MuJoCo Weekly: Tips and Tricks for MuJoCo Users, Every Week

A weekly "newsletter" introducing MuJoCo users to new features, tips, and tricks.

• MuJoCo Weekly: Tips and Tricks for MuJoCo Users, Every Week
  • Week 1: Gravity Compensation
  • Week 2: Ghostifying an Object

Week 1: Gravity Compensation

Here are two ways to implement gravity compensation in MuJoCo:

• Inject fake upwards forces at the center-of-mass (CoM) of each body in a kinematic chain such that each force exactly counteracts the body’s weight (weight = gravity x mass). Write these forces to xfrc_applied which are user-defined Cartesian wrenches that MuJoCo will apply to body CoMs.

def compensate_gravity(self, model: mjcf.RootElement, physics: mjcf.Physics) -> None:
    gravity = np.hstack([physics.model.opt.gravity, [0, 0, 0]])
    physics_bodies = physics.bind(model.find_all("body"))
    physics_bodies.xfrc_applied[:] = -gravity * physics_bodies.mass[..., None]

• Use the body’s gravcomp attribute, which is a new feature added as of version 2.3.1. gravcomp=1.0 will exactly counteract gravity, values greater than 1 will create a buoyancy effect. The default value for gravcomp is 0.

def compensate_gravity(model: mjcf.RootElement) -> None:
    for body in model.find_all("body"):
        body.gravcomp = 1.0

Week 2: Ghostifying an Object

To disable contacts with a body in a scene, iterate through all its geoms and set their contype and conaffinity to 0.

for geom in prop.find_all("geom"):
    geom.contype = 0
    geom.conaffinity = 0

There are many use cases for disabling contacts (one being speeding up the physics pipeline), but let's talk about one particular useful instance: "ghostifying" an object for use as a visual hint. For example, in manipulation, you might want to visually render an object's goal pose. The MuJoCo way to achieve this is to:

1. Duplicate the object of interest
2. Iterate through all its geoms and set their contype and conaffinity to 0
3. Iterate through all its bodies and set their mocap attribute to True
4. Optional: make each geom transparent

Note that mocap=True effectively disables the object's dynamics (it is treated as fixed by the simulator).