Observation Pipeline

1. Entry Point & Environment Setup

File: env_factory.py
make_env() loads pal_kangaroo_flat_env_cfg() and builds the ManagerBasedRlEnv.

2. Configuration of Observation Terms

File: kangaroo_env_cfg.py

pal_kangaroo_flat_env_cfg() is based on pal_kangaroo_rough_env_cfg()
pal_kangaroo_rough_env_cfg() overrides observation terms:
Removed: base_lin_vel, projected_gravity
Added: imu_projected_gravity (IMU quaternion), base_lin_acc (IMU linear acceleration)

File: velocity_env_cfg.py
Defines policy_terms and critic_terms as ObservationTermCfg.

3. Observation Functions (Data Sources)

File: observations.py

Term	Source
`base_lin_vel`	Root-link linear velocity (simulator state)
`base_ang_vel`	Root-link angular velocity (simulator state)
`projected_gravity`	Gravity in the body frame
`joint_pos_rel`	Joint positions relative to the default position
`joint_vel_rel`	Joint velocities
`last_action`	from `ActionManager`
`generated_commands`	from `CommandManager`
`builtin_sensor`	MuJoCo sensor data
`imu_projected_gravity`	IMU quaternion → projected gravity

4. Sensor Source (MuJoCo)

File: builtin_sensor.py
BuiltinSensor reads sensordata directly from MuJoCo.

File: scene.py
Binds sensors from the scene/entity XML via BuiltinSensor.from_existing().

5. Robot State (from the simulator)

File: data.py

All raw values come from mjwarp.Data (simulator state):

projected_gravity_b
root_link_lin_vel_b
root_link_ang_vel_b
joint_pos
joint_vel

6. Observation Pipeline (Processing Steps)

File: observation_manager.py

_prepare_terms() computes shapes and initializes delay/history buffers.

compute_group() processes each term in this order:

term_cfg.func(...)   →  raw observation
    │
  ▼  Noise          (NoiseCfg / NoiseModelCfg)
  ▼  Clip           (term_cfg.clip)
  ▼  Scale          (term_cfg.scale)
  ▼  NaN check
  ▼  Delay          (DelayBuffer)
  ▼  History        (CircularBuffer, optional flatten)
  ▼  Concatenate    (if the group is configured that way)

7. Observation Groups

Policy Group (Shape: `(86,)`)

Index	Term	Shape
0	`base_ang_vel`	`(3,)`
1	`joint_pos`	`(26,)`
2	`joint_vel`	`(26,)`
3	`actions`	`(22,)`
4	`command`	`(3,)`
5	`imu_projected_gravity`	`(3,)`
6	`base_lin_acc`	`(3,)`

base_lin_vel and projected_gravity are disabled in this config (None).

Critic Group (Shape: `(104,)`)

Index	Term	Shape
0	`base_lin_vel`	`(3,)`
1	`base_ang_vel`	`(3,)`
2	`projected_gravity`	`(3,)`
3	`joint_pos`	`(26,)`
4	`joint_vel`	`(26,)`
5	`actions`	`(22,)`
6	`command`	`(3,)`
7	`foot_height`	`(2,)`
8	`foot_air_time`	`(2,)`
9	`foot_contact`	`(2,)`
10	`foot_contact_forces`	`(6,)`
11	`imu_projected_gravity`	`(3,)`
12	`base_lin_acc`	`(3,)`

foot_height, foot_air_time, foot_contact, and foot_contact_forces depend on the ContactSensor.
The critic has access to privileged information that is not available to the policy.