Introduce trilinear flex parametrization.

These flexes use only 24 DOFs (3 per vertex of the bounding box), while colliding with the full high resolution mesh.

On an 8x8x8 cube, the performance using DOFs at all vertices is

```
 Simulation time      : 18.74 s
 Steps per second     : 533
 Realtime factor      : 0.53 x
 Time per step        : 1874.4 µs

 Contacts per step    : 114.88
 Constraints per step : 3322.51
 Degrees of freedom   : 1536
```

With the new implementation, it is the following:

```
 Simulation time      : 1.82 s
 Steps per second     : 5507
 Realtime factor      : 5.51 x
 Time per step        : 181.6 µs

 Contacts per step    : 38.84
 Constraints per step : 155.36
 Degrees of freedom   : 24
```

PiperOrigin-RevId: 721008829
Change-Id: I833df027527db578d86667cc4b24295bcf6f7d22

src/xml/xml_native_reader.cc

@@ -311,7 +311,7 @@ const char* MJCF[nMJCF][mjXATTRNUM] = {
               {"config", "*", "2", "key", "value"},
             {">"},
         {">"},
-        {"flexcomp", "*", "25", "name", "type", "group", "dim",
+        {"flexcomp", "*", "26", "name", "type", "group", "dim", "dof",
             "count", "spacing", "radius", "rigid", "mass", "inertiabox",
             "scale", "file", "point", "element", "texcoord", "material", "rgba",
             "flatskin", "pos", "quat", "axisangle", "xyaxes", "zaxis", "euler", "origin"},
@@ -331,8 +331,8 @@ const char* MJCF[nMJCF][mjXATTRNUM] = {
 
     {"deformable", "*", "0"},
     {"<"},
-        {"flex", "*", "11", "name", "group", "dim", "radius", "material",
-            "rgba", "flatskin", "body", "vertex", "element", "texcoord"},
+        {"flex", "*", "12", "name", "group", "dim", "radius", "material",
+            "rgba", "flatskin", "body", "vertex", "element", "texcoord", "node"},
         {"<"},
             {"contact", "?", "13", "contype", "conaffinity", "condim", "priority",
                 "friction", "solmix", "solref", "solimp", "margin", "gap",
@@ -803,6 +803,14 @@ const mjMap fcomp_map[mjNFCOMPTYPES] = {
 };
 
 
+// flexcomp dof type
+const mjMap fdof_map[mjNFCOMPDOFS] = {
+  {"full",        mjFCOMPDOF_FULL},
+  {"radial",      mjFCOMPDOF_RADIAL},
+  {"trilinear",   mjFCOMPDOF_TRILINEAR}
+};
+
+
 // flex selfcollide type
 const mjMap flexself_map[5] = {
   {"none",        mjFLEXSELF_NONE},
@@ -1324,7 +1332,7 @@ void mjXReader::Statistic(XMLElement* section) {
 
 // flex element parser
 void mjXReader::OneFlex(XMLElement* elem, mjsFlex* flex) {
-  string text, name, material;
+  string text, name, material, nodebody;
   int n;
 
   // read attributes
@@ -1347,6 +1355,9 @@ void mjXReader::OneFlex(XMLElement* elem, mjsFlex* flex) {
   if (ReadAttrTxt(elem, "body", text, true)) {
     mjs_setStringVec(flex->vertbody, text.c_str());
   }
+  if (ReadAttrTxt(elem, "node", nodebody)) {
+    mjs_setStringVec(flex->nodebody, nodebody.c_str());
+  }
   auto vert = ReadAttrVec<double>(elem, "vertex");
   if (vert.has_value()) {
     mjs_setDouble(flex->vert, vert->data(), vert->size());
@@ -2678,6 +2689,11 @@ void mjXReader::OneFlexcomp(XMLElement* elem, mjsBody* body, const mjVFS* vfs) {
     fcomp.texcoord = std::move(texcoord.value());
   }
 
+  // dof type
+  if (MapValue(elem, "dof", &n, fdof_map, mjNFCOMPDOFS)) {
+    fcomp.doftype = (mjtDof)n;
+  }
+
   // edge
   XMLElement* edge = FirstChildElement(elem, "edge");
   if (edge) {