Remove distance constraints.

- `distance` constraints were not very useful and increased the contact buffer within the constraint solver, violating a core design principle. Therefore they have been removed.
- The enum value `mjEQ_DISTANCE` is retained, but errors will be thrown in the parser and engine if these constraints are selected.

PiperOrigin-RevId: 467050101
Change-Id: Ie72a07e4fe338eea5107004a9fc0f5e47eb0c3b7

doc/APIreference.rst

@@ -324,7 +324,7 @@ mjtEq
        mjEQ_WELD,                      // fix relative position and orientation of two bodies
        mjEQ_JOINT,                     // couple the values of two scalar joints with cubic
        mjEQ_TENDON,                    // couple the lengths of two tendons with cubic
-       mjEQ_DISTANCE                   // fix the contact distance betweent two geoms
+       mjEQ_DISTANCE                   // unsupported, will cause an error if used
    } mjtEq;
 
 | Defined in `mjmodel.h <https://github.com/deepmind/mujoco/blob/main/include/mujoco/mjmodel.h>`_

doc/XMLreference.rst

@@ -4061,20 +4061,8 @@ This element constrains the length of one tendon to be a quartic polynomial of a
 :el-prefix:`equality/` **distance** (*)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-This element constrains the nearest distance between two geoms. When the distance attribute is set to 0 the two geom
-surfaces slide over each other, otherwise they slide over a virtual cushion with depth equal to the specified distance.
-This mechanism is implemented as a modification to the collision detector. For geom pairs handled by the general-purpose
-convex collider, large distance values in this constraint are handled approximately, due to the nature of the underlying
-collision algorithm.
-
-:at:`name`, :at:`class`, :at:`active`, :at:`solref`, :at:`solimp`
-   Same as in :ref:`connect <equality-connect>` element.
-:at:`geom1`: :at-val:`string, required`
-   Name of the first geom.
-:at:`geom2`: :at-val:`string, required`
-   Name of the second geom.
-:at:`distance`: :at-val:`real, "0"`
-   Desired distance between the two geom surfaces. The constraint solver enforces this distance softly.
+Distance equality constraints were removed in MuJoCo version 2.2.2. If you are using an earlier version, please switch
+to the corresponding version of the documentation.
 
 .. _tendon:
 

doc/changelog.rst

@@ -18,9 +18,13 @@ General
   inaccurate. If this occurs, open the mesh in MeshLab or Blender and recalculate the faces.
 - Added ``azimuth`` and ``elevation`` attributes to :ref:`visual/global<global>`, defining the initial orientation of
   the free camera at model load time.
- - Added ``mjv_defaultFreeCamera`` which sets the default free camera, respecting the above attributes.
+- Added ``mjv_defaultFreeCamera`` which sets the default free camera, respecting the above attributes.
 - ``simulate`` now supports taking a screenshot via a button in the File section or via ``Ctrl-P``.
 
+Deleted/deprecated features
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+- Removed ``distance`` constraints.
+
 Bug fixes
 ^^^^^^^^^
 - Fixed rendering of some transparent geoms in reflection.