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。

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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。

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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。

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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();

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4 仿真流程概述