Implementation:Google deepmind Mujoco MJX Collision Convex

Knowledge Sources	Google_deepmind_Mujoco MJX Documentation
Domains	Physics_Simulation, JAX, Collision_Detection
Last Updated	2026-02-15 04:00 GMT

Overview

JAX-based convex collision detection functions for MJX, implementing contact resolution between convex geom shapes including boxes, spheres, capsules, meshes, and height fields.

Description

This module provides the core convex collision algorithms used by MJX's collision pipeline. It implements the GJK/SAT-based approach for convex-convex intersections, manifold generation for box-box contacts, and specialized routines for plane-convex, sphere-convex, and capsule-convex pairs. Height field collision is also supported via subgrid decomposition into convex prisms. All functions are designed to be JAX-compatible and differentiable, operating on vectorized geom data.

Usage

These collision functions are called by the collision driver (collision_driver.py) during the contact detection phase of mjx.step(). The collider decorator wraps each function so that it can be dispatched by geom type and vmapped over geom pairs.

Code Reference

Source Location

• Repository: Google_deepmind_Mujoco
• File: mjx/mujoco/mjx/_src/collision_convex.py
• Lines: 1-1117

Key Functions

def collider(ncon: int)
def plane_convex(plane: GeomInfo, convex: ConvexInfo) -> Collision
def sphere_convex(sphere: GeomInfo, convex: ConvexInfo) -> Collision
def capsule_convex(cap: GeomInfo, convex: ConvexInfo) -> Collision
def box_box(b1: ConvexInfo, b2: ConvexInfo) -> Collision
def convex_convex(c1: ConvexInfo, c2: ConvexInfo) -> Collision
def hfield_sphere(hfield: HFieldInfo, sphere: GeomInfo, ...) -> Collision
def hfield_capsule(hfield: HFieldInfo, cap: GeomInfo, ...) -> Collision
def hfield_convex(hfield: HFieldInfo, convex: ConvexInfo, ...) -> Collision

Import

from mujoco.mjx._src.collision_convex import plane_convex
from mujoco.mjx._src.collision_convex import convex_convex
from mujoco.mjx._src.collision_convex import box_box

I/O Contract

Inputs

Name	Type	Required	Description
m	mjx.Model	Yes	JAX model (used in collider wrapper)
d	mjx.Data	Yes	JAX simulation data with geom positions/orientations
plane / sphere / cap / convex	GeomInfo / ConvexInfo	Yes	Geom shape info including position, rotation matrix, and size
key	FunctionKey	Yes	Collision function key specifying geom types and data IDs
geom	jax.Array	Yes	Array of geom index pairs to check for collision

Outputs

Name	Type	Description
dist	jax.Array	Penetration distance (negative means overlap)
pos	jax.Array	Contact point position (midpoint between geoms)
frame	jax.Array	Contact frame (3x3); normal in row 0, tangents in rows 1-2

Related Pages

• Environment:Google_deepmind_Mujoco_MJX_JAX_Environment