NPZ Viewer / Player — Motion Viewer for URDF Humanoid Robots

Motion Viewer is a free, browser-based NPZ viewer and NPZ player for visualizing humanoid robot motion. Drag a robot folder containing a URDF file and STL meshes into the window, then drag one or more .npz motion files to play them back with full timeline scrubbing, variable playback speed, and frame-by-frame stepping.

Features

• Browser-based NPZ file viewer — no installation, no upload, runs locally via WebGL and Three.js
• Drag-and-drop loading of URDF + STL mesh bundles
• Plays back NumPy .npz motion trajectories at arbitrary FPS
• Supports Unitree G1, Unitree H1, and any custom humanoid URDF
• Compatible with Isaac Lab / IsaacSim motion data and the whole_body_tracking NPZ convention
• Playback controls: play/pause, scrub, 0.25x–2x speed, frame stepping
• Camera follow mode and ground-plane trajectory trail
• Multi-motion switching with keyboard shortcuts

Frequently asked questions

What is an NPZ viewer?

An NPZ viewer is a tool that visualizes motion data stored in NumPy .npz files. Motion Viewer reads body pose trajectories (body_pos_w, body_quat_w) and animates them on a URDF robot model directly in the browser.

How do I play an .npz motion file?

Open Motion Viewer, drag a robot folder containing a URDF and STL meshes into the window, then drag one or more .npz motion files. Use the timeline and play button to scrub through the motion.

Does it support Unitree G1?

Yes. Motion Viewer works with Unitree G1, Unitree H1, and any other humanoid robot described by a standard URDF.

What .npz format does it expect?

The viewer follows the whole_body_tracking convention. Required keys: fps, body_pos_w with shape (N, num_bodies, 3), and body_quat_w with shape (N, num_bodies, 4) in wxyz order, using Z-up coordinates.

Source code and documentation: github.com/zelin-tao/motion-viewer