MuJoCo解析之 mjData

在 MuJoCo 中，mjData 用于存放仿真数据，即给定模型后，当前的状态数据。比如，各个对象的位置、各关节的角度、碰撞信息等等。下面，详细解析 mjData 中各个变量的含义。

1. mjContact

mjContact 是表示碰撞结果的类，其定义如下：

struct mjContact_ {                // result of collision detection functions
  // contact parameters set by near-phase collision function
  mjtNum  dist;                    // distance between nearest points; neg: penetration
  mjtNum  pos[3];                  // position of contact point: midpoint between geoms
  mjtNum  frame[9];                // normal is in [0-2], points from geom[0] to geom[1]

  // contact parameters set by mj_collideGeoms
  mjtNum  includemargin;           // include if dist<includemargin=margin-gap
  mjtNum  friction[5];             // tangent1, 2, spin, roll1, 2
  mjtNum  solref[mjNREF];          // constraint solver reference, normal direction
  mjtNum  solreffriction[mjNREF];  // constraint solver reference, friction directions
  mjtNum  solimp[mjNIMP];          // constraint solver impedance

  // internal storage used by solver
  mjtNum  mu;                      // friction of regularized cone, set by mj_makeConstraint
  mjtNum  H[36];                   // cone Hessian, set by mj_constraintUpdate

  // contact descriptors set by mj_collideXXX
  int     dim;                     // contact space dimensionality: 1, 3, 4 or 6
  int     geom1;                   // id of geom 1; deprecated, use geom[0]
  int     geom2;                   // id of geom 2; deprecated, use geom[1]
  int     geom[2];                 // geom ids; -1 for flex
  int     flex[2];                 // flex ids; -1 for geom
  int     elem[2];                 // element ids; -1 for geom or flex vertex
  int     vert[2];                 // vertex ids;  -1 for geom or flex element

  // flag set by mj_setContact or mj_instantiateContact
  int     exclude;                 // 0: include, 1: in gap, 2: fused, 3: no dofs

  // address computed by mj_instantiateContact
  int     efc_address;             // address in efc; -1: not included
};
typedef struct mjContact_ mjContact;

其中，dist 是碰撞深度/距离，即 A、B 两物体之间最近的距离；pos[3] 是碰撞点，是 A、B 两物体相交处的中点；frame[9] 则依次存放了法向量，A 上的碰撞点，B 上的碰撞点。这部分信息的示意图如下：
（待补充）