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How to create a custom object

Objects, such as boxes and cans, are essential to building manipulation environments. We designed the MujocoObject interfaces to standardize and simplify the procedure for importing 3D models into the scene or procedurally generate new objects. MuJoCo defines models via the MJCF xml format. These MJCF files can either be stored as XML files on disk and loaded into simulator, or be created on-the-fly by code prior to simulation. Based on these two mechanisms of how MJCF models are created, we offer two main ways of creating your own object:

  • Define an object in an MJCF XML file;
  • Use procedural generation APIs to dynamically create an MJCF model.

The MujocoObject class

class MujocoObject:
    def __init__(...):
        pass

MujocoObject is the base class of all objects. One must note that it is not a subclass of MujocoXML. The XML of an object is generated through the call to get_collision and get_visual. Both calls take an argument name which assigns a name to the generated <body> tag to help locate the object in simulation. They also take argument site, which adds a site to the center of the object. A site is helpful for locating the object in simulation.

    def get_collision(self, name=None, site=False):
        pass

    def get_visual(self, name=None, site=False):
        pass

Additionally, objects are usually placed relatively. For example, we want to put an object on a table or place a cube on top of another. Instance methods get_bottom_offset, get_top_offset, get_horizontal_radius provide the necessary information to place objects properly.

    def get_bottom_offset(self):
        pass

    def get_top_offset(self):
        pass

    def get_horizontal_radius(self):
        pass

This allows us to do things like the following.

table_top = np.array([0, 1, 0])
bottom_offset = obj.get_bottom_offset()
pos = table_top - bottom_offset # pos + bottom_offset = table_top
obj_xml = obj.get_visual().set("pos", array_to_string(pos))

Creating a XMLObject

One can use MuJoCo MJCF xml to generate an object, either as a composition of primitive geoms or imported from STL files of triangulated meshes. An example is robosuite.models.objects.xml_objects.BreadObject. Its python definition is short.

class BreadObject(MujocoXMLObject):
    def __init__(self):
        super().__init__(xml_path_completion("objects/bread.xml"))

So all the important definitions are in the bread.xml file.

<mujoco model="bread">
  <asset>
    <mesh file="meshes/bread.stl" name="bread_mesh" scale="0.8 0.8 0.8"/>
    <texture file="../textures/bread.png" type="2d" name="tex-bread" />
    <material name="bread" reflectance="0.7" texrepeat="15 15" texture="tex-bread" texuniform="true"/>
  </asset>
  <worldbody>
    <body>
      <body name="collision">
        <geom pos="0 0 0" mesh="bread_mesh" type="mesh" solimp="0.998 0.998 0.001" solref="0.001 1" density="50" friction="0.95 0.3 0.1"  material="bread" group="1" condim="4"/>
      </body>
      <body name="visual">
        <geom pos="0 0 0" mesh="bread_mesh" type="mesh" material="bread"  conaffinity="0" contype="0"  group="0" mass="0.0001"/>
        <geom pos="0 0 0" mesh="bread_mesh" type="mesh" material="bread"  conaffinity="0" contype="0"  group="1" mass="0.0001"/>
      </body>
      <site rgba="0 0 0 0" size="0.005" pos="0 0 -0.045" name="bottom_site"/>
      <site rgba="0 0 0 0" size="0.005" pos="0 0 0.03" name="top_site"/>
      <site rgba="0 0 0 0" size="0.005" pos="0.03 0.03 0" name="horizontal_radius_site"/>
    </body>
  </worldbody>
</mujoco>
  • get_collision of MujocoXmlObject would look for the <body> tag with name collision.
  • get_visual of MujocoXmlObject would look for the <body> tag with name visual.
  • bottom_site should be the bottom of the object, i.e. contact point with the surface it is placed on.
  • top_site should be the top of the object, i.e. contact point if something is placed on it.
  • horizontal_radius_site can be any point on a circle in the x-y plane that does not intersect the object. This allows us to place multiple objects without having them collide into one another.
  • These attributes will be parsed by the MujocoXMLObject class and conform to the MujocoObject interface.

Creating a procedurally generated object

Proceudally generated objects have been used in several recent works to train control policies with improved robustness and generalization. Here you can programmatically generate a MJCF xml of an object from scratch using xml.etree.ElementTree. The implementation is straightforward and interested readers should refer to _get_collision and get_visual method of MujocoGeneratedObject, defined here. For instance, the BaxterLift environment used a pot with two side handles. This object is created with the procedural generation APIs, see here.