Clean-up flex assumptions.

Do not allow a mix of `elastic2d != none` with `dof = trilinear` since the latter assumes 3d elasticity.

Also, do not assume that `flex_interp > 0` in the engine. This will enable to use, e.g., `flex_interp = -1` to mean a linear surface finite element instead of a 3d finite element which is currently identified with `flex_interp = 1`.

PiperOrigin-RevId: 903852035
Change-Id: Ia6290b4a05e9e510ffb7f36d141cd525b40d3110

doc/XMLreference.rst

@@ -4328,7 +4328,7 @@ stress-strain relationship. See also :ref:`deformable <CDeformable>` objects and
 
 :at:`elastic2d`: :at-val:`[none, bend, stretch, both], "none"`
    Elastic contribution to passive forces of 2D flexes. "none": none, "bend": bending only, "stretch": stretching only,
-   "both": bending and stretching.
+   "both": bending and stretching. Not yet supported by :ref:`dof<body-flexcomp-dof>` **trilinear** and **quadratic**.
 
 .. _flex-contact:
 

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" elastic2d="stretch"/>
+      <elasticity young="1e3" poisson="0.1" damping="0.001" elastic2d="none"/>
       <contact selfcollide="none" internal="false"/>
     </flexcomp>
   </worldbody>

model/flex/bunny_multicell.xml

@@ -31,7 +31,7 @@
     </body>
     <flexcomp type="mesh" file="bunny.obj" pos="0 0 0" dim="2" euler="90 0 0" cellcount="3 3 3"
               radius=".001" rgba="0 .7 .7 1" mass=".05" name="softbody" dof="trilinear">
-      <elasticity young="1e3" poisson="0.1" damping="0.01" elastic2d="stretch"/>
+      <elasticity young="1e3" poisson="0.1" damping="0.01" elastic2d="none"/>
       <contact selfcollide="none" internal="false"/>
     </flexcomp>
   </worldbody>

model/flex/bunny_quadratic.xml

@@ -31,7 +31,7 @@
     </body>
     <flexcomp type="mesh" file="bunny.obj" pos="0 0 -.01" dim="2" euler="90 0 0"
               radius=".002" rgba="0 .7 .7 1" mass=".05" name="softbody" dof="quadratic">
-      <elasticity young="1e3" poisson="0.1" damping="0.0001" elastic2d="stretch"/>
+      <elasticity young="1e3" poisson="0.1" damping="0.0001" elastic2d="none"/>
       <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" elastic2d="stretch"/>
+      <elasticity young="1e3" poisson="0.1" damping="0.001" elastic2d="none"/>
       <contact selfcollide="none" internal="false"/>
     </flexcomp>
   </worldbody>

src/engine/engine_core_constraint.c

@@ -275,6 +275,7 @@ static int mj_vertBodyWeight(const mjModel* m, const mjData* d, int f, int* v,
   }
 
   int order = m->flex_interp[f];
+  order = order < 0 ? -order : order;
   int npc = (order+1)*(order+1)*(order+1);  // number of nodes per cell
 
   // cell lookup: get local coords and node indices
@@ -709,6 +710,7 @@ void mj_instantiateEquality(const mjModel* m, mjData* d) {
       int f = id[0];
       int nodenum = m->flex_nodenum[f];
       int order = m->flex_interp[f];
+      order = order < 0 ? -order : order;
 
       // skip if not interpolated (order == 0 or no nodes)
       if (!order || !nodenum) {
@@ -1676,6 +1678,7 @@ void mj_diagApprox(const mjModel* m, mjData* d) {
         int flex_id = m->eq_obj1id[id];
         int nstart = m->flex_nodeadr[flex_id];
         int order = m->flex_interp[flex_id];
+        order = order < 0 ? -order : order;
         int npc = (order+1)*(order+1)*(order+1);
 
         // per-cell constraint count
@@ -2296,6 +2299,7 @@ static int mj_ne(const mjModel* m, mjData* d, int* nnz) {
       // per-cell strain constraints: each equality is one cell
       int f = id[0];
       int order = m->flex_interp[f];
+      order = order < 0 ? -order : order;
       if (!order || !m->flex_nodenum[f]) {
         break;
       }

src/engine/engine_core_smooth.c

@@ -595,7 +595,8 @@ void mj_flex(const mjModel* m, mjData* d) {
         }
       }
 
-      int order = m->flex_interp[f];
+      int interp = m->flex_interp[f];
+      int order = interp < 0 ? -interp : interp;
       int cx = m->flex_cellnum[3*f+0];
       int cy = m->flex_cellnum[3*f+1];
       int cz = m->flex_cellnum[3*f+2];
@@ -2624,7 +2625,8 @@ void mj_rnePostConstraint(const mjModel* m, mjData* d) {
     case mjEQ_FLEXSTRAIN: {
       // increment: trilinear uses 2 center (I1,J-1) + 3*ngauss shear, quadratic uses 6*ngauss
       k = m->eq_obj1id[id];
-      int order = m->flex_interp[k];
+      int interp_k = m->flex_interp[k];
+      int order = interp_k < 0 ? -interp_k : interp_k;
       int nodenum = m->flex_nodenum[k];
       if (order && nodenum) {
         int nquad = order + 1;

src/engine/engine_derivative.c

@@ -887,6 +887,7 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
     if (!m->flex_interp[f]) continue;
     if (m->flex_rigid[f]) continue;
     int order = m->flex_interp[f];
+    order = order < 0 ? -order : order;
     int npc = (order+1)*(order+1)*(order+1);
     if (npc > max_npc) max_npc = npc;
     if (m->flex_nodenum[f] > max_nodenum) max_nodenum = m->flex_nodenum[f];
@@ -966,6 +967,7 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
     }
 
     int order = m->flex_interp[f];
+    order = order < 0 ? -order : order;
     int npc = (order+1)*(order+1)*(order+1);
     int cx = m->flex_cellnum[3*f+0];
     int cy = m->flex_cellnum[3*f+1];

src/engine/engine_passive.c

@@ -236,6 +236,7 @@ static void mj_springdamper(const mjModel* m, mjData* d) {
 
     if (m->flex_interp[f]) {
       int order = m->flex_interp[f];
+      order = order < 0 ? -order : order;
       int npc = (order+1)*(order+1)*(order+1);  // nodes per cell
       int cx = m->flex_cellnum[3*f+0];
       int cy = m->flex_cellnum[3*f+1];

src/engine/engine_setconst.c

@@ -714,6 +714,7 @@ static void makeFlexBandwidth(mjModel* m, mjData* d) {
     for (int f = 0; f < m->nflex; f++) {
       if (!m->flex_interp[f]) continue;
       int order = m->flex_interp[f];
+      order = order < 0 ? -order : order;
       int nodeadr = m->flex_nodeadr[f];
       int nodenum = m->flex_nodenum[f];
       int cx = m->flex_cellnum[3*f+0];

src/engine/engine_vis_interact.c

@@ -864,6 +864,7 @@ int mjv_select(const mjModel* m, const mjData* d, const mjvOption* vopt,
         if (m->flex_interp[i]) {
           mjtNum* coord = m->flex_vert0 + 3*(m->flex_vertadr[i] + vertid);
           int order = m->flex_interp[i];
+          order = order < 0 ? -order : order;
           int npc = (order+1)*(order+1)*(order+1);
 
           // cell lookup: get local coords and node indices

src/engine/engine_vis_visualize.c

@@ -1452,6 +1452,7 @@ static void addFlexBvhGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
     int cy = m->flex_cellnum[3*f+1];
     int cz = m->flex_cellnum[3*f+2];
     int order = m->flex_interp[f];
+    order = order < 0 ? -order : order;
     int NX = cx * order + 1;
     int NY = cy * order + 1;
     int NZ = cz * order + 1;

src/user/user_mesh.cc

@@ -4100,6 +4100,19 @@ void mjCFlex::Compile(const mjVFS* vfs) {
   }
   nelem = (int)elem_.size()/(dim+1);
 
+  // elastic2d checks
+  if (elastic2d) {
+    if (thickness <= 0) {
+      throw mjCError(this, "2d elasticity requires positive thickness");
+    }
+    if (interpolated) {
+      throw mjCError(this, "interpolated flex does not yet support 2d elasticity");
+    }
+    if (dim != 2 && !interpolated) {
+      throw mjCError(this, "2d elasticity requires 2d flex");
+    }
+  }
+
   // set nvert, rigid, centered; check size
   if (vert_.empty()) {
     centered = true;
@@ -4342,9 +4355,6 @@ void mjCFlex::Compile(const mjVFS* vfs) {
 
     // bending stiffness (2D only)
     if (dim == 2 && (elastic2d == 1 || elastic2d == 3)) {
-      if (thickness < 0) {
-        throw mjCError(this, "thickness must be positive for bending stiffness");
-      }
       bending.assign(nedge*17, 0);
 
       for (unsigned int e = 0; e < nedge; e++) {

test/user/user_mesh_test.cc

@@ -819,6 +819,58 @@ TEST_F(MjCMeshTest, VolumeSmallAllowedShell) {
   mj_deleteModel(model);
 }
 
+TEST_F(MjCMeshTest, Flex2DElasticityRequiresPositiveThickness) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <worldbody>
+      <flexcomp name="f" type="grid" count="3 3 1" spacing="1 1 1" dim="2" dof="2d">
+        <elasticity young="1" thickness="0" elastic2d="bend"/>
+      </flexcomp>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* model = LoadModelFromString(xml, error.data(), error.size());
+  EXPECT_THAT(model, testing::IsNull());
+  EXPECT_THAT(error.data(),
+              HasSubstr("2d elasticity requires positive thickness"));
+}
+
+TEST_F(MjCMeshTest, InterpolatedFlexDoesNotSupport2DElasticity) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <worldbody>
+      <flexcomp name="f" type="grid" count="3 3 1" spacing="1 1 1" dim="2" dof="trilinear">
+        <contact selfcollide="none"/>
+        <elasticity young="1" thickness="1" elastic2d="bend"/>
+      </flexcomp>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* model = LoadModelFromString(xml, error.data(), error.size());
+  EXPECT_THAT(model, testing::IsNull());
+  EXPECT_THAT(
+      error.data(),
+      HasSubstr("interpolated flex does not yet support 2d elasticity"));
+}
+
+TEST_F(MjCMeshTest, Flex2DElasticityRequires2DFlex) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <worldbody>
+      <flexcomp name="f" type="grid" count="3 3 3" spacing="1 1 1" dim="3" dof="2d">
+        <elasticity young="1" thickness="1" elastic2d="bend"/>
+      </flexcomp>
+    </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* model = LoadModelFromString(xml, error.data(), error.size());
+  EXPECT_THAT(model, testing::IsNull());
+  EXPECT_THAT(error.data(), HasSubstr("2d elasticity requires 2d flex"));
+}
+
 TEST_F(MjCMeshTest, VolumeNegativeThrowsError) {
   static constexpr char xml[] = R"(
   <mujoco>