Copy texture coordinates from OBJ into flex. Fixes #2464.

PiperOrigin-RevId: 732109960
Change-Id: I506b3540ab5d489ce8f10d0c473919611ccb654a

src/user/user_mesh.cc

@@ -3184,6 +3184,7 @@ void mjCFlex::PointToLocal() {
   spec.vert = &spec_vert_;
   spec.node = &spec_node_;
   spec.texcoord = &spec_texcoord_;
+  spec.facetexcoord = &spec_facetexcoord_;
   spec.elem = &spec_elem_;
   spec.info = &info;
   material = nullptr;
@@ -3192,6 +3193,7 @@ void mjCFlex::PointToLocal() {
   vert = nullptr;
   node = nullptr;
   texcoord = nullptr;
+  facetexcoord = nullptr;
   elem = nullptr;
 }
 
@@ -3217,6 +3219,7 @@ void mjCFlex::CopyFromSpec() {
   vert_ = spec_vert_;
   node_ = spec_node_;
   texcoord_ = spec_texcoord_;
+  facetexcoord_ = spec_facetexcoord_;
   elem_ = spec_elem_;
 
   // clear precompiled asset. TODO: use asset cache
@@ -3320,10 +3323,16 @@ void mjCFlex::Compile(const mjVFS* vfs) {
   }
 
   // check texcoord
-  if (!texcoord_.empty() && texcoord_.size()!=2*nvert) {
+  if (!texcoord_.empty() && texcoord_.size()!=2*nvert && facetexcoord_.empty()) {
     throw mjCError(this, "two texture coordinates per vertex expected");
   }
 
+  // no facetexcoord: copy from faces
+  if (facetexcoord_.empty() && !texcoord_.empty()) {
+    facetexcoord_.assign(3*nelem, 0);
+    memcpy(facetexcoord_.data(), elem_.data(), 3*nelem*sizeof(int));
+  }
+
   // resolve material name
   mjCBase* pmat = model->FindObject(mjOBJ_MATERIAL, material_);
   if (pmat) {