T-Rex Model
A dinosaur-inspired bipedal simulation model with an oversized head and vestigial forelimbs. Its stage 3 task uses contact from a fixed head geom as a bite proxy; the model has no articulated jaw.
Generated specifications
These values are generated from the environment and compiled MJCF model. “Dynamic mass” is a simulator property, not an anatomical weight estimate.
| Gait | Bipedal |
|---|---|
| Observation dimension | 83 |
| Action dimension / actuators | 21 |
| Generalized coordinates (nq) | 40 |
| Generalized velocities (nv) | 37 |
| Compiled dynamic model mass | 85.4 kg |
| Plant contract revisions | Policy r1 · physics r1 · visual r1 |
| Model | environments/trex/assets/trex.xml |
Current curriculum configuration
The Stable-Baselines3 budgets and early-advancement gates below come from the current TOML files and can differ from older published runs. A stage also ends when its configured budget is exhausted. JAX/MJX uses the same task sequence, but its CLI and notebook gate behavior is not yet equivalent.
| Stage | Objective | SB3 configured budget | SB3 early-advancement gate |
|---|---|---|---|
| 1 — Balance | Learn to stand and balance without falling | 6M | reward ≥ 100; episode length ≥ 750; ≥ 10 episodes/evaluation; 3 consecutive passes |
| 2 — Locomotion | Learn forward walking/running | 8M | reward ≥ 100; episode length ≥ 750; avg. velocity ≥ 2 m/s; ≥ 10 episodes/evaluation; 3 consecutive passes |
| 3 — Bite | Sprint to prey and make contact with the head bite proxy | 8M | reward ≥ 100; task success ≥ 50.0%; ≥ 10 episodes/evaluation; 3 consecutive passes |
Backend-specific task-success semantics
- Stable-Baselines3 — Head-contact bite proxy: The head-bite geom contacts the prey geom while the bite reward is enabled; the model has no articulated jaw.
- JAX/MJX — Head-tip proximity bite proxy: The head-tip site comes within 0.35 m of the prey target position while the bite bonus is enabled; physical geom contact is not required and the model has no articulated jaw.
Published run summaries
These are historical experiment records. A result is not evidence for the current model revision unless its provenance is explicitly marked current and verified.
PPO · Stable-Baselines3 · 2026-03-18
Provenance: Historical model · unverified · evaluation episode count not recorded · Stable-Baselines3 (version not recorded)
| Stage | Trained steps | Best eval reward | Avg. forward velocity | Task success (Stable-Baselines3) | Passed |
|---|---|---|---|---|---|
| 1 — balance | 6M | 3008.66 | 0.02 m/s | — | Yes |
| 2 — locomotion | 8M | 1936.01 | 3.47 m/s | — | Yes |
| 3 — bite | 8M | 1294.28 | 1.68 m/s | 96.7% | Yes |
Current Stable-Baselines3 definition for this task label: The head-bite geom contacts the prey geom while the bite reward is enabled; the model has no articulated jaw.
Anatomy
The T-Rex model includes:
- Torso - Forward-leaning body (~30 deg from horizontal) with ribcage and belly
- Neck + Skull - Short neck assembly with an elongated skull shape and brow-ridge geoms
- Head - Fixed contact geom used as a bite proxy; there is no articulated jaw
- Legs - Digitigrade-style hind limbs (hip pitch/roll, knee, ankle, 3 toe digits per leg)
- Arms - Tiny vestigial forelimbs with 2-fingered hands (passive, not actuated)
- Tail - 5-segment heavy counterbalance to skull (4 actuated segments)
Diagnostic Metrics (Stage 3)
After each training stage the eval command reports:
| Metric | Description |
|---|---|
mean_forward_velocity | Average forward speed (m/s) |
gait_symmetry | Left/right stride symmetry ∈ [0, 1] |
stride_frequency | Step frequency (Hz) |
cost_of_transport | Energy efficiency (lower is better) |
mean_pelvis_height | Upright stability (m) |
mean_heading_alignment | cos θ toward prey ∈ [-1, 1] |
success_rate | Fraction of episodes where the fixed head-contact proxy touches the prey target |
min_prey_distance | Closest approach to prey (m) |
Training Videos
Published videos for T-Rex, where available. Artifact provenance is shown on each stage.
Balance
Learn to stand and balance without falling
PPO · Stable-Baselines3 artifact · historical model · unverified · backend version not recorded. It is not evidence for the current model or stage config.
Locomotion
Learn forward walking/running
PPO · Stable-Baselines3 artifact · historical model · unverified · backend version not recorded. It is not evidence for the current model or stage config.
Bite
Sprint to prey and make contact with the head bite proxy
PPO · Stable-Baselines3 artifact · historical model · unverified · backend version not recorded. It is not evidence for the current model or stage config.
Usage
cd environments/trex
# View the model
python scripts/view_model.py
# Train stage 1 using its current TOML-configured budget
python scripts/train_sb3.py train --stage 1