From 745375f6e5b1023a0c84fcbd337e8397fa84c54a Mon Sep 17 00:00:00 2001
From: Kallinteris Andreas
 <30759571+Kallinteris-Andreas@users.noreply.github.com>
Date: Wed, 15 Mar 2023 00:00:31 +0200
Subject: [PATCH] `MuJoCo/Ant-v4` DOC fix contact_forces observation (#387)

---
 gymnasium/envs/mujoco/ant_v4.py | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/gymnasium/envs/mujoco/ant_v4.py b/gymnasium/envs/mujoco/ant_v4.py
index 620a9b7e8..582226412 100644
--- a/gymnasium/envs/mujoco/ant_v4.py
+++ b/gymnasium/envs/mujoco/ant_v4.py
@@ -81,13 +81,31 @@ class AntEnv(MujocoEnv, utils.EzPickle):
     | 23  | angular velocity of angle between torso and back left link   | -Inf   | Inf    | hip_3 (back_leg)                       | hinge | angle (rad)              |
     | 24  | angular velocity of the angle between back left links        | -Inf   | Inf    | ankle_3 (back_leg)                     | hinge | angle (rad)              |
     | 25  | angular velocity of angle between torso and back right link  | -Inf   | Inf    | hip_4 (right_back_leg)                 | hinge | angle (rad)              |
-    | 26  |angular velocity of the angle between back right links        | -Inf   | Inf    | ankle_4 (right_back_leg)               | hinge | angle (rad)              |
+    | 26  | angular velocity of the angle between back right links       | -Inf   | Inf    | ankle_4 (right_back_leg)               | hinge | angle (rad)              |
 
 
     If version < `v4` or `use_contact_forces` is `True` then the observation space is extended by 14*6 = 84 elements, which are contact forces
     (external forces - force x, y, z and torque x, y, z) applied to the
-    center of mass of each of the links. The 14 links are: the ground link,
-    the torso link, and 3 links for each leg (1 + 1 + 12) with the 6 external forces.
+    center of mass of each of the objects. The 14 object are:
+
+    in `v4` or earlier:
+    | id | object |
+    |  ---  |  ------------  |
+    | 0 | worldObject (note: forces are always full of zeros) |
+    | 1 | torso |
+    | 2 | front_left_leg |
+    | 3 | aux_1 (front left leg) |
+    | 4 | ankle_1 (front left leg) |
+    | 5 | front_right_leg |
+    | 6 | aux_2 (front right leg) |
+    | 7 | ankle_2 (front right leg) |
+    | 8 | back_leg (back left leg) |
+    | 9 | aux_3 (back left leg) |
+    | 10 | ankle_3 (back left leg) |
+    | 11 | right_back_leg |
+    | 12 | aux_4 (back right leg) |
+    | 13 | ankle_4 (back right leg) |
+
 
     The (x,y,z) coordinates are translational DOFs while the orientations are rotational
     DOFs expressed as quaternions. One can read more about free joints on the [Mujoco Documentation](https://mujoco.readthedocs.io/en/latest/XMLreference.html).