Implementation:Google deepmind Dm control MJCF Export

Metadata
Knowledge Sources	dm_control
Domains	Physics Simulation, Robotics, Model Serialization
Last Updated	2026-02-15 00:00 GMT

Overview

Concrete tool for exporting MJCF model trees and their binary assets to directories or zip archives using export_with_assets(), export_with_assets_as_zip(), and mjcf_model.to_xml_string().

Description

The MJCF Export implementation provides three serialization paths:

1. mjcf_model.to_xml_string(*, precision=17, zero_threshold=0)

Serializes the in-memory MJCF tree to an XML string. The method:

1. Calls to_xml() on the root element, which recursively serialises every element and its attributes into an lxml.etree tree.
2. During serialization, identifiers are prefixed according to the name scope hierarchy (e.g., robot/elbow). Reference attributes are resolved to their fully qualified names. File attributes are replaced with content-addressed VFS filenames.
3. The etree is converted to a byte string via etree.tostring() with optional pretty-printing.
4. The precision parameter controls the number of significant digits for floating-point values (default 17 for full double precision). The zero_threshold parameter snaps small values to zero.

2. export_with_assets(mjcf_model, out_dir, out_file_name=None, *, precision=17, zero_threshold=0)

Writes the model and all its assets as individual files in a directory:

1. Determines the output file name (defaults to mjcf_model.model + '.xml'). Raises ValueError if the name does not end with .xml.
2. Calls mjcf_model.get_assets() to collect all binary assets. This method traverses all File attributes in the model's namescope (and recursively in attached models) and returns a {vfs_filename: bytes} dictionary.
3. Adds the XML string to the assets dictionary.
4. Creates out_dir if it does not exist.
5. Writes each entry as a binary file.

3. export_with_assets_as_zip(mjcf_model, out_dir, model_name=None, *, precision=17, zero_threshold=0)

Writes the model and all its assets into a single zip file:

1. Determines the model name (defaults to mjcf_model.model). The zip file is named model_name.zip and contains a directory named model_name/.
2. Collects assets and the XML string in the same way as the directory export.
3. Creates out_dir if it does not exist.
4. Writes all files into the zip archive under the model_name/ prefix.
5. Returns the full path to the created zip file.

Usage

Use to_xml_string() for in-memory serialization. Use export_with_assets() to write a model to a directory for standalone use. Use export_with_assets_as_zip() to create distributable archives.

Code Reference

Property	Value
Source Location	dm_control/mjcf/export_with_assets.py:L24-61, dm_control/mjcf/export_with_assets_as_zip.py:L23-67, dm_control/mjcf/element.py:L817-862 (to_xml_string)
Signature (export_with_assets)	export_with_assets(mjcf_model, out_dir, out_file_name=None, *, precision=17, zero_threshold=0)
Signature (export_with_assets_as_zip)	export_with_assets_as_zip(mjcf_model, out_dir, model_name=None, *, precision=17, zero_threshold=0)
Signature (to_xml_string)	to_xml_string(self, prefix_root=None, self_only=False, pretty_print=True, *, precision=17, zero_threshold=0, filename_with_hash=True) -> str
Import	from dm_control.mjcf import export_with_assets, from dm_control.mjcf import export_with_assets_as_zip

I/O Contract

Inputs (export_with_assets):

Parameter	Type	Description
mjcf_model	mjcf.RootElement	The model to export.
out_dir	str	Directory to write the XML and assets. Created if it does not exist.
out_file_name	str or None	Name of the output XML file. Defaults to model_name + '.xml'. Must end with .xml.
precision	int	Number of significant digits for float output (default 17).
zero_threshold	float	Values with absolute value below this are written as zero (default 0).

Inputs (export_with_assets_as_zip):

Parameter	Type	Description
mjcf_model	mjcf.RootElement	The model to export.
out_dir	str	Directory for the output zip file. Created if it does not exist.
model_name	str or None	Base name for the zip file, internal directory, and XML file. Defaults to mjcf_model.model.
precision	int	Number of significant digits for float output (default 17).
zero_threshold	float	Values with absolute value below this are written as zero (default 0).

Inputs (to_xml_string):

Parameter	Type	Description
precision	int	Significant digits for floats (default 17).
zero_threshold	float	Zero-snap threshold (default 0).
pretty_print	bool	Whether to indent the XML (default True).

Outputs:

Output	Type	Description
return value (export_with_assets)	None	Files are written to out_dir.
return value (export_with_assets_as_zip)	str	Full path to the created zip file.
return value (to_xml_string)	str	The XML content as a string.

Usage Examples

from dm_control import mjcf
from dm_control.mjcf import export_with_assets as ewa
from dm_control.mjcf import export_with_assets_as_zip as ewa_zip

# Build or load a model
model = mjcf.RootElement(model='my_scene')
body = model.worldbody.add('body', name='box')
body.add('geom', type='box', size=[0.1, 0.1, 0.1])

# 1. Export to XML string
xml_string = model.to_xml_string()
print(xml_string)

# 2. Export to XML string with reduced precision
xml_compact = model.to_xml_string(precision=5, zero_threshold=1e-10)

# 3. Export model and assets to a directory
ewa.export_with_assets(model, out_dir='/tmp/my_scene')
# Creates /tmp/my_scene/my_scene.xml and any asset files

# 4. Export with a custom file name
ewa.export_with_assets(model, out_dir='/tmp/my_scene',
                       out_file_name='scene_v2.xml')

# 5. Export model and assets as a zip archive
zip_path = ewa_zip.export_with_assets_as_zip(
    model, out_dir='/tmp/archives')
print(zip_path)  # /tmp/archives/my_scene.zip
# The zip contains: my_scene/my_scene.xml and my_scene/<asset files>

# 6. Export with a custom model name
zip_path = ewa_zip.export_with_assets_as_zip(
    model, out_dir='/tmp/archives', model_name='release_v1')
print(zip_path)  # /tmp/archives/release_v1.zip

Related Pages

• Principle:Google_deepmind_Dm_control_MJCF_Model_Export
• Implementation:Google_deepmind_Dm_control_MJCF_RootElement_Parser
• Implementation:Google_deepmind_Dm_control_MJCF_Physics