Add mjModel.tendon_armature (in preparation, not yet implemented)

PiperOrigin-RevId: 743649968
Change-Id: I1d893e3e06afc48b652575bc7837750ab3573670

src/user/user_model.cc

@@ -3390,6 +3390,7 @@ void mjCModel::CopyObjects(mjModel* m) {
     m->tendon_margin[i] = (mjtNum)pte->margin;
     m->tendon_stiffness[i] = (mjtNum)pte->stiffness;
     m->tendon_damping[i] = (mjtNum)pte->damping;
+    m->tendon_armature[i] = (mjtNum)pte->armature;
     m->tendon_frictionloss[i] = (mjtNum)pte->frictionloss;
     m->tendon_lengthspring[2*i] = (mjtNum)pte->springlength[0];
     m->tendon_lengthspring[2*i+1] = (mjtNum)pte->springlength[1];
@@ -4931,6 +4932,7 @@ bool mjCModel::CopyBack(const mjModel* m) {
     tendons_[i]->margin = (double)m->tendon_margin[i];
     tendons_[i]->stiffness = (double)m->tendon_stiffness[i];
     tendons_[i]->damping = (double)m->tendon_damping[i];
+    tendons_[i]->armature = (double)m->tendon_armature[i];
     tendons_[i]->frictionloss = (double)m->tendon_frictionloss[i];
 
     if (nuser_tendon) {

src/user/user_objects.cc

@@ -5616,6 +5616,12 @@ void mjCTendon::Compile(void) {
 
     // spatial path
     else {
+      if (armature < 0) {
+        throw mjCError(this,
+                       "tendon '%s' (id = %d): tendon armature cannot be negative",
+                        name.c_str(), id);
+      }
+
       switch (path[i]->type) {
         case mjWRAP_PULLEY:
           // pulley should not follow other pulley
@@ -5657,6 +5663,12 @@ void mjCTendon::Compile(void) {
                            name.c_str(), id, i);
           }
 
+          if (armature > 0) {
+            throw mjCError(this,
+                           "tendon '%s' (id = %d): geom wrapping not supported by tendon armature",
+                           name.c_str(), id);
+          }
+
           // mark geoms as non visual
           model->Geoms()[path[i]->obj->id]->SetNotVisual();
           break;

src/xml/xml_native_reader.cc

@@ -363,18 +363,18 @@ const char* MJCF[nMJCF][mjXATTRNUM] = {
 
     {"tendon", "*", "0"},
     {"<"},
-        {"spatial", "*", "18", "name", "class", "group", "limited", "range",
+        {"spatial", "*", "19", "name", "class", "group", "limited", "range",
             "solreflimit", "solimplimit", "solreffriction", "solimpfriction",
             "frictionloss", "springlength", "width", "material",
-            "margin", "stiffness", "damping", "rgba", "user"},
+            "margin", "stiffness", "damping", "armature", "rgba", "user"},
         {"<"},
             {"site", "*", "1", "site"},
             {"geom", "*", "2", "geom", "sidesite"},
             {"pulley", "*", "1", "divisor"},
         {">"},
-        {"fixed", "*", "15", "name", "class", "group", "limited", "range",
+        {"fixed", "*", "16", "name", "class", "group", "limited", "range",
             "solreflimit", "solimplimit", "solreffriction", "solimpfriction",
-            "frictionloss", "springlength", "margin", "stiffness", "damping", "user"},
+            "frictionloss", "springlength", "margin", "stiffness", "damping", "armature", "user"},
         {"<"},
             {"joint", "*", "2", "joint", "coef"},
         {">"},
@@ -2061,6 +2061,7 @@ void mjXReader::OneTendon(XMLElement* elem, mjsTendon* tendon) {
   ReadAttr(elem, "margin", 1, &tendon->margin, text);
   ReadAttr(elem, "stiffness", 1, &tendon->stiffness, text);
   ReadAttr(elem, "damping", 1, &tendon->damping, text);
+  ReadAttr(elem, "armature", 1, &tendon->armature, text);
   ReadAttr(elem, "frictionloss", 1, &tendon->frictionloss, text);
   // read springlength, either one or two values; if one, copy to second value
   if (ReadAttr(elem, "springlength", 2, tendon->springlength, text, false, false) == 1) {
@@ -3800,7 +3801,7 @@ void mjXReader::Tendon(XMLElement* section) {
       def = mjs_getSpecDefault(spec);
     }
 
-    // create equality constraint and parse
+    // create tendon and parse
     mjsTendon* tendon = mjs_addTendon(spec, def);
     OneTendon(elem, tendon);
 

src/xml/xml_native_writer.cc

@@ -729,6 +729,7 @@ void mjXWriter::OneTendon(XMLElement* elem, const mjCTendon* tendon, mjCDef* def
   WriteAttr(elem, "margin", 1, &tendon->margin, &def->Tendon().margin);
   WriteAttr(elem, "stiffness", 1, &tendon->stiffness, &def->Tendon().stiffness);
   WriteAttr(elem, "damping", 1, &tendon->damping, &def->Tendon().damping);
+  WriteAttr(elem, "armature", 1, &tendon->armature, &def->Tendon().armature);
   WriteAttr(elem, "frictionloss", 1, &tendon->frictionloss, &def->Tendon().frictionloss);
   if (tendon->springlength[0] != tendon->springlength[1] ||
       def->Tendon().springlength[0] != def->Tendon().springlength[1]) {