T-Rex Model

The Tyrannosaurus Rex is a large bipedal predator with a massive skull, powerful jaws, and vestigial forelimbs. It hunts by sprinting toward prey and delivering a bite.

Specifications

Property	Value
Hip height	~1.1m (simulated)
Total mass	~80 kg (simulated, scaled)
Hinge joints	25
Ball joints	2 (shoulders, passive)
Actuators	21
Observation dim	83
Action dim	21

Anatomy

The T-Rex model includes:

• Torso - Forward-leaning body (~30 deg from horizontal) with ribcage and belly
• Neck + Skull - Short muscular neck with massive elongated skull and brow ridges
• Head - Bite contact geom for prey interaction
• Legs - Powerful digitigrade 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)

Action Space (21 dims)

Index	Actuator	Type
0-2	Neck pitch, neck yaw, head pitch	Position
3-9	Right leg (hip pitch/roll, knee, ankle, toe d2/d3/d4)	Position
10-16	Left leg (hip pitch/roll, knee, ankle, toe d2/d3/d4)	Position
17-20	Tail (pitch 1, yaw 1, pitch 2, pitch 3)	Position

Training Stages

1. Balance - Stable bipedal stance
2. Locomotion - Walk and run toward prey
3. Hunting - Sprint and bite prey with jaws

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 steps with jaw-bite event
min_prey_distance	Closest approach to prey (m)

Usage

cd environments/trex

# View the model
python scripts/view_model.py

# Train stage 1
python scripts/train_sb3.py train --stage 1 --timesteps 1000000