Implementation:Google deepmind Mujoco MJWarp Passive

Knowledge Sources	Google_deepmind_Mujoco MJWarp Documentation
Domains	Physics_Simulation, GPU_Computing, NVIDIA_Warp
Last Updated	2026-02-15 04:00 GMT

Overview

MJWarp Passive computes passive forces including spring-damper, gravity, fluid drag, and flex elasticity on the GPU.

Description

This module computes all passive forces in the simulation: joint spring-damper forces, tendon spring-damper forces, gravitational forces, fluid interaction forces (viscosity, density, lift, drag), and flex element elasticity and bending forces. It combines these into the qfrc_passive generalized force vector. The fluid model supports both inertia-based and ellipsoid-based drag computation depending on geometry type.

Usage

Called during the forward dynamics pipeline via passive(m, d) to compute passive forces that are independent of actuator inputs. These forces are subtracted from the equations of motion.

Code Reference

Source Location

• Repository: Google_deepmind_Mujoco
• File: mjx/mujoco/mjx/third_party/mujoco_warp/_src/passive.py
• Lines: 1-848

Key Functions

# Primary entry point
def passive(m: Model, d: Data)

# Spring-damper forces
def _spring_damper_dof_passive(...)   # Joint spring-damper
def _spring_damper_tendon_passive(...)  # Tendon spring-damper

# External forces
def _gravity_force(...)
def _fluid_force(...)
def _fluid(m: Model, d: Data)

# Flex forces
def _flex_elasticity(...)
def _flex_bending(...)

# Passive force assembly
def _qfrc_passive(...)

# Utility
def _geom_semiaxes(size, geom_type) -> wp.vec3
def _ellipsoid_max_moment(size, dir) -> float

Import

from mujoco.mjx.third_party.mujoco_warp._src.passive import passive

I/O Contract

Inputs

Name	Type	Required	Description
m	Model	Yes	Warp model with spring, damping, gravity, and fluid parameters
d	Data	Yes	Warp simulation data with current positions and velocities

Outputs

Name	Type	Description
d.qfrc_passive	wp.array2d	Generalized passive forces (spring, damper, gravity, fluid, flex)

Related Pages

• Environment:Google_deepmind_Mujoco_MJX_Warp_CUDA_Environment