如何方便地在MuJoCo中动态创建两个物体之间的约束？

在MuJoCo中，我们可以使用Constraint模块动态地创建约束。具体示例如下：

import mujoco_py as mujoco

# 创建MuJoCo的模拟器对象和世界对象
model = mujoco.load_model_from_path('model.xml')
sim = mujoco.MjSim(model)

# 创建两个物体（例如两个球）并添加到世界中
sim.model.geom_names.append('ball1')
sim.model.geom_size.append(0.05)
sim.model.geom_type.append(2)
sim.model.geom_rgba.append([1, 0, 0, 1])
sim.model.geom_pos.append([0.1, 0, 0])

sim.model.geom_names.append('ball2')
sim.model.geom_size.append(0.05)
sim.model.geom_type.append(2)
sim.model.geom_rgba.append([0, 1, 0, 1])
sim.model.geom_pos.append([-0.1, 0, 0])

# 创建一个球与球之间的约束
joint = mujoco.MjJntBall()
joint.name = 'ball_joint'
joint.attach(sid=0, frameid=0, type=mujoco.mjOBJECT_BODY)
joint.attach(sid=1, frameid=0, type=mujoco.mjOBJECT_BODY)
joint.range = [0, 0, 0]
joint.solref[0] = 0.9
joint.solref[1] = 0.2
joint.solref[2] = 0.2
joint.solref[3] = 0.9

# 添加约束到模型中
constraint = mujoco.MjConstraint()
constraint.type = mujoco.mjCONSTRAINT_BALL
constraint.name = 'constraint_ball'
constraint.jntnum = 1
constraint.jointid.append(joint.id)
constraint.rho = 1
constraint.limited[0] = 1
constraint.limited[1] = 1
constraint.limited[2] = 1
constraint.limited[3] = 0
constraint.limited[4] = 0
constraint.limited[5] = 0
constraint.lower = [-1