Core Concepts¶

mjswan uses a four-level hierarchy to describe a browser application: Builder → Project → Scene → Policy/Splat. Understanding this structure is the fastest way to get oriented before looking at the API.

Builder
  └── Project
        └── Scene
              ├── Splat
              └── Policy
                    └── Motion

Builder¶

Builder is the entry point. It collects everything you define and, when you call build(), compiles it into a self-contained web application written to dist/ (or a directory you choose).

import mjswan

builder = mjswan.Builder()
# … add projects …
app = builder.build()
app.launch()

Two optional constructor arguments matter for deployment:

Project¶

A project groups related scenes under a single URL. The first project added becomes the root (/); additional projects are reachable at /<project-id>/.

project = builder.add_project(name="My Robots")

# Explicit URL slug — accessible at /demo/
demo = builder.add_project(name="Demo", id="demo")

If you omit id, mjswan derives it automatically from the project name (spaces and hyphens become underscores, lowercased). The first project is always the root regardless of id.

Scene¶

A scene contains exactly one MuJoCo model. You supply either an MjSpec or an MjModel:

import mujoco

# Compressed .mjz — smaller output, recommended for large deployments
scene = project.add_scene(
    spec=mujoco.MjSpec.from_file("robot/scene.xml"),
    name="My Robot",
)

# Binary .mjb — loads slightly faster in the browser, produces larger files
scene = project.add_scene(
    model=mujoco.MjModel.from_xml_path("robot/scene.xml"),
    name="My Robot",
)

Splat¶

A splat is a Gaussian Splat background rendered behind the MuJoCo simulation. Splats are stored as .spz files and give scenes a photorealistic real-world environment without affecting physics.

Add one or more splats to a scene using add_splat(). You must supply exactly one of source or url:

# Recommended: bundle the .spz file into the app
scene.add_splat(
    "Lab Environment",
    source="lab.spz",        # copied into dist/ at build time
    scale=1.35,              # converts splat units → meters
    z_offset=0.71,           # vertical shift to align ground planes
)

# Alternative: reference an external URL (not bundled)
scene.add_splat(
    "Outdoor",
    url="https://example.com/outdoor.spz",
    scale=3.0,
    z_offset=0.5,
)

When multiple splats are attached to the same scene, the viewer shows a selector so users can switch between them at runtime.

Source vs URL¶

Alignment controls¶

Set control=True to expose these alignment controls as live sliders in the viewer — useful while calibrating a new capture:

scene.add_splat("Lab", source="lab.spz", scale=1.35, control=True)

Splat selector without pre-configured splats¶

By default, the Splat selector only appears when at least one splat is attached to the scene. Call enable_splat_section() to show the selector unconditionally — this lets viewers paste an arbitrary .spz URL directly in the control panel at runtime:

scene.enable_splat_section()

Policy¶

A policy is an ONNX model that runs inference inside the browser. Attach one or more policies to a scene:

import onnx

policy = scene.add_policy(
    name="Locomotion",
    policy=onnx.load("locomotion.onnx"),
    config_path="locomotion.json",  # optional: observation/action config
)

Policies are purely client-side: inference runs in the browser via onnxruntime-web, so no server is needed at runtime.

You can also build the observation / action / termination config entirely from Python by passing observations=, actions=, commands=, and terminations= to add_policy() — config_path becomes optional in that case. See examples/tutorial/minimum_policy.py for a fully-Python example.

Commands¶

Commands let users interact with a running policy — for example, steering a walking robot with velocity sliders. Pass a commands= dict to add_policy():

scene.add_policy(
    name="Locomotion",
    policy=onnx.load("locomotion.onnx"),
    commands={
        "velocity": mjswan.ui_command([
            mjswan.Slider("lin_vel_x", "Forward Velocity", range=(-1.0, 1.0), default=0.5),
            mjswan.Slider("lin_vel_y", "Lateral Velocity", range=(-0.5, 0.5), default=0.0),
            mjswan.Slider("ang_vel_z", "Yaw Rate",         range=(-1.0, 1.0), default=0.0),
        ]),
    },
)

Available command inputs:

Motion¶

Motion-tracking policies need one or more reference motions (.npz files) loaded alongside the ONNX model. Attach them to the PolicyHandle:

policy = scene.add_policy(name="Tracker", policy=onnx.load("tracker.onnx"))

# Local .npz bundled into dist/ at build time
policy.add_motion(
    name="default",
    source="motions/walk.npz",
    fps=50.0,
    anchor_body_name="pelvis",
    body_names=("pelvis",),
    default=True,
    loop=False,
)

# Or fetch from a W&B run
policy.add_motion_wandb(
    run_path="<entity>/<project>/<run_id>",
    anchor_body_name="pelvis",
    body_names=("pelvis",),
)

anchor_body_name and body_names are required — they tell the browser-side tracker which bodies in the MuJoCo model correspond to the dataset. default=True marks the motion as the one selected on load; when multiple motions are attached the viewer shows a selector. See the API reference for the full parameter list.

Output structure¶

builder.build() writes the following layout to the output directory:

dist/
├── index.html
├── logo.svg
├── manifest.json
├── robots.txt
├── assets/
│   ├── config.json          ← project/scene/policy manifest
│   └── …                    ← compiled JS/CSS
├── _headers                 ← only when Builder(mt=True)
├── coi-serviceworker.js     ← only when Builder(mt=True)
└── <project-id>/            ← "main" for the first project
    ├── index.html
    ├── logo.svg
    ├── manifest.json
    └── assets/
        └── <scene-id>/
            ├── scene.mjz    ← or scene.mjb
            ├── <policy>.onnx
            ├── <policy>.json  ← present when config_path / commands / observations / actions / terminations are set
            ├── <policy>_<motion>.npz   ← per motion attached to the policy
            └── <splat>.spz    ← only when source= is used

The result is a fully static site: copy dist/ to any static host (GitHub Pages, Netlify, S3, …) and it works without a server.

Environment variables¶