Different behavior from gym.Mujoco: the walker body penetrate the ground

[lkevinzc]

I am testing the walker2d environment with brax. I provide no action (all-zero vector) and just let the gravity pull the walker down. However, I found the walker body will penetrate the ground, which gives very different physics behaviors from the one with gym.

brax	gym

I'm using the following codes for brax:

import jax
import jax.numpy as jnp

from brax.envs import walker2d
from brax.io.html import render

env = walker2d.Walker2d()
state = env.reset(jax.random.PRNGKey(0))

env.step = jax.jit(env.step)

no_action = jnp.zeros(env.action_size)

rollout = []
for _ in range(500):
    rollout.append(state.pipeline_state)
    state = env.step(state, no_action)

html_str = render(env.sys, rollout)
with open("./walker_no_control.html", "w") as f:
    f.write(html_str)

and for gym:

import gymnasium as gym
import numpy as np
import skvideo.io

env = gym.make("Walker2d-v4", render_mode="rgb_array")
env.reset()
frames = []

no_action = np.zeros(np.cumprod(env.action_space.shape))
for _ in range(500):
    frames.append(env.render())
    env.step(no_action)

frames_np = np.stack(frames)
writer = skvideo.io.FFmpegWriter(f"walker_no_control.mp4", verbosity=1)
for i in range(len(frames_np)):
    writer.writeFrame(frames_np[i, :, :, :])
writer.close()

Could anyone help to explain why, and give some solutions?

[lkevinzc]

Solved. In the xml file, for the floor geom and all the body geom, modify the contype or conaffinity such that they are compatible to pass the collision detection filtering.

Ref: [1] https://mujoco.readthedocs.io/en/stable/XMLreference.html#body-geom [2] https://mujoco.readthedocs.io/en/stable/computation/index.html#collision-detection