Principle:Google deepmind Dm control MJCF Model Creation
| Metadata | |
|---|---|
| Knowledge Sources | dm_control |
| Domains | Physics Simulation, Robotics, Model Authoring |
| Last Updated | 2026-02-15 00:00 GMT |
Overview
Programmatic construction or parsing of physics simulation model definitions enables flexible, reproducible creation of articulated bodies and environments without manual XML editing.
Description
MJCF Model Creation is the foundational step in any physics simulation workflow built on top of the MuJoCo XML format (MJCF). Rather than hand-writing XML files, a programmatic approach allows users to construct model definitions entirely in code or to load existing XML files into a mutable in-memory object tree.
The core idea is that every MJCF model is rooted by a single top-level element (analogous to the <mujoco> XML tag). This root element serves as the entry point for all subsequent model building operations: adding bodies, geoms, joints, actuators, sensors, and every other component defined in the MJCF schema. The root element also owns a name scope that tracks every named identifier in the model, ensuring uniqueness and enabling cross-referencing.
There are two complementary paths to obtaining a root element:
- Construction from scratch -- create an empty root element and populate it programmatically.
- Parsing from XML -- load an existing MJCF file (from a file path, an XML string, or a zip archive) and obtain a fully populated root element that can be inspected and mutated.
Both paths yield the same kind of mutable object tree, so downstream code does not need to distinguish between a model that was built programmatically and one that was parsed from disk.
Usage
Model creation is typically the very first operation in any workflow that uses the PyMJCF library:
- Create a blank model when authoring a new robot, object, or arena from scratch.
- Parse an existing model when loading a pre-authored asset (e.g., a robot URDF that has been converted to MJCF) for further modification or composition.
- Parse from a zip archive when models and their binary assets (meshes, textures) are distributed together in a single compressed file.
The resulting root element is then passed to structure-building, attachment, querying, compilation, or export operations.
Theoretical Basis
The creation step can be described in pseudocode:
FUNCTION create_model(name):
scope = new NameScope(name)
root = new RootElement(spec=MUJOCO_SCHEMA, scope=scope)
RETURN root
FUNCTION parse_model(xml_source):
xml_tree = parse_xml(xml_source)
ASSERT xml_tree.root_tag starts with "mujoco"
root = create_model(xml_tree.get("model"))
FOR EACH child IN xml_tree.children:
recursively_add(root, child)
resolve_all_references(root)
RETURN root
The key invariant is that after creation (whether from scratch or parsed), the resulting root element always has a valid name scope and satisfies the MJCF schema constraints. All identifiers are unique within their respective namespaces, and all reference attributes (e.g., a joint referencing a body) are resolved to the actual element objects.