcher, btBroadphaseInterface* pairCache, btMultiBodyConstraintSolver* constraintSolver, btCollisionConfiguration* collisionConfiguration)
: btDiscreteDynamicsWorld(dispatcher, pairCache, constraintSolver, collisionConfiguration),
m_multiBodyConstraintSolver(constraintSolver)
{
//split impulse is not yet supported for Featherstone hierarchies
// getSolverInfo().m_splitImpulse = false;
getSolverInfo().m_solverMode |= SOLVER_USE_2_FRICTION_DIRECTIONS;
m_solverMultiBodyIslandCallback = new MultiBodyInplaceSolverIslandCallback(constraintSolver, dispatcher);
}
其中，碰撞检测相关的： narrow phase 和 Broad phase 还是用了 btCollisionDispatcher 和 btDbvtBroadphase ，而碰撞检测算法的分配（算法矩阵）用了 btSoftBodyRigidBodyCollisionConfiguration 进行分配。
软体仿真相关的用到了 btDeformableBodySolver 和 btDeformableMultiBodyConstraintSolver DeformableBodyInplaceSolverIslandCallback
MutiBody 相关的用到了 MultiBodyInplaceSolverIslandCallback
2.2 仿真场景的构建
在函数 GraspDeformable::initPhysics() 中，还搭建了仿真场景中的物体。
（1）新建了 btMultiBody 类，加入到了场景中；（2）添加 ground shape；（3）新建了软体 btSoftBody ，并向 m_deformableBodySolver 中添加了 btDeformableGravityForce 和 btDeformableNeoHookeanForce 等作用力。
3 仿真计算流程 PipeLine
仿真主要步骤包含在了 btDeformableMultiBodyDynamicsWorld::stepSimulation(...) 中，具体拆分如下：
[1] btDiscreteDynamicsWorld::saveKinematicState(..);
[2] btDiscreteDynamicsWorld::applyGravity();
[3] btDeformableMultiBodyDynamicsWorld::internalSingleStepSimulation(..);
[3.1] btDeformableMultiBodyDynamicsWorld::reinitialize(t);
[3.2] btDeformableMultiBodyDynamicsWorld::applyRigidBodyGravity(t);
[3.3] btDeformableMultiBodyDynamicsWorld::predictUnconstraintMotion(..);
[3.3.1] btMultiBodyDynamicsWorld::predictUnconstraintMotion(timeStep);
[3.3.2] m_deformableBodySolver->predictMotion(timeStep);
[3.4] btMultiBodyDynamicsWorld::performDiscreteCollisionDetection();
[3.5] btDeformableMultiBodyDynamicsWorld::softBodySelfCollision() 即 遍历 btDeformableMultiBodyDynamicsWorld.m_softBodies ，执行 psb->defaultCollisionHandler(psb);
[3.6] btMultiBodyDynamicsWorld::calculateSimulationIslands();
[3.7] btDeformableMultiBodyDynamicsWorld::beforeSolverCallbacks(timeStep);
[3.8] btDeformableMultiBodyDynamicsWorld::solveConstraints(timeStep);
[3.9] btDeformableMultiBodyDynamicsWorld::afterSolverCallbacks(timeStep);
[3.10] btDeformableMultiBodyDynamicsWorld::applyRepulsionForce(timeStep);
[3.11] btDeformableMultiBodyDynamicsWorld::performGeometricCollisions(timeStep);
[3.12] btDeformableMultiBodyDynamicsWorld::integrateTransforms(timeStep);
[3.13] btMultiBodyDynamicsWorld::updateActions(timeStep);
[3.14] btDeformableMultiBodyDynamicsWorld::updateActivationState(timeStep);
[4] btDiscreteDynamicsWorld::synchronizeMotionStates();
[5] btMultiBodyDynamicsWorld::clearForces() 即 btDiscreteDynamicsWorld::clearForces();
