Remove the Shell plugin and integrate it into the engine.

PiperOrigin-RevId: 760688502
Change-Id: Ia70988d42b7edf571d7cb4a4f48f8fc50b51667d

doc/XMLreference.rst

@@ -4073,9 +4073,9 @@ stress-strain relationship.. See also :ref:`deformable <CDeformable>` objects.
 
 .. _flex-elasticity-elastic2d:
 
-:at:`elastic2d`: :at-val:`int, "1"`
+:at:`elastic2d`: :at-val:`[none, bend, stretch, both], "none"`
-   Elastic contribution to passive forces of 2D flexes. 0: none, 1: bending only, 2: stretching only, 3: bending and
+   Elastic contribution to passive forces of 2D flexes. "none": none, "bend": bending only, "stretch": stretching only,
-   stretching
+   "both": bending and stretching.
 
 .. _flex-contact:
 

doc/changelog.rst

@@ -9,6 +9,8 @@ General
 ^^^^^^^
 - Refactored island implementation so that island data is memory-contiguous. This speeds up island processing in the
   solver and clears the way for the addition of the Newton and PGS solvers (currently only CG is supported).
+- Removed the :at:`shell` plugin. This is now supported by :ref:`flexcomp<body-flexcomp>` and is active depending on
+  the :ref:`elastic2d<body-flexcomp-elasticity-elastic2d>` attribute (on by default).
 
 simulate
 ^^^^^^^^

model/flex/bunny.xml

@@ -31,7 +31,7 @@
     </body>
     <flexcomp type="mesh" file="bunny.obj" pos="0 0 .1" dim="2" euler="90 0 0"
               radius=".001" rgba="0 .7 .7 1" mass=".05" name="softbody" dof="trilinear">
-      <elasticity young="1e3" poisson="0.1" damping="0.001"/>
+      <elasticity young="1e3" poisson="0.1" damping="0.001" elastic2d="stretch"/>
       <contact selfcollide="none" internal="false"/>
     </flexcomp>
   </worldbody>

model/flex/bunny_with_uv.xml

@@ -37,7 +37,7 @@
     </body>
     <flexcomp type="mesh" file="bunny_with_uv.obj" pos="0 0 .1" dim="2" euler="90 0 0"
               radius=".001" material="matsponge" mass=".05" name="softbody" dof="trilinear">
-      <elasticity young="1e3" poisson="0.1" damping="0.001"/>
+      <elasticity young="1e3" poisson="0.1" damping="0.001" elastic2d="stretch"/>
       <contact selfcollide="none" internal="false"/>
     </flexcomp>
   </worldbody>

model/flex/flag.xml

@@ -26,10 +26,6 @@
   <option wind="5 5 0" density="10" solver="CG" tolerance="1e-6">
   </option>
 
-  <extension>
-    <plugin plugin="mujoco.elasticity.shell"/>
-  </extension>
-
   <worldbody>
     <geom name="floor" type="plane" size="0 0 .1"/>
     <light diffuse=".6 .6 .6" specular="0.2 0.2 0.2" pos="0 0 4" dir="0 0 -1"/>
@@ -37,8 +33,7 @@
       <flexcomp type="grid" count="9 19 1" spacing=".05 .05 .05" mass="10"
                 name="flag" radius="0.001">
         <edge equality="true" damping="0.001"/>
-        <elasticity poisson="0" thickness="1e-2" young="3e6"/>
+        <elasticity poisson="0" thickness="1e-2" young="3e6" elastic2d="none"/>
-        <plugin plugin="mujoco.elasticity.shell"/>
       </flexcomp>
     </body>
   </worldbody>

model/flex/pancake.xml

@@ -16,10 +16,6 @@
 <mujoco model="Pancake">
   <include file="scene.xml"/>
 
-  <extension>
-    <plugin plugin="mujoco.elasticity.shell"/>
-  </extension>
-
   <option timestep="0.001" solver="CG" tolerance="1e-6" integrator="implicitfast"/>
 
   <size memory="10M"/>
@@ -39,7 +35,7 @@
               radius=".01" mass=".5" name="plate" dim="2">
       <contact condim="3" solref="0.01 1" solimp=".95 .99 .0001"/>
       <edge equality="true" damping="10"/>
-      <elasticity poisson="0" thickness="8e-3" young="3e5"/>
+      <elasticity poisson="0" thickness="8e-3" young="3e5" elastic2d="bend"/>
     </flexcomp>
   </worldbody>
 </mujoco>

model/flex/plate.xml

@@ -16,10 +16,6 @@
 <mujoco model="Plate">
   <include file="scene.xml"/>
 
-  <extension>
-    <plugin plugin="mujoco.elasticity.shell"/>
-  </extension>
-
   <option solver="CG" tolerance="1e-6" integrator="implicitfast"/>
 
   <size memory="10M"/>
@@ -42,8 +38,7 @@
               radius=".001" mass="100" name="plate">
       <contact condim="3" solref="0.01 1" solimp=".95 .99 .0001"/>
       <edge equality="true" damping="0.001"/>
-      <elasticity poisson="0" thickness="1e-2" young="3e7"/>
+      <elasticity poisson="0" thickness="1e-2" young="3e7" elastic2d="bend"/>
-      <plugin plugin="mujoco.elasticity.shell"/>
     </flexcomp>
   </worldbody>
 </mujoco>

model/flex/poncho.xml

@@ -19,10 +19,6 @@
     <flag energy="enable"/>
   </option>
 
-  <extension>
-    <plugin plugin="mujoco.elasticity.shell"/>
-  </extension>
-
   <visual>
     <map force="0.1" zfar="30"/>
     <rgba haze="0.15 0.25 0.35 1"/>
@@ -1418,9 +1414,8 @@
                 398 399 418
                 398 376 378">
       <edge equality="true" damping="0.1"/>
-      <elasticity young="3e5" poisson="0" thickness="8e-3"/>
+      <elasticity young="3e5" poisson="0" thickness="8e-3" elastic2d="bend"/>
       <contact solref="0.003"/>
-      <plugin plugin="mujoco.elasticity.shell"/>
     </flexcomp>
 
   </worldbody>

model/flex/poncho_vertcollide.xml

@@ -19,10 +19,6 @@
     <flag energy="enable"/>
   </option>
 
-  <extension>
-    <plugin plugin="mujoco.elasticity.shell"/>
-  </extension>
-
   <visual>
     <map force="0.1" zfar="30"/>
     <rgba haze="0.15 0.25 0.35 1"/>
@@ -1418,9 +1414,8 @@
                 398 399 418
                 398 376 378">
       <edge equality="true" damping="0.1"/>
-      <elasticity poisson="0.3" young="3e5" thickness="8e-3"/>
+      <elasticity poisson="0.3" young="3e5" thickness="8e-3" elastic2d="bend"/>
       <contact solref="0.003" contype="0" conaffinity="0" vertcollide="true"/>
-      <plugin plugin="mujoco.elasticity.shell"/>
     </flexcomp>
 
   </worldbody>

model/flex/trampoline.xml

@@ -39,7 +39,7 @@
               radius=".001" mass="10" name="plate" dim="2">
       <contact condim="3" solref="0.01 1" solimp=".95 .99 .0001"/>
       <edge equality="false" damping="10"/>
-      <elasticity young="3e5" poisson="0" thickness="1e-2" elastic2d="2"/>
+      <elasticity young="3e5" poisson="0" thickness="1e-2" elastic2d="stretch"/>
       <pin id="0 15 240 255"/>
     </flexcomp>
   </worldbody>

plugin/elasticity/CMakeLists.txt

@@ -22,8 +22,6 @@ set(MUJOCO_ELASTICITY_SRCS
     elasticity.cc
     elasticity.h
     register.cc
-    shell.cc
-    shell.h
 )
 
 add_library(elasticity SHARED)

plugin/elasticity/README.md

@@ -18,15 +18,3 @@ Parameters:
    - `bend` [Pa]: bending stiffness.
    - `flat` [bool]: if true, the stress-equilibrium configuration is that of a straight cable; if false or unspecified, it is the configuration defined in the XML.
    - `vmax` [N/m^2]: If greater than zero, the cable is colored using mechanical stresses; the value represent the maximum stress in the color scale.
-
-### Shell
-
-Implemented in [shell.cc](shell.cc).
-
-The shell plugin discretizes an inextensible 2D continuum. It is intended to simulate the bending of plates where the stretching is negligible compared to other deformation modes.
-
-Parameters:
-
-   - `young` [Pa]: Young's modulus.
-   - `poisson` [Pa]: Poisson's ratio; if 0, then the material only opposed shear deformations; if near 0.5, then the material is nearly incompressible (rubber-like).
-   - `thickness` [m]: shell thickness, used to scale the bending stiffness.

plugin/elasticity/register.cc

@@ -14,13 +14,11 @@
 
 #include <mujoco/mjplugin.h>
 #include "cable.h"
-#include "shell.h"
 
 namespace mujoco::plugin::elasticity {
 
 mjPLUGIN_LIB_INIT {
   Cable::RegisterPlugin();
-  Shell::RegisterPlugin();
 }
 
 }  // namespace mujoco::plugin::elasticity

plugin/elasticity/shell.cc

@@ -1,154 +0,0 @@
-// Copyright 2023 DeepMind Technologies Limited
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include <cassert>
-#include <cmath>
-#include <cstdint>
-#include <cstdlib>
-#include <optional>
-#include <utility>
-#include <vector>
-
-#include <mujoco/mjplugin.h>
-#include <mujoco/mjtnum.h>
-#include <mujoco/mujoco.h>
-#include "elasticity.h"
-#include "shell.h"
-
-
-namespace mujoco::plugin::elasticity {
-namespace {
-
-// local tetrahedron numbering
-constexpr int kNumVerts = Stencil2D::kNumVerts;
-
-
-
-}  // namespace
-
-// factory function
-std::optional<Shell> Shell::Create(const mjModel* m, mjData* d, int instance) {
-    return Shell(m, d, instance);
-}
-
-// plugin constructor
-Shell::Shell(const mjModel* m, mjData* d, int instance)
-    : f0(-1) {
-  // count plugin bodies
-  nv = 0;
-  for (int i = 1; i < m->nbody; i++) {
-    if (m->body_plugin[i] == instance) {
-      if (!nv++) {
-        i0 = i;
-      }
-    }
-  }
-
-  // count flexes
-  for (int i = 0; i < m->nflex; i++) {
-    for (int j = 0; j < m->flex_vertnum[i]; j++) {
-      if (m->flex_vertbodyid[m->flex_vertadr[i]+j] == i0) {
-        f0 = i;
-        nv = m->flex_vertnum[f0];
-        if (m->flex_dim[i] != 2) {  // SHOULD NOT OCCUR
-          mju_error("mujoco.elasticity.shell requires a 2D mesh");
-        }
-      }
-    }
-  }
-
-  // loop over all triangles
-  for (int t = 0; t < m->flex_elemnum[f0]; t++) {
-    int* v = m->flex_elem + 3*(t+m->flex_elemadr[f0]);
-    for (int i = 0; i < kNumVerts; i++) {
-      if (m->body_plugin[i0+v[i]] != instance) {
-        mju_error("This body does not have the requested plugin instance");
-      }
-    }
-  }
-
-  // allocate array
-  position.assign(nv*3, 0);
-
-  // store previous positions
-  mju_copy(position.data(), m->body_pos+3*i0, 3*nv);
-}
-
-void Shell::Compute(const mjModel* m, mjData* d, int instance) {
-  for (int e = 0; e < m->flex_edgenum[f0]; e++) {
-    int* edge = m->flex_edge + 2*(e+m->flex_edgeadr[f0]);
-    int* flap = m->flex_edgeflap + 2*(e+m->flex_edgeadr[f0]);
-    int v[4] = {edge[0], edge[1], flap[0], flap[1]};
-    mjtNum force[12] = {0};
-    if (v[3] == -1) {
-      // skip boundary edges
-      continue;
-    }
-    mjtNum* k = m->flex_bending + 16*m->flex_edgeadr[f0];
-    for (int i = 0; i < StencilFlap::kNumVerts; i++) {
-      for (int j = 0; j < StencilFlap::kNumVerts; j++) {
-        for (int x = 0; x < 3; x++) {
-          force[3*i+x] += k[16*e+4*i+j] * d->xpos[3*(i0+v[j])+x];
-        }
-      }
-    }
-
-    // update stored positions
-    mju_copy(position.data(), d->xpos+3*i0, 3*nv);
-
-    // insert into global force
-    for (int i = 0; i < StencilFlap::kNumVerts; i++) {
-      for (int x = 0; x < 3; x++) {
-        d->qfrc_passive[m->body_dofadr[i0]+3*v[i]+x] -= force[3*i+x];
-      }
-    }
-  }
-}
-
-
-
-void Shell::RegisterPlugin() {
-  mjpPlugin plugin;
-  mjp_defaultPlugin(&plugin);
-
-  plugin.name = "mujoco.elasticity.shell";
-  plugin.capabilityflags |= mjPLUGIN_PASSIVE;
-
-  const char* attributes[] = {"damping"};
-  plugin.nattribute = sizeof(attributes) / sizeof(attributes[0]);
-  plugin.attributes = attributes;
-  plugin.nstate = +[](const mjModel* m, int instance) { return 0; };
-
-  plugin.init = +[](const mjModel* m, mjData* d, int instance) {
-    auto elasticity_or_null = Shell::Create(m, d, instance);
-    if (!elasticity_or_null.has_value()) {
-      return -1;
-    }
-    d->plugin_data[instance] = reinterpret_cast<uintptr_t>(
-        new Shell(std::move(*elasticity_or_null)));
-    return 0;
-  };
-  plugin.destroy = +[](mjData* d, int instance) {
-    delete reinterpret_cast<Shell*>(d->plugin_data[instance]);
-    d->plugin_data[instance] = 0;
-  };
-  plugin.compute = +[](const mjModel* m, mjData* d, int instance, int type) {
-    auto* elasticity = reinterpret_cast<Shell*>(d->plugin_data[instance]);
-    elasticity->Compute(m, d, instance);
-  };
-
-  mjp_registerPlugin(&plugin);
-}
-
-}  // namespace mujoco::plugin::elasticity

plugin/elasticity/shell.h

@@ -1,61 +0,0 @@
-// Copyright 2023 DeepMind Technologies Limited
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef MUJOCO_PLUGIN_ELASTICITY_SHELL_H_
-#define MUJOCO_PLUGIN_ELASTICITY_SHELL_H_
-
-#include <optional>
-#include <vector>
-
-#include <mujoco/mjdata.h>
-#include <mujoco/mjmodel.h>
-#include <mujoco/mjtnum.h>
-#include "elasticity.h"
-
-
-namespace mujoco::plugin::elasticity {
-
-struct StencilFlap {
-  static constexpr int kNumVerts = 4;
-  int vertices[kNumVerts];
-};
-
-class Shell {
- public:
-  // Returns a new Shell instance or nullopt on failure.
-  static std::optional<Shell> Create(const mjModel* m, mjData* d,
-                                     int instance);
-  Shell(Shell&&) = default;
-
-  Shell& operator=(Shell&& other) = default;
-
-  void Compute(const mjModel* m, mjData* d, int instance);
-
-  static void RegisterPlugin();
-
-  int i0;  // index of first body
-  int f0;  // index of corresponding flex
-  int nc;  // number of quads in the grid
-  int nv;  // number of vertices (bodies) in the Shell
-
-  // precomputed quantities
-  std::vector<mjtNum> position;             // previous-step positions (nv x 3)
-
- private:
-  Shell(const mjModel* m, mjData* d, int instance);
-};
-
-}  // namespace mujoco::plugin::elasticity
-
-#endif  // MUJOCO_PLUGIN_ELASTICITY_SHELL_H_

src/engine/engine_passive.c

@@ -116,9 +116,50 @@ static void mj_springdamper(const mjModel* m, mjData* d) {
   // flex elasticity
   for (int f=0; f < m->nflex; f++) {
     mjtNum* k = m->flex_stiffness + 21*m->flex_elemadr[f];
+    mjtNum* b = m->flex_bending + 16*m->flex_edgeadr[f];
     int dim = m->flex_dim[f];
 
-    if (dim == 1 || m->flex_rigid[f] || k[0] == 0) {
+    if (dim == 1 || m->flex_rigid[f]) {
+      continue;
+    }
+
+    // add bending forces to qfrc_spring
+    if (dim == 2) {
+      mjtNum* xpos = d->flexvert_xpos + 3*m->flex_vertadr[f];
+      int* bodyid = m->flex_vertbodyid + m->flex_vertadr[f];
+
+      for (int e = 0; e < m->flex_edgenum[f]; e++) {
+        const int* edge = m->flex_edge + 2*(e+m->flex_edgeadr[f]);
+        const int* flap = m->flex_edgeflap + 2*(e+m->flex_edgeadr[f]);
+        int v[4] = {edge[0], edge[1], flap[0], flap[1]};
+        if (v[3] == -1) {
+          // skip boundary edges
+          continue;
+        }
+        mjtNum force[12] = {0};
+        for (int i = 0; i < 4; i++) {
+          for (int j = 0; j < 4; j++) {
+            for (int x = 0; x < 3; x++) {
+              force[3*i+x] += b[16*e+4*i+j] * xpos[3*v[j]+x];
+            }
+          }
+        }
+
+        // TODO: add damping
+
+        // insert into global force
+        for (int i = 0; i < 4; i++) {
+          int bid = bodyid[v[i]];
+          int body_dofnum = m->body_dofnum[bid];
+          int body_dofadr = m->body_dofadr[bid];
+          for (int x = 0; x < body_dofnum; x++) {
+            d->qfrc_spring[body_dofadr+x] -= force[3*i+x];
+          }
+        }
+      }
+    }
+
+    if (k[0] == 0) {
       continue;
     }
 

src/user/user_init.c

@@ -238,7 +238,6 @@ void mjs_defaultFlex(mjsFlex* flex) {
   flex->rgba[0] = flex->rgba[1] = flex->rgba[2] = 0.5f;
   flex->rgba[3] = 1.0f;
   flex->thickness = -1;
-  flex->elastic2d = 1;
 }
 
 

src/user/user_mesh.cc

@@ -3382,9 +3382,13 @@ void mjCFlex::ResolveReferences(const mjCModel* m) {
   vertbodyid.clear();
   nodebodyid.clear();
   for (const auto& vertbody : vertbody_) {
-    mjCBase* pbody = m->FindObject(mjOBJ_BODY, vertbody);
+    mjCBody* pbody = static_cast<mjCBody*>(m->FindObject(mjOBJ_BODY, vertbody));
     if (pbody) {
       vertbodyid.push_back(pbody->id);
+      if (pbody->joints.size() != 3 && dim == 2 && (elastic2d == 1 || elastic2d == 3)) {
+        // TODO(quaglino): add support for pins
+        throw mjCError(this, "pins are not supported for bending");
+      }
     } else {
       throw mjCError(this, "unknown body '%s' in flex", vertbody.c_str());
     }
@@ -3658,7 +3662,10 @@ void mjCFlex::Compile(const mjVFS* vfs) {
     }
 
     // bending stiffness (2D only)
-    if (dim == 2 && (elastic2d == 1 || elastic2d == 3) && thickness > 0) {
+    if (dim == 2 && (elastic2d == 1 || elastic2d == 3)) {
+      if (thickness < 0) {
+        throw mjCError(this, "thickness must be positive for bending stiffness");
+      }
       bending.assign(nedge*16, 0);
 
       for (unsigned int e = 0; e < nedge; e++) {
@@ -3668,7 +3675,7 @@ void mjCFlex::Compile(const mjVFS* vfs) {
     }
   }
 
-  // add plugins
+  // placeholder for setting plugins parameters, currently not used
   for (const auto& vbodyid : vertbodyid) {
     if (vbodyid < 0) {
       continue;
@@ -3676,8 +3683,8 @@ void mjCFlex::Compile(const mjVFS* vfs) {
     if (model->Bodies()[vbodyid]->plugin.element) {
       mjCPlugin* plugin_instance =
         static_cast<mjCPlugin*>(model->Bodies()[vbodyid]->plugin.element);
-      if (damping > 0) {
+      if (!plugin_instance) {
-        plugin_instance->config_attribs["damping"] = std::to_string(damping);
+        throw mjCError(this, "plugin instance not found");
       }
     }
   }

src/user/user_model.cc

@@ -3165,7 +3165,7 @@ void mjCModel::CopyObjects(mjModel* m) {
     for (int k=0; k < pfl->nedge; k++) {
       m->flex_edge[2*(edge_adr+k)] = pfl->edge[k].first;
       m->flex_edge[2*(edge_adr+k)+1] = pfl->edge[k].second;
-      if (pfl->dim == 2) {
+      if (pfl->dim == 2 && (pfl->elastic2d == 1 || pfl->elastic2d == 3)) {
         m->flex_edgeflap[2*(edge_adr+k)+0] = pfl->flaps[k].vertices[2];
         m->flex_edgeflap[2*(edge_adr+k)+1] = pfl->flaps[k].vertices[3];
       } else {

src/xml/xml_base.h

@@ -71,6 +71,7 @@ extern const mjMap datatype_map[];
 extern const mjMap meshtype_map[];
 extern const mjMap meshinertia_map[];
 extern const mjMap flexself_map[];
+extern const mjMap elastic2d_map[];
 
 
 //---------------------------------- Base XML class ------------------------------------------------

src/xml/xml_native_reader.cc

@@ -807,6 +807,15 @@ const mjMap flexself_map[5] = {
 };
 
 
+// flex elastic 2d type
+const mjMap elastic2d_map[5] = {
+  {"none",        0},
+  {"bend",        1},
+  {"stretch",     2},
+  {"both",        3},
+};
+
+
 
 //---------------------------------- class mjXReader implementation --------------------------------
 
@@ -1401,7 +1410,7 @@ void mjXReader::OneFlex(XMLElement* elem, mjsFlex* flex) {
     ReadAttr(elasticity, "poisson", 1, &flex->poisson, text);
     ReadAttr(elasticity, "thickness", 1, &flex->thickness, text);
     ReadAttr(elasticity, "damping", 1, &flex->damping, text);
-    ReadAttr(elasticity, "elastic2d", 1, &flex->elastic2d, text);
+    MapValue(elasticity, "elastic2d", &flex->elastic2d, elastic2d_map, 4);
   }
 
   // write error info
@@ -2665,7 +2674,7 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjsBody* body, const mjVFS* vfs) {
     ReadAttr(elasticity, "poisson", 1, &dflex.poisson, text);
     ReadAttr(elasticity, "damping", 1, &dflex.damping, text);
     ReadAttr(elasticity, "thickness", 1, &dflex.thickness, text);
-    ReadAttr(elasticity, "elastic2d", 1, &dflex.elastic2d, text);
+    MapValue(elasticity, "elastic2d", &dflex.elastic2d, elastic2d_map, 4);
   }
 
   // check errors

src/xml/xml_native_writer.cc

@@ -194,7 +194,7 @@ void mjXWriter::OneFlex(XMLElement* elem, const mjCFlex* flex) {
   WriteAttr(elastic, "poisson", 1, &flex->poisson, &defflex.poisson);
   WriteAttr(elastic, "thickness", 1, &flex->thickness, &defflex.thickness);
   WriteAttr(elastic, "damping", 1, &flex->damping, &defflex.damping);
-  WriteAttr(elastic, "elastic2d", 1, &flex->elastic2d, &defflex.elastic2d);
+  WriteAttrKey(elastic, "elastic2d", elastic2d_map, 2, flex->elastic2d, defflex.elastic2d);
 
   // edge subelement
   XMLElement* edge = InsertEnd(elem, "edge");

test/benchmark/engine_util_sparse_benchmark_test.cc

@@ -343,7 +343,7 @@ void ABSL_ATTRIBUTE_NOINLINE mulMatVecSparse_8(mjtNum* res,
 // ----------------------------- benchmark ------------------------------------
 
 static void BM_MatVecSparse(benchmark::State& state, int unroll) {
-  static mjModel* m = LoadModelFromPath("plugin/elasticity/flag_flex.xml");
+  static mjModel* m = LoadModelFromPath("flex/flag.xml");
   mjData* d = mj_makeData(m);
 
   // warm-up rollout to get a typical state

test/benchmark/parse_benchmark_test.cc

@@ -73,7 +73,7 @@ static void run_parse_benchmark(const std::string xml_path,
 // run_parse_benchmark).
 
 void ABSL_ATTRIBUTE_NO_TAIL_CALL BM_ParseFlagPlugin(benchmark::State& state) {
-  run_parse_benchmark(GetModelPath("plugin/elasticity/flag_flex.xml"), state);
+  run_parse_benchmark(GetModelPath("flex/flag.xml"), state);
 }
 BENCHMARK(BM_ParseFlagPlugin);
 

test/benchmark/step_benchmark_test.cc

@@ -73,7 +73,7 @@ static void run_step_benchmark(const mjModel* model, benchmark::State& state) {
 
 void ABSL_ATTRIBUTE_NO_TAIL_CALL BM_StepFlagPlugin(benchmark::State& state) {
   MujocoErrorTestGuard guard;
-  static mjModel* model = LoadModelFromPath("plugin/elasticity/flag_flex.xml");
+  static mjModel* model = LoadModelFromPath("flex/flag.xml");
   run_step_benchmark(model, state);
 }
 BENCHMARK(BM_StepFlagPlugin);

test/engine/engine_plugin_test.cc

@@ -37,7 +37,7 @@ using ::testing::DoubleNear;
 using ::testing::HasSubstr;
 using ::testing::NotNull;
 
-constexpr int kNumTruePlugins = 11;
+constexpr int kNumTruePlugins = 10;
 constexpr int kNumFakePlugins = 30;
 constexpr int kNumTestPlugins = 4;
 

test/plugin/elasticity/elasticity_test.cc

@@ -22,7 +22,6 @@
 #include <gtest/gtest.h>
 #include <mujoco/mujoco.h>
 #include "test/fixture.h"
-#include "plugin/elasticity/shell.h"
 
 namespace mujoco {
 namespace {
@@ -58,15 +57,10 @@ TEST_F(ElasticityTest, FlexCompatibility) {
 TEST_F(ElasticityTest, ElasticEnergyShell) {
   static constexpr char cantilever_xml[] = R"(
   <mujoco>
-  <extension>
-    <plugin plugin="mujoco.elasticity.shell"/>
-  </extension>
-
   <worldbody>
     <flexcomp type="grid" count="8 8 1" spacing="1 1 1"
               radius=".025" name="test" dim="2">
       <elasticity young="2" poisson="0" thickness="1"/>
-      <plugin plugin="mujoco.elasticity.shell"/>
     </flexcomp>
   </worldbody>
   </mujoco>
@@ -76,7 +70,8 @@ TEST_F(ElasticityTest, ElasticEnergyShell) {
   mjModel* m = LoadModelFromString(cantilever_xml, error, sizeof(error));
   ASSERT_THAT(m, testing::NotNull()) << error;
   mjData* d = mj_makeData(m);
-  auto* shell = reinterpret_cast<plugin::elasticity::Shell*>(d->plugin_data[0]);
+  mj_kinematics(m, d);
+  mj_flex(m, d);
 
   // check that a plane is in the kernel of the energy
   for (mjtNum scale = 1; scale < 4; scale++) {
@@ -92,8 +87,8 @@ TEST_F(ElasticityTest, ElasticEnergyShell) {
       for (int i = 0; i < 4; i++) {
         for (int j = 0; j < 4; j++) {
           for (int x = 0; x < 3; x++) {
-            mjtNum elongation1 = scale * shell->position[3*v[i]+x];
+            mjtNum elongation1 = scale * d->flexvert_xpos[3*v[i]+x];
-            mjtNum elongation2 = scale * shell->position[3*v[j]+x];
+            mjtNum elongation2 = scale * d->flexvert_xpos[3*v[j]+x];
             energy += m->flex_bending[16*e+4*i+j] * elongation1 * elongation2;
           }
         }
@@ -114,7 +109,7 @@ TEST_F(PluginTest, ElasticEnergyMembrane) {
   <worldbody>
     <flexcomp type="grid" count="8 8 1" spacing="1 1 1"
               radius=".025" name="test" dim="2">
-      <elasticity young="2" poisson="0" thickness="1" elastic2d="2"/>
+      <elasticity young="2" poisson="0" thickness="1" elastic2d="stretch"/>
       <edge equality="false"/>
     </flexcomp>
   </worldbody>