【Bullet引擎】刚体类 —— btRigidBody

btRigidBody类主要用于刚体数据的计算。

在模拟刚体动画过程中，可以使用btRigidBody类获取所保存的刚体对象，进而控制刚体对象的旋转和位移。进行刚体模拟计算需要经常用到此类。

API：http://bulletphysics.org/Bullet/BulletFull/classbtRigidBody.html

创建刚体对象

    btCollisionShape* colShape = new btBoxShape(btVector3(5, 5, 5));//创建一个基本几何体

    /// Create Dynamic Objects
    btTransform startTransform;
    startTransform.setIdentity();
    btScalar mass(1.f);

    //rigidbody is dynamic if and only if mass is non zero, otherwise static
    bool isDynamic = (mass != 0.f);

    btVector3 localInertia(0, 0, 0);
    if (isDynamic)
        colShape->calculateLocalInertia(mass, localInertia);

    startTransform.setOrigin(btVector3(2, 10, 0));//刚体初始位置

    //using motionstate is recommended, it provides interpolation capabilities, and only synchronizes 'active' objects
    btDefaultMotionState* myMotionState = new btDefaultMotionState(startTransform);
    btRigidBody::btRigidBodyConstructionInfo rbInfo(mass, myMotionState, colShape, localInertia);
    btRigidBody* body = new btRigidBody(rbInfo);

    dynamicsWorld->addRigidBody(body);

从场景中获取刚体对象

　　btCollisionObject* obj = dynamicsWorld->getCollisionObjectArray()[i];//i为场景中刚体对象的索引
　　btRigidBody* body = btRigidBody::upcast(obj);

常用方法：

刚体质心坐标：          

　　body->getCenterOfMassPosition();

获取刚体四元数：      

　　btQuaternion qua = body->getOrientation();

刚体旋转矩阵：         

　　btTransform trans;
　　body->getMotionState()->getWorldTransform(trans);
　　btMatrix3x3 matrix33 = trans.getBasis();

向刚体添加外力：

　　body->applyCentralForce(btVector3(2, 1, 0));

向刚体添加转矩：

　　body->applyTorque(btVector3(0, 30, 0));

设置刚体变换：

　　btTransform trans;
　　trans.setIdentity();
　　trans.setOrigin(body->getCenterOfMassPosition() - dx);
　　body->setCenterOfMassTransform(trans);

 刚体对象