Understanding Mujoco

xpos v/s pos

So mjModel.body_pos, mjModel.body_quat and all other spatial quantities in mjModel are expressed in local coordinates. The job of forward kinematics is to accumulate the joint transformations and static offsets along the kinematic tree and compute all positions and orientations in global coordinates. The quantities in mjData that start with "x" are expressed in global coordinates. These are mjData.xpos, mjData.geom_xpos etc.