Add vertex equality option.

PiperOrigin-RevId: 860299454
Change-Id: I7befd8e5f4c8bac6d74684a96a9cc3cee9ae1e29

doc/XMLreference.rst

@@ -3820,9 +3820,10 @@ element is used to adjust the properties of all edges in the flex.
 
 .. _flexcomp-edge-equality:
 
-:at:`equality`: :at-val:`[true, false], "false"`
-   When enabled, an equality constraint of :ref:`type flex<equality-flex>` is added to the model, referencing the
-   automatically-generated flex by name.
+:at:`equality`: :at-val:`[false, true, vert], "false"`
+   The type of equality constraint applied to this edge. If **false**, no equality constraint is applied. If **true**,
+   then edge constraints are enforced. If **vert**, an averaged constraint is used, see
+   :ref:`flexvert<equality-flexvert>`.
 
 .. _flexcomp-edge-solref:
 .. _flexcomp-edge-solimp:
@@ -4801,6 +4802,31 @@ as there are edges in the specified flex.
    Name of the flex whose edges are being constrained.
 
 
+.. _equality-flexvert:
+
+:el-prefix:`equality/` |-| **flexvert** |m|
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This element constrains the trace and the derminant of the strain tensor to that of the identity matrix as in Chen, Kry,
+and Vouga, "Locking-free Simulation of Isometric Thin Plates", 2019. The strain tensor is computed per triangle and
+averaged over all triangles adjacent to a vertex. This reduces the number of constraints from 2T to 2V, freeing
+V degrees of freedom to avoid locking. It is only supported for dimension 2, i.e., cloth-like flexes.
+
+.. _equality-flexvert-name:
+.. _equality-flexvert-class:
+.. _equality-flexvert-active:
+.. _equality-flexvert-solref:
+.. _equality-flexvert-solimp:
+
+:at:`name`, :at:`class`, :at:`active`, :at:`solref`, :at:`solimp`
+   Same as in :ref:`connect <equality-connect>` element.
+
+.. _equality-flexvert-flex:
+
+:at:`flex`: :at-val:`string, required`
+   Name of the flex whose vertices are being constrained.
+
+
 .. _equality-distance:
 
 :el-prefix:`equality/` |-| **distance** |m|

doc/XMLschema.rst

@@ -2021,6 +2021,30 @@
                :ref:`solimp<equality-flex-solimp>`
 
 
+      .. dropdown:: flexvert |m|
+
+         .. grid:: 2 3 4 4
+            :gutter: 0
+
+            .. grid-item::
+               :ref:`name<equality-flexvert-name>`
+
+            .. grid-item::
+               :ref:`class<equality-flexvert-class>`
+
+            .. grid-item::
+               :ref:`flex<equality-flexvert-flex>`
+
+            .. grid-item::
+               :ref:`active<equality-flexvert-active>`
+
+            .. grid-item::
+               :ref:`solref<equality-flexvert-solref>`
+
+            .. grid-item::
+               :ref:`solimp<equality-flexvert-solimp>`
+
+
    .. dropdown:: tendon |m|
 
 

doc/changelog.rst

@@ -32,6 +32,15 @@ Upcoming version (not yet released)
 General
 ^^^^^^^
 
+.. image:: images/changelog/poncho.png
+   :width: 45%
+   :align: right
+   :target: https://github.com/google-deepmind/mujoco/blob/main/model/flex/poncho.xml
+
+- Added new :ref:`flexvert<equality-flexvert>` equality constraints that enable cloth simulations with coarser meshes.
+  This adds a new option ``vert`` to flexcomp edge :ref:`equality<flexcomp-edge-equality>` and the new equality type
+  :ref:`flexvert<equality-flexvert>`.
+
 .. image:: images/XMLreference/rfcamera.png
    :width: 45%
    :align: right
@@ -65,6 +74,7 @@ General
   - Allcating sizes in :ref:`mjModel` now use 64-bit rather than 32-bit integers to accommodate larger
     scenes.
 
+
 MJX
 ^^^
 - Added ``actuator_length``, ``cdof`` and ``cdof_dof`` fields to ``mjx.Data``.

doc/includes/references.h

@@ -603,6 +603,7 @@ typedef enum mjtEq_ {             // type of equality constraint
   mjEQ_JOINT,                     // couple the values of two scalar joints with cubic
   mjEQ_TENDON,                    // couple the lengths of two tendons with cubic
   mjEQ_FLEX,                      // fix all edge lengths of a flex
+  mjEQ_FLEXVERT,                  // fix all vertex lengths of a flex
   mjEQ_DISTANCE                   // unsupported, will cause an error if used
 } mjtEq;
 typedef enum mjtWrap_ {           // type of tendon wrap object