walkersim/sim/isaac/README.md

Isaac Model

Converted from sim/mujoco/model/walker.xml.

Current Status

• URDF conversion pipeline is in place and regenerates sim/isaac/model/walker.urdf from MuJoCo XML.
• Standalone browser demo runs via uv run python -m http.server 8000 and loads in urdf-loader.
• Cranks rotate continuously through full 360+ cycles in playback.
• Leg/tendon linkage is kinematically solved in playback and stays ground-facing on both near and far sides.
• Anchor diagnostics are enabled in demo: cyan line segments + console distance logs.

Generate URDF (Option A)

python sim/isaac/mjcf_to_urdf_option_a.py \
  --input sim/mujoco/model/walker.xml \
  --output sim/isaac/model/walker.urdf

Output

• URDF: sim/isaac/model/walker.urdf
• Converter: sim/isaac/mjcf_to_urdf_option_a.py

Constraint Notes

• URDF is a tree format and cannot represent MuJoCo equality/connect closed-loop constraints as true physical constraints.
• urdf-loaders supports standard URDF features (including mimic joints), but mimic is kinematic coupling and does not create a physics loop constraint.
• The generated URDF does not include mimic tags; linkage coupling is handled in demo playback code.
• The generated URDF also adds small magenta marker spheres at each dropped MuJoCo equality/connect anchor so pivot locations are visible in URDF viewers.
• For Isaac Sim fidelity, keep the URDF tree and add constraints after import (USD/PhysX joints or an Isaac-side constraints config).

Validation

• XML syntax check:

python -c "import xml.etree.ElementTree as ET; ET.parse('sim/isaac/model/walker.urdf'); print('ok')"

• Visual check: drag and drop the URDF in the online viewer: https://gkjohnson.github.io/urdf-loaders/javascript/example/bundle/index.html

urdf-loaders Playback

• A browser playback helper is provided at sim/isaac/urdf_loaders_play.js.
• Playback sets joint ignoreLimits=true so crank joints can rotate continuously through 360+ degrees.
• In playback, tendon joints are solved from crank + leg kinematics each frame (approximate closure for cleaner visualization).
• After loading the model in a urdf-loader app and getting a robot instance:

import { playWalker } from './sim/isaac/urdf_loaders_play.js';
const stop = playWalker(robot);

• Call stop() to halt animation.

Standalone Demo (uv)

• Files:
  • sim/isaac/demo/index.html
  • sim/isaac/demo/main.js
• Dependencies are loaded from CDN in the browser (three and urdf-loader).
• The demo draws cyan debug line segments between each paired equality-anchor marker and logs pair distances in the browser console every 0.25s.
• The demo overlays near-side part labels for quick inspection: CF, CG, AD, BE.
• Run a local server from repo root:

uv run python -m http.server 8000

• Open: http://localhost:8000/sim/isaac/demo/

Next

• Add an Isaac constraints companion file to recreate the four MuJoCo loop closures after URDF import.