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#!/usr/bin/env python3
import gym
import numpy as np
from gym.envs.mujoco.mujoco_env import MujocoEnv
from learn2learn.gym.envs.meta_env import MetaEnv
class AntForwardBackwardEnv(MetaEnv, MujocoEnv, gym.utils.EzPickle):
This environment requires the ant to learn to run forward or backward.
At each time step the ant receives a signal composed of a
control cost and a reward equal to its average velocity in the direction
of the plane. The tasks are Bernoulli samples on {-1, 1} with probability 0.5, where -1 indicates the ant should
move backward and +1 indicates the ant should move forward.
The velocity is calculated as the distance (in the direction of the plane) of the ant's torso
position before and after taking the specified action divided by a small value dt.
As noted in [1], a small positive bonus is added to the reward to stop the ant from
prematurely ending the episode.
Adapted from Jonas Rothfuss' implementation.
1. Finn et al. 2017. "Model-Agnostic Meta-Learning for Fast Adaptation of Deep Networks." arXiv [cs.LG].
2. Rothfuss et al. 2018. "ProMP: Proximal Meta-Policy Search." arXiv [cs.LG].
def __init__(self, task=None):
MetaEnv.__init__(self, task)
MujocoEnv.__init__(self, 'ant.xml', 5)
# -------- MetaEnv Methods --------
def set_task(self, task):
MetaEnv.set_task(self, task)
self.goal_direction = task['direction']
def sample_tasks(self, num_tasks):
directions = np.random.choice((-1, 1), (num_tasks,))
tasks = [{'direction': direction} for direction in directions]
return tasks
# -------- Mujoco Methods --------
def _get_obs(self):
return np.concatenate([[2:],,
np.clip(, -1, 1).flat,
def viewer_setup(self):
camera_id = self.model.camera_name2id('track') = 2 = camera_id = self.model.stat.extent * 0.5
# Hide the overlay
self.viewer._hide_overlay = True
def reset_model(self):
qpos = self.init_qpos + self.np_random.uniform(size=self.model.nq, low=-.1, high=.1)
qvel = self.init_qvel + self.np_random.randn(self.model.nv) * .1
self.set_state(qpos, qvel)
# -------- Gym Methods --------
def step(self, action):
xposbefore = np.copy(self.get_body_com("torso")[0])
self.do_simulation(action, self.frame_skip)
xposafter = self.get_body_com("torso")[0]
forward_reward = self.goal_direction * (xposafter - xposbefore) / self.dt
ctrl_cost = .5 * np.square(action).sum()
contact_cost = 0.5 * 1e-3 * np.sum(
np.square(np.clip(, -1, 1)))
survive_reward = 1.0
reward = forward_reward - ctrl_cost - contact_cost + survive_reward
state = self.state_vector()
notdone = np.isfinite(state).all() and 1.0 >= state[2] >= 0.
done = not notdone
ob = self._get_obs()
return ob, reward, done, dict(
def reset(self, *args, **kwargs):
MujocoEnv.reset(self, *args, **kwargs)
return self._get_obs()
def render(self, mode='human'):
if mode == 'rgb_array':
# window size used for old mujoco-py:
width, height = 500, 500
data = self._get_viewer(mode).read_pixels(width,
return data
elif mode == 'human':
if __name__ == '__main__':
env = AntForwardBackwardEnv()
for task in [env.get_task(), env.sample_tasks(1)[0]]:
action = env.action_space.sample()
You can’t perform that action at this time.