Fish

A 3D free-swimming fish with five actuated joints in a water-like medium.

FishSwim

Property	Value
Canonical ID	mjx/fish_swim-v0
Action space	Box(-1.0, 1.0, (5,), float32)
Observation space	Box(-inf, inf, (24,), float32)
Episode length	1000
Config	{"ctrl_dt": 0.04, "sim_dt": 0.002, "naconmax": 0, "njmax": 25}

Description

The fish swims toward a randomised target in 3D space. Five actuators across its body produce undulatory locomotion through fluid drag — a noticeably different control regime from the rigid-body locomotion envs in the suite. Lateral roll makes target-tracking harder, so a stable upright posture matters as much as raw forward thrust.

Rewards

Uses a dense reward that combines an "in target" tolerance with torso uprightness as a weighted sum, with most of the weight on the target term:

in_target  = tolerance(
    norm(mouth_to_target_local),
    bounds=(0, target_radius),
    margin=2 * target_radius,
)
is_upright = 0.5 * (torso_z_up + 1)
reward = (7 * in_target + is_upright) / 8

The two terms capture different concerns:

• in_target — the smooth tolerance indicator over mouth-to-target distance: 1.0 inside target_radius, decaying smoothly out to 2 * target_radius, zero further.
• is_upright — the dot product of the torso's up-axis with world-z, normalised to [0, 1]. 1.0 when upright, 0.5 on its side, 0.0 upside-down.

Starting state

obs = [ 0.8535  0.1007  0.1161 -0.129  -0.121  -0.1377  0.1608  0.023
        0.3777  0.1225  0.0365  0.      0.      0.      0.      0.
        0.      0.      0.      0.      0.      0.      0.      0.    ]

(joint positions and orientations followed by velocities — fish initialised at the origin with the target randomised, body at rest.)

Termination

Episode ends when step >= max_steps (default 1000). No early termination.

Usage

import envrax
env = envrax.make("mjx/fish_swim-v0")

Reference

Upstream: mujoco_playground/_src/dm_control_suite/fish.py.