Add support for loading 1D flex components from OBJ line segments.

The obj_decoder plugin is updated to parse line segments ('l') from OBJ files and represent them as degenerate triangles. The user_flexcomp compiler now supports flex dimensions of 1, using these parsed line segments to define the 1D flex elements. The compile sample's error reporting is also improved.

PiperOrigin-RevId: 928566870
Change-Id: I6920729d1b1d5aeb391401c3245ebb1d4784c190

doc/changelog.rst

@@ -16,6 +16,7 @@ General
 - Added :ref:`mjs_makeFlex`, a new C API function equivalent to the :ref:`flexcomp<body-flexcomp>` element for
   programmatically creating flex objects with auto-generated bodies, joints, and equality constraints. Exposed as
   ``body.make_flex()`` in Python.
+- Added support for loading 1D flex components from OBJ line segments
 - Significantly improved the quality of coarse convex hulls produced by the :ref:`maxhullvert<asset-mesh-maxhullvert>`
   attribute by invoking Qhull's `Q9 <http://www.qhull.org/html/qh-optq.htm#Q9>`__ option.
 

plugin/obj_decoder/obj_decoder.cc

@@ -53,41 +53,80 @@ mjSpec* Decode(mjResource* resource, const mjVFS* vfs) {
   std::vector<int> userfacetexcoord;
 
   if (!obj_reader.GetShapes().empty()) {
-    const auto& obj_mesh = obj_reader.GetShapes()[0].mesh;
+    const auto& obj_shape = obj_reader.GetShapes()[0];
+    if (!obj_shape.mesh.indices.empty()) {
+      const auto& obj_mesh = obj_shape.mesh;
 
-    std::vector<tinyobj::index_t> face_indices;
-    for (size_t face = 0, idx = 0; idx < obj_mesh.indices.size();) {
-      int nfacevert = obj_mesh.num_face_vertices[face];
-      if (nfacevert < 3 || nfacevert > 4) {
-        mju_warning(
-            "obj_decoder: only tri or quad meshes are supported (file '%s')",
-            resource->name);
-        mj_deleteSpec(spec);
-        return nullptr;
+      size_t num_face_indices = 0;
+      for (size_t face = 0; face < obj_mesh.num_face_vertices.size(); ++face) {
+        int nfacevert = obj_mesh.num_face_vertices[face];
+        if (nfacevert == 3) {
+          num_face_indices += 3;
+        } else if (nfacevert == 4) {
+          num_face_indices += 6;
+        }
       }
 
-      face_indices.push_back(obj_mesh.indices[idx]);
-      face_indices.push_back(obj_mesh.indices[idx + 1]);
-      face_indices.push_back(obj_mesh.indices[idx + 2]);
-
-      if (nfacevert == 4) {
-        face_indices.push_back(obj_mesh.indices[idx]);
-        face_indices.push_back(obj_mesh.indices[idx + 2]);
-        face_indices.push_back(obj_mesh.indices[idx + 3]);
-      }
-      idx += nfacevert;
-      ++face;
-    }
-
-    for (const auto& mesh_index : face_indices) {
-      userface.push_back(mesh_index.vertex_index);
-
+      std::vector<tinyobj::index_t> face_indices;
+      face_indices.reserve(num_face_indices);
+      userface.reserve(num_face_indices);
       if (!usernormal.empty()) {
-        userfacenormal.push_back(mesh_index.normal_index);
+        userfacenormal.reserve(num_face_indices);
+      }
+      if (!usertexcoord.empty()) {
+        userfacetexcoord.reserve(num_face_indices);
       }
 
-      if (!usertexcoord.empty()) {
-        userfacetexcoord.push_back(mesh_index.texcoord_index);
+      for (size_t face = 0, idx = 0; idx < obj_mesh.indices.size();) {
+        int nfacevert = obj_mesh.num_face_vertices[face];
+        if (nfacevert < 3 || nfacevert > 4) {
+          mju_warning(
+              "obj_decoder: only tri or quad meshes are supported (file '%s')",
+              resource->name);
+          mj_deleteSpec(spec);
+          return nullptr;
+        }
+
+        face_indices.push_back(obj_mesh.indices[idx]);
+        face_indices.push_back(obj_mesh.indices[idx + 1]);
+        face_indices.push_back(obj_mesh.indices[idx + 2]);
+
+        if (nfacevert == 4) {
+          face_indices.push_back(obj_mesh.indices[idx]);
+          face_indices.push_back(obj_mesh.indices[idx + 2]);
+          face_indices.push_back(obj_mesh.indices[idx + 3]);
+        }
+        idx += nfacevert;
+        ++face;
+      }
+
+      for (const auto& mesh_index : face_indices) {
+        userface.push_back(mesh_index.vertex_index);
+
+        if (!usernormal.empty()) {
+          userfacenormal.push_back(mesh_index.normal_index);
+        }
+
+        if (!usertexcoord.empty()) {
+          userfacetexcoord.push_back(mesh_index.texcoord_index);
+        }
+      }
+    } else if (!obj_shape.lines.indices.empty()) {
+      // encode line segments as degenerate triangles (i1, i2, i2) in userface,
+      // since mjsMesh has no native edge field; decoded back by flexcomp
+      const auto& obj_lines = obj_shape.lines;
+      userface.reserve(3 * (obj_lines.indices.size() - obj_lines.num_line_vertices.size()));
+      size_t idx = 0;
+      for (size_t l = 0; l < obj_lines.num_line_vertices.size(); l++) {
+        int nlinevert = obj_lines.num_line_vertices[l];
+        for (int v = 0; v < nlinevert - 1; v++) {
+          int i1 = obj_lines.indices[idx + v].vertex_index;
+          int i2 = obj_lines.indices[idx + v + 1].vertex_index;
+          userface.push_back(i1);
+          userface.push_back(i2);
+          userface.push_back(i2);
+        }
+        idx += nlinevert;
       }
     }
   }
@@ -101,8 +140,10 @@ mjSpec* Decode(mjResource* resource, const mjVFS* vfs) {
   mjs_setFloat(mesh->usernormal, usernormal.data(), usernormal.size());
   mjs_setFloat(mesh->usertexcoord, usertexcoord.data(), usertexcoord.size());
   mjs_setInt(mesh->userface, userface.data(), userface.size());
-  mjs_setInt(mesh->userfacenormal, userfacenormal.data(), userfacenormal.size());
-  mjs_setInt(mesh->userfacetexcoord, userfacetexcoord.data(), userfacetexcoord.size());
+  mjs_setInt(mesh->userfacenormal, userfacenormal.data(),
+             userfacenormal.size());
+  mjs_setInt(mesh->userfacetexcoord, userfacetexcoord.data(),
+             userfacetexcoord.size());
 
   return spec;
 }

src/user/user_flexcomp.cc

@@ -1334,8 +1334,8 @@ bool mjCFlexcomp::MakeMesh(mjCModel* model, mjsCompiler* compiler, char* error,
   }
 
   // check dim
-  if (def.spec.flex->dim < 2) {
-    return comperr(error, "Flex dim must be at least 2 for mesh", error_sz);
+  if (def.spec.flex->dim < 1) {
+    return comperr(error, "Flex dim must be at least 1 for mesh", error_sz);
   }
 
   // load resource
@@ -1381,6 +1381,15 @@ bool mjCFlexcomp::MakeMesh(mjCModel* model, mjsCompiler* compiler, char* error,
   // copy faces or create 3D mesh
   if (def.spec.flex->dim == 2) {
     element = mesh.Face();
+  } else if (def.spec.flex->dim == 1) {
+    // extract edge pairs from degenerate triangles (i1, i2, i2)
+    const std::vector<int>& face = mesh.Face();
+    element.clear();
+    element.reserve(face.size() * 2 / 3);
+    for (size_t i = 0; i < face.size(); i += 3) {
+      element.push_back(face[i]);
+      element.push_back(face[i+1]);
+    }
   } else {
     point.insert(point.begin() + 0, origin[0]);
     point.insert(point.begin() + 1, origin[1]);

test/user/testdata/flex_line_obj.xml

@@ -0,0 +1,5 @@
+<mujoco>
+  <worldbody>
+    <flexcomp name="line" type="mesh" file="line.obj" dim="1" radius="0.1"/>
+  </worldbody>
+</mujoco>

test/user/testdata/line.obj

@@ -0,0 +1,6 @@
+v 0 0 0
+v 2 0 0
+v 1 0 0
+v 3 0 0
+l 1 3 2
+l 2 4

test/user/user_flex_test.cc

@@ -1399,6 +1399,18 @@ TEST_F(UserFlexTest, Vert0RotationInvariant) {
   mj_deleteModel(m2);
 }
 
+TEST_F(UserFlexTest, Load1DFlexFromOBJ) {
+  const std::string xml_path =
+      GetTestDataFilePath("user/testdata/flex_line_obj.xml");
+  std::array<char, 1024> error;
+  mjModel* m = mj_loadXML(xml_path.c_str(), 0, error.data(), error.size());
+  ASSERT_THAT(m, NotNull()) << error.data();
+  EXPECT_EQ(m->nflexvert, 4);
+  EXPECT_EQ(m->nflexelem, 3);
+  EXPECT_EQ(m->flex_dim[0], 1);
+  mj_deleteModel(m);
+}
+
 }  // namespace
 }  // namespace mujoco