Remove the Shell plugin and integrate it into the engine.

PiperOrigin-RevId: 760688502
Change-Id: Ia70988d42b7edf571d7cb4a4f48f8fc50b51667d

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"/>
-        <plugin plugin="mujoco.elasticity.shell"/>
+        <elasticity poisson="0" thickness="1e-2" young="3e6" elastic2d="none"/>
       </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"/>
-      <plugin plugin="mujoco.elasticity.shell"/>
+      <elasticity poisson="0" thickness="1e-2" young="3e7" elastic2d="bend"/>
     </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>