Domain randomization

Randomize by writing the model arrays in place, then notify the solver with the flag for what changed. Arrays are per element, so a per-world randomization writes that world’s bodies, shapes and DOFs. notify_model_changed(flags, world_mask=...) refreshes cached data for every world; the mask only limits which worlds lose their warm-start impulses.

Property	Model array	Notify with	When Steel reads it	Tutorial
Mass, inertia	body_mass, body_inertia and inverses	BODY_INERTIAL_PROPERTIES	Notify by contract. With the default update_mass_matrix_interval=1 the mass matrix is rebuilt from these arrays every step, so it happens to work without the notification; with a longer interval the old values stay until the next rebuild.	dr-mass, dr-inertia
Center of mass	body_com	BODY_INERTIAL_PROPERTIES	Cached (body-to-COM transforms): ignored without the notification.	dr-com
Friction	shape_material_mu	SHAPE_PROPERTIES	Copied on notify (scaled by contact_friction_scale, combined by friction_combine); ignored without it.	dr-friction
Floor friction per world	a per-world floor shape’s shape_material_mu	SHAPE_PROPERTIES	A ground plane in the global world is one shape with one coefficient for every world. Give each world its own floor shape; with the default average combine the other shape’s coefficient dilutes the spread.	dr-terrain-friction
Restitution	shape_material_restitution	SHAPE_PROPERTIES	Copied on notify; ignored without it. Positive restitution at construction excludes the sparse mass factors; if the solver already uses sparse factors, introducing positive restitution raises and needs a new solver. That check reads the host, so notify outside capture.	dr-restitution
Spin and rolling friction	shape_material_mu_torsional, _rolling	SHAPE_PROPERTIES	On notify, for free-body contacts with steel:condim 4 or 6 (integration branch); a solver built without condim above 3 ignores them.
Drive gains	joint_target_ke, joint_target_kd	JOINT_DOF_FORCE_PROPERTIES	Implicit gains are folded into the mass matrix; notify by contract so it is rebuilt with them (with interval 1 the rebuild happens every step anyway).	dr-drive-gains
Effort limits	joint_effort_limit	JOINT_DOF_FORCE_PROPERTIES (optional)	Read every step.	dr-effort-limits
Joint damping, spring stiffness	joint_damping, joint_stiffness	JOINT_DOF_FORCE_PROPERTIES	Copied on notify; ignored without it.	dr-joint-damping
Armature	joint_armature	JOINT_DOF_INERTIAL_PROPERTIES	Cached effective armature: ignored without the notification.	dr-armature
Spring rest position	joint_rest_q	JOINT_REFERENCE_POSE_PROPERTIES	Copied on notify.
Joint friction	joint_friction, steel:joint_friction_load_coefficient	JOINT_DOF_FORCE_PROPERTIES	Rows exist only with enable_joint_friction=True at construction; values are read every step. The notification wakes sleeping islands and validates values. The load term exists only if some coefficient was positive at construction.	dr-joint-friction
Velocity limits	joint_velocity_limit	none needed	Rows exist only with enable_joint_velocity_limits=True at construction; values are read every step.	dr-velocity-limits
Position limits	joint_limit_lower, joint_limit_upper	none needed	Read every step; enforced only with enable_joint_limits=True.
Gravity	model.set_gravity(g, world=w)	MODEL_PROPERTIES with reuse_kinematics	Per world; read every step. Without the notification a change lags one step under reuse_kinematics.	dr-gravity
Pushes	state.joint_qd (velocity kick) or state.body_f (force)	none (state)	A free joint’s joint_qd is the COM velocity and angular velocity. Write body_f on the input state of every step the push lasts. Writes to state.body_qd are ignored: Steel recomputes it from joint_qd.	dr-pushes
Adhesion	steel:adhesion	none needed	Read every step (integration branch); a solver built with no positive value ignores it.
Kinematic flag	body_flags	BODY_PROPERTIES	Read on the host: notify outside graph capture. A kinematic free body that the solver left out of the response at construction cannot become dynamic later.
Collision geometry and offsets	shape_transform, shape_scale, margin, gap	SHAPE_PROPERTIES	Covered by the notification (which also discards friction anchors of the changed bodies), but it changes contact generation and has no tutorial yet. Shape variants use SHAPE_TYPE_AND_SOURCE at reset.	not covered

Several of these also have runnable tutorials, including dr-per-world-mixed, which randomizes base mass, drive stiffness, foot friction and gravity per world with one combined notification and a world_mask partial reset.

Pitfalls

• A missing notification is silent. Center of mass, friction, restitution, joint damping and armature changes are ignored without their notification. Mass, inertia and drive gains appear to work without one at update_mass_matrix_interval=1; notify anyway. Log the value the solver uses, not only the array you wrote.
• Feature switches are fixed at construction. enable_joint_friction, enable_joint_velocity_limits, enable_joint_limits, combine modes, drive_mode and row capacities cannot be randomized; randomize the values those features read.
• Capture. Joint, DOF, inertial and shape notifications run on the device and can be captured into the reset graph; they read the arrays they were captured with. Body-flag notifications, sleeping and some validity checks read the host and must run outside capture.
• Construction-time gates. Joint-friction load coefficients, adhesion and torsion radii must be positive somewhere, and steel:condim must be 4 or 6 somewhere, when the solver is built for their rows to exist. Restitution decides the mass-factor layout at construction. To randomize these later, build the solver with the feature already on.
• Independent per-shape materials split friction patches. A region needs matching friction coefficients, so randomizing each shape separately makes more, smaller regions and more friction rows. Re-measure capacities with randomization on.
• One factor at a time. Validate each randomized property alone against the baseline before combining them; interactions (restitution in particular) are task-dependent.
• Reset order. Write the arrays, notify, then reset the state of the selected worlds; the notification and reset discard warm-start impulses and friction anchors that no longer apply.