Auto-generate 3D mesh from convex OBJ if dim=3 in flex.

This requires the user to explicitly specify an origin.

PiperOrigin-RevId: 712483526
Change-Id: Ie316a5eccb676bb918d4bb746795019145c10f5b

src/xml/xml_native_reader.cc

@@ -311,10 +311,10 @@ const char* MJCF[nMJCF][mjXATTRNUM] = {
               {"config", "*", "2", "key", "value"},
             {">"},
         {">"},
-        {"flexcomp", "*", "24", "name", "type", "group", "dim",
+        {"flexcomp", "*", "25", "name", "type", "group", "dim",
             "count", "spacing", "radius", "rigid", "mass", "inertiabox",
             "scale", "file", "point", "element", "texcoord", "material", "rgba",
-            "flatskin", "pos", "quat", "axisangle", "xyaxes", "zaxis", "euler"},
+            "flatskin", "pos", "quat", "axisangle", "xyaxes", "zaxis", "euler", "origin"},
         {"<"},
             {"edge", "?", "5", "equality", "solref", "solimp", "stiffness", "damping"},
             {"elasticity", "?", "4", "young", "poisson", "damping", "thickness"},
@@ -2643,6 +2643,10 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjsBody* body, const mjVFS* vfs) {
   ReadAttrInt(elem, "dim", &dflex.dim);
   ReadAttr(elem, "radius", 1, &dflex.radius, text);
   ReadAttrInt(elem, "group", &dflex.group);
+  if (!ReadAttr(elem, "origin", 3, fcomp.origin, text) &&
+      fcomp.type == mjFCOMPTYPE_MESH && dflex.dim == 3) {
+    throw mjXError(elem, "origin must be specified for mesh flexcomps if dim=3");
+  }
 
   // pose
   ReadAttr(elem, "pos", 3, fcomp.pos, text);