FEMFX 仿真之 刚体类 FmRigidBody

在 FEMFX 中，刚体对象为 FmRigidBody，详见 FEMFX/amd_femfx/src/Simulation/FEMFXRigidBody.h

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FmRigidBody 类的定义为：

struct FmRigidBody
{
    uint                  objectId;
    uint                  rbIslandId;         // Island id assigned by rigid body engine; The FEM library will group FEM objects in the same island if connected to the same rigid body island.
    uint                  femIslandId;        // Id of island that contains this body in FEM scene.
    FmRigidBodyState      state;              // Dynamic state of rigid body
    float                 mass;               // Mass of rigid body
    float                 frictionCoeff;      // Used for contacts with this body
    FmMatrix3             bodyInertiaTensor;  // Inertial tensor in rigid body's local space
    FmMatrix3             worldInertiaTensor; // Current inertial tensor in world space
    FmVector3             gravityVector;      // Gravity acceleration, 0 by default; added with FmSceneControlParams::gravityVector

    float                 dims[3];            // Shape dimensions (box half-widths)
    float                 maxRadius;          // Largest distance from center to point on rigid body, used to bound motion for CCD
    FmTetMeshBuffer*      collisionObj;       // Used for intersection with FEM tet meshes
    FmAabb                aabb;               // AABB for rigid body
    uint8_t               collisionGroup;     // Collision group index < 32

    // These are updated after each pass of solving or stabilization.
    // In a solver pass, deltaVel is change in velocity and deltaAngVel is change in angular velocity.
    // In a stabilization pass, deltaPos is change in position, deltaAngPos is change in "angular position", which can be seen treated as integrated angular velocity.
    // When a user solves external constraints, these should be updated prior to running the next pass.
    FmVector3             deltaVel;
    FmVector3             deltaAngVel;
    FmVector3             deltaPos;
    FmVector3             deltaAngPos;

    bool                  foundInConstraint;  // Used to decide whether to add rigid body to constraint island

    float                 sleepMaxSpeedThreshold;              // Max speed must be under this threshold for sleeping 
    float                 sleepAvgSpeedThreshold;              // Average speed must be under this threshold for sleeping
    uint                  sleepStableCount;                    // Number of steps that speeds must fall under thresholds to initiate sleeping

    FmVelStats            velStats;  // Statistics for sleeping test

    void*                 userData;  // User data pointer
    uint16_t              flags;     // Bitwise or of FM_OBJECT_FLAG* values
}

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对各部分成员函数的解释：

objectId 是该刚体全局独一无二的 id。相关的地方有 FmScene::awakeRigidBodyIds sleepingRigidBodyIds rigidBodyIdxFromId 等

rbIslandId femIslandId 关于 island 这个概念，应该是在碰撞检测中常用的，记得在 Bullet Real-Time Physics Simulation 中接触过这个概念。