MJCF Support

What URLab parses from MJCF and writes back to a MuJoCo spec. This is a reference for which elements and attributes round-trip, so you can tell ahead of time whether a model will import cleanly.

Checked against MuJoCo upstream main (header version 3010000, that is 3.10.0-dev). Status terms:

• Supported parses on import and exports back to the spec.
• Missing is not parsed or written; MuJoCo's own default applies if the model relies on it.

URLab models MJCF through UMjComponent subclasses. Most attribute-to-field mapping is generated from the MuJoCo schema; see Codegen for how coverage is kept in sync with each MuJoCo version. The import flow itself is in the Importing guide.

Coverage at a glance

Category	Supported	Missing	Total
body	7	3	10
joint	18	2	20
geom	22	5	27
site	10	2	12
actuator (common)	14	2	16
actuator types	10	0	10
sensor types	41	0	41
tendon	16	1	17
equality	10	0	10
default	6	5	11
compiler	5	2	7
option	17	3	20
keyframe	6	0	6
contact	2	0	2
asset	3	1	4
flexcomp	22	2	24

The per-element notes below list only the attributes worth flagging (the gaps and the import-specific behaviour). Anything not called out as Missing is supported.

body

Supported: name, childclass, pos, quat, axisangle, xyaxes, zaxis, euler, mocap, gravcomp, sleep. All orientation forms resolve through OrientationToMjQuat, honouring the <compiler> angle and eulerseq settings. mocap maps to bDrivenByUnreal so Unreal can drive the body kinematically.

Missing: user.

joint

Supported includes type, axis, range, springdamper, stiffness and damping (up to three polynomial coefficients each), armature, frictionloss, actuatorfrcrange, and the full sol*limit / sol*friction set. Slide-joint range and ref auto-convert metres and centimetres.

Missing: actuatorgravcomp, user.

geom

Supported includes type, size, fromto, contype, conaffinity, condim, priority, friction, mass, density, material, rgba, mesh, all orientation forms, and the sol* pair. Type maps to a UE visualizer where one exists: box, sphere, cylinder, and capsule (an engine cylinder plus two sphere caps). Ellipsoid, plane, hfield, and sdf use the base UMjGeom with no visualizer mesh.

Missing: hfield (the geom type is recognised but the hfield asset is not imported from <asset>), fitscale, shellinertia, solmix (bound in the runtime view but not imported / exported via XML), fluidshape, fluidcoef, user.

fromto resolution

fromto on geoms and sites is decomposed at import into pos, quat, and the half-length size slot, leaving the radius to come from the default chain. See the Geometry guide.

site

Supported: name, class, type, group, pos, all orientation forms, size, fromto, rgba.

Missing: material, user.

actuator

Common attributes supported: class, group, ctrllimited, forcelimited, ctrlrange, forcerange, lengthrange, gear, cranklength, the four transmission targets (joint / tendon / site / body), actearly, actlimited, actrange.

Missing common: damping (not on the base actuator, though the damper subtype's kv covers that case), armature, user.

All ten actuator types are supported: general, motor, position, velocity, intvelocity, damper, cylinder, muscle, adhesion, dcmotor. Preset types use the matching mjs_setTo* function; general writes raw gainprm / biasprm / dynprm.

biastype / gaintype / dyntype

<general> actuators round-trip their biastype, gaintype, and dyntype so affine PD loops survive import. See the Controllers guide.

sensor

All 41 sensor types are supported, imported via a tag-name map and exported through mjsSensor->type. This spans touch / IMU sensors, joint / tendon / actuator readouts, frame and subtree quantities, geometric queries (geomdist, geomnormal, geomfromto), contact, energy, clock, tactile, plugin, and user.

Common attributes supported: noise, cutoff, target resolution (objname / site / joint), refname, objtype, reftype, dim, class. Missing: user.

tendon

Spatial and fixed tendons are supported, including stiffness and damping (three-coefficient polynomials), springlength, armature, range, actfrcrange, the sol* set, width, and rgba. All four wrap types work: joint, site, geom (with sidesite), pulley.

Missing: user.

equality

All types supported: connect, weld, joint, tendon, plus the flex constraints flex, flexvert, and flexstrain. Common attributes active, solref, solimp round-trip.

default

The <default> class hierarchy (including nesting) is supported, and these children export: geom, joint, site, camera, tendon, and all actuator subtypes (polymorphic).

Missing from default export: mesh, material, pair, equality, light.

compiler

Supported: angle, eulerseq, meshdir, texturedir, assetdir, autolimits. Missing: coordinate (always treated as local, the MuJoCo 3.x default), settotalmass.

option

Supported: timestep, gravity, wind, magnetic, density, viscosity, impratio, tolerance, iterations, ls_iterations, integrator, cone, solver, the noslip_* and ccd_* pairs, and the MultiCCD / Sleep enable flags.

Missing: o_margin, o_solref, o_solimp, the mpr_* and sdf_* solver tuning groups.

keyframe, contact, asset

• keyframe: name, time, qpos (auto-padded for free joints), qvel, act, ctrl, mpos, mquat all round-trip.
• contact: pair and exclude both import and export fully.
• asset: mesh, material, and texture import (GLB / STL / OBJ meshes; PNG / JPG / BMP / TGA textures applied to material instances). Missing: hfield is not imported from <asset>.

flexcomp

<flexcomp> is parsed into a UMjFlexcomp and, at registration, serialised back to an MJCF fragment that MuJoCo's own parser expands via mjs_attach. All geometry types (grid / box / cylinder / ellipsoid / square / disc / circle / mesh / direct) and DOF modes (full / radial / trilinear / quadratic) work without a plugin-side reimplementation. Sub-elements <contact>, <edge>, <elasticity>, and <pin> are supported through bOverride_* rules.

Missing: the pingrid and pinrange pin selectors (only id and gridrange are supported).

The compiled <flex> element is read back at runtime for visualization; UMjFlexcomp::Bind caches flex_vertadr, flex_vertnum, and the triangle index list and updates from mjData.flexvert_xpos each tick.