[Question] Discrepancy between Humanoid-v3 and Humanoid-v4 about gymnasium

[Kallinteris-Andreas]

Identical behavior is not guaranteed across different gymnasium environment versions, in fact that is the point of environment versions, that being said if you want closer performance, you could try using an earlier version of mujoco library with the Humanoid-v4 or Humanoid-v5 environment

[jjyyxx]

@Kallinteris-Andreas Thanks for your prompt reply! I fully acknowledge the guarantee that identical behavior is only guaranteed within the same gymnasium environment version. However, your statement may be a little bit "self-contradictory", that:

a. the environment behavior could change across different underlying mujoco version, but
b. the environment behavior should keep consistent with fixed environment version

Regarding the specific Humanoid environment, I am quite confident that the program logic on the Gymnasium side of V3 and V4 are identical, and only the called API differs. So, I'm pretty sure that Humanoid-V4 would exhibit V3-like behavior with old enough mujoco binding (e.g., 2.1.5), and the environment behavior silently changes as mujoco upgrades.

[Kallinteris-Andreas]

Humanoid version 3 and humanoid version 4 are definitely not identical, They have different reward functions because version 4 contact cost is always 0, (due to a bug)

regarding MuJoCo version differences:
https://gymnasium.farama.org/main/environments/mujoco/#exact-reproducibility

[jjyyxx]

Sorry for my negligence and inclarity. The core logic to call mujoco simulation is what I refer to. I noticed the minor contact reward difference. Your point about exact reproducibility is fair enough, though I was not expecting obvious numerical difference since the first step of simulation between V3 and V4 with same seed (maximum absolute difference ~0.75).