Move flex damping to the engine and remove membrane and solid plugins.

PiperOrigin-RevId: 676434954
Change-Id: I24e8dbaa90afcffd613a9cf106ef8b7262195328

doc/XMLreference.rst

@@ -4154,6 +4154,12 @@ these mechanisms to be combined as desired.
 :el-prefix:`flex/` |-| **elasticity** (?)
 '''''''''''''''''''''''''''''''''''''''''
 
+The elasticity model is a `Saint Venant-Kirchhoff
+<https://en.wikipedia.org/wiki/Hyperelastic_material#Saint_Venant%E2%80%93Kirchhoff_model>`__ model discretized with
+piecewise linear finite elements, intended to simulate the compression or elongation of hyperelastic materials subjected
+to large displacements (finite rotations) and small strains, since it uses a nonlinear strain-displacement but a linear
+stress-strain relationship.. See also :ref:`deformable <CDeformable>` objects.
+
 .. _flex-elasticity-young:
 
 :at:`young`: :at-val:`real(1), "0"`

doc/changelog.rst

@@ -2,6 +2,17 @@
 Changelog
 =========
 
+Upcoming version (not yet released)
+-----------------------------------
+
+General
+^^^^^^^
+
+- Removed the :at:`solid` and :at:`membrane` plugins and moved the associated computations into the engine. See `3D
+  example model <https://github.com/google-deepmind/mujoco/blob/main/model/flex/floppy.xml>`__ and `2D example model
+  <https://github.com/google-deepmind/mujoco/blob/main/src/model/trampoline.xml>`__ for examples of flex objects
+  that previously required these plugins.
+
 Version 3.2.3 (Sep 16, 2024)
 ----------------------------
 

doc/includes/references.h

@@ -1172,6 +1172,7 @@ struct mjModel_ {
   mjtNum*   flexedge_invweight0;  // edge inv. weight in qpos0                (nflexedge x 1)
   mjtNum*   flex_radius;          // radius around primitive element          (nflex x 1)
   mjtNum*   flex_stiffness;       // finite element stiffness matrix          (nflexelem x 21)
+  mjtNum*   flex_damping;         // Rayleigh's damping coefficient           (nflex x 1)
   mjtNum*   flex_edgestiffness;   // edge stiffness                           (nflex x 1)
   mjtNum*   flex_edgedamping;     // edge damping                             (nflex x 1)
   mjtByte*  flex_edgeequality;    // is edge equality constraint defined      (nflex x 1)

doc/programming/extension.rst

@@ -274,12 +274,11 @@ There are several first-party plugin directories:
   `README <https://github.com/google-deepmind/mujoco/blob/main/plugin/actuator/README.md>`__ for details.
 * **elasticity:** The plugins in the `elasticity/
   <https://github.com/google-deepmind/mujoco/tree/main/plugin/elasticity>`__ directory are passive forces based on
-  continuum mechanics for 1-dimensional and 3-dimensional bodies. The 1D model is invariant under rotations and captures
-  the large deformation of elastic cables, decoupling twisting and bending strains. The 3D solid is a `Saint
-  Venant-Kirchhoff <https://en.wikipedia.org/wiki/Hyperelastic_material#Saint_Venant%E2%80%93Kirchhoff_model>`__ model
-  discretized with piecewise linear finite elements, which is suitable for large deformations with small strains. See
-  also :ref:`composite <CComposite>` and :ref:`deformable <CDeformable>` objects. For more information, please see the
-  `README <https://github.com/google-deepmind/mujoco/blob/main/plugin/elasticity/README.md>`__.
+  continuum mechanics for 1-dimensional and 2-dimensional bodies. The 1D model is invariant under rotations and captures
+  the large deformation of elastic cables, decoupling twisting and bending strains. The 2D model is a suitable for
+  computing the bending stiffness of thin elastic plates (i.e. shells having a flat stress-free configuration). In this
+  case, the elastic energy is quadratic and therefore the stiffness matrix is constant. For more information, please see
+  the `README <https://github.com/google-deepmind/mujoco/blob/main/plugin/elasticity/README.md>`__.
 * **sensor:** The plugins in the `sensor/ <https://github.com/google-deepmind/mujoco/tree/main/plugin/sensor>`__
   directory implement custom sensors. Currently the sole sensor plugin is the touch grid sensor, see the
   `README <https://github.com/google-deepmind/mujoco/blob/main/plugin/sensor/README.md>`__ for details.