Implementation:Google deepmind Mujoco Engine Core Constraint
| Knowledge Sources | |
|---|---|
| Domains | Physics Simulation, Constraint Dynamics, Contact Mechanics |
| Last Updated | 2026-02-15 04:00 GMT |
Overview
Implements constraint construction, Jacobian computation, impedance assignment, and constraint projection for all constraint types in MuJoCo's forward dynamics pipeline.
Description
This is one of the largest and most critical engine modules. It handles the entire constraint pipeline: counting constraints, allocating EFC (equality-friction-contact) arrays on the arena, instantiating equality constraints (mj_instantiateEquality), friction losses (mj_instantiateFriction), joint limits (mj_instantiateLimit), and contacts (mj_instantiateContact). The module computes constraint Jacobians, builds impedance parameters via mj_makeImpedance, performs diagonal approximation of the constraint force mixing matrix (mj_diagApprox), and constructs the full constraint system (mj_makeConstraint). It also provides mj_projectConstraint for constraint force projection and mj_constraintUpdate for updating constraint forces during solving.
Usage
Called during the forward dynamics pipeline after collision detection. mj_makeConstraint is the main entry point that sets up the constraint system, followed by mj_projectConstraint which solves for constraint forces. mj_constraintUpdate is called iteratively by the constraint solver.
Code Reference
Source Location
- Repository: Google_deepmind_Mujoco
- File: src/engine/engine_core_constraint.c
- Lines: 1-2598
Key Functions
// Determine solver type (dual if PGS or noslip)
int mj_isDual(const mjModel* m);
// Assign friction, reference, impedance, and margin parameters
void mj_assignFriction(const mjModel* m, mjtNum* target, const mjtNum* source);
void mj_assignRef(const mjModel* m, mjtNum* target, const mjtNum* source);
void mj_assignImp(const mjModel* m, mjtNum* target, const mjtNum* source);
mjtNum mj_assignMargin(const mjModel* m, mjtNum source);
// Add a contact to the arena
int mj_addContact(const mjModel* m, mjData* d, const mjContact* con);
// Jacobian multiplication
void mj_mulJacVec(const mjModel* m, const mjData* d, mjtNum* res, const mjtNum* vec);
void mj_mulJacTVec(const mjModel* m, const mjData* d, mjtNum* res, const mjtNum* vec);
// Constraint instantiation functions
void mj_instantiateEquality(const mjModel* m, mjData* d);
void mj_instantiateFriction(const mjModel* m, mjData* d);
void mj_instantiateLimit(const mjModel* m, mjData* d);
void mj_instantiateContact(const mjModel* m, mjData* d);
// Contact Jacobian computation
int mj_contactJacobian(const mjModel* m, mjData* d, const mjContact* con, int dim,
mjtNum* jacdifp, mjtNum* jacdifr);
// Diagonal approximation of the EFC system
void mj_diagApprox(const mjModel* m, mjData* d);
// Build impedance from constraint parameters
void mj_makeImpedance(const mjModel* m, mjData* d);
// Main constraint construction entry point
void mj_makeConstraint(const mjModel* m, mjData* d);
// Project constraint forces
void mj_projectConstraint(const mjModel* m, mjData* d);
// Reference constraint computation
void mj_referenceConstraint(const mjModel* m, mjData* d);
// Update constraint forces during solving
void mj_constraintUpdate(const mjModel* m, mjData* d, const mjtNum* jar,
mjtNum cost[1], int flg_coneHessian);
Import
#include "engine/engine_core_constraint.h"
I/O Contract
Inputs
| Name | Type | Required | Description |
|---|---|---|---|
| m | const mjModel* | Yes | Physics model with constraint definitions, solver parameters |
| d | mjData* | Yes | Simulation state with contact array and kinematic data |
Outputs
| Name | Type | Description |
|---|---|---|
| d->efc_J | mjtNum* | Constraint Jacobian (sparse or dense) |
| d->efc_aref | mjtNum* | Reference constraint acceleration |
| d->efc_force | mjtNum* | Constraint forces |
| d->nefc | int | Total number of scalar constraints |
| d->ne, d->nf, d->nl | int | Equality, friction-loss, and limit constraint counts |