USD export: convert active UV map to st.

Updated the logic of the convert_uv_to_st option to rename the
active UV set to 'st', to allow specifying the default UV set
when there are multiple UVs.  Previously, this option assumed
a single UV set.

source/blender/editors/io/io_usd.c

@@ -698,8 +698,7 @@ void WM_OT_usd_export(struct wmOperatorType *ot)
       "convert_uv_to_st",
       false,
       "Convert uv to st",
-      "When checked, the USD exporter will convert all uv map names to interchangeable 'st'"
-      "(Assumes one uv layout per mesh)");
+      "Export the active uv map as USD primvar named 'st'");
 
   RNA_def_boolean(ot->srna,
                   "convert_orientation",

source/blender/io/usd/intern/usd_writer_abstract.cc

@@ -236,6 +236,9 @@ pxr::UsdShadeMaterial USDAbstractWriter::ensure_usd_material(const HierarchyCont
   }
   if (material->use_nodes && this->usd_export_context_.export_params.generate_preview_surface) {
     std::string active_uv = get_mesh_active_uvlayer_name(context.object);
+    if (usd_export_context_.export_params.convert_uv_to_st && !active_uv.empty()) {
+      active_uv = "st";
+    }
     create_usd_preview_surface_material(
         this->usd_export_context_, material, usd_material, active_uv);
   }

source/blender/io/usd/intern/usd_writer_mesh.cc

@@ -193,10 +193,18 @@ struct USDMeshData {
 void USDGenericMeshWriter::write_custom_data(const Mesh *mesh, pxr::UsdGeomMesh usd_mesh)
 {
   const CustomData *ldata = &mesh->ldata;
+
+  /* Index of the UV layer to be renamed "st", set to the active UV layer index if
+   * the convert_uv_to_st option is enabled and set to -1 otherwise. */
+  const int st_layer_idx = usd_export_context_.export_params.convert_uv_to_st ?
+                               CustomData_get_active_layer_index(ldata, CD_MLOOPUV) :
+                               -1;
+
   for (int layer_idx = 0; layer_idx < ldata->totlayer; layer_idx++) {
     const CustomDataLayer *layer = &ldata->layers[layer_idx];
     if (layer->type == CD_MLOOPUV && usd_export_context_.export_params.export_uvmaps) {
-      write_uv_maps(mesh, usd_mesh, layer);
+      const char *name_override = st_layer_idx == layer_idx ? "st" : nullptr;
+      write_uv_maps(mesh, usd_mesh, layer, name_override);
     }
     else if (layer->type == CD_MLOOPCOL &&
              usd_export_context_.export_params.export_vertex_colors) {
@@ -207,7 +215,8 @@ void USDGenericMeshWriter::write_custom_data(const Mesh *mesh, pxr::UsdGeomMesh
 
 void USDGenericMeshWriter::write_uv_maps(const Mesh *mesh,
                                          pxr::UsdGeomMesh usd_mesh,
-                                         const CustomDataLayer *layer)
+                                         const CustomDataLayer *layer,
+                                         const char *name_override)
 {
   pxr::UsdTimeCode timecode = get_export_time_code();
 
@@ -215,7 +224,8 @@ void USDGenericMeshWriter::write_uv_maps(const Mesh *mesh,
    * The primvar name is the same as the UV Map name. This is to allow the standard name "st"
    * for texture coordinates by naming the UV Map as such, without having to guess which UV Map
    * is the "standard" one. */
-  pxr::TfToken primvar_name(pxr::TfMakeValidIdentifier(layer->name));
+  pxr::TfToken primvar_name(name_override ? name_override :
+                                            pxr::TfMakeValidIdentifier(layer->name));
 
   if (usd_export_context_.export_params.author_blender_name) {
     // Store original layer name in blender
@@ -228,9 +238,6 @@ void USDGenericMeshWriter::write_uv_maps(const Mesh *mesh,
         .Set(std::string(layer->name), pxr::UsdTimeCode::Default());
   }
 
-  if (usd_export_context_.export_params.convert_uv_to_st)
-    primvar_name = pxr::TfToken("st");
-
   pxr::UsdGeomPrimvar uv_coords_primvar = usd_mesh.CreatePrimvar(
       primvar_name, pxr::SdfValueTypeNames->TexCoord2fArray, pxr::UsdGeomTokens->faceVarying);
 

source/blender/io/usd/intern/usd_writer_mesh.h

@@ -34,7 +34,9 @@ class USDGenericMeshWriter : public USDAbstractWriter {
                         pxr::UsdGeomMesh usd_mesh,
                         const MaterialFaceGroups &usd_face_groups);
   void write_custom_data(const Mesh *mesh, pxr::UsdGeomMesh usd_mesh);
-  void write_uv_maps(const Mesh *mesh, pxr::UsdGeomMesh usd_mesh, const CustomDataLayer *layer);
+  void write_uv_maps(const Mesh *mesh, pxr::UsdGeomMesh usd_mesh,
+                     const CustomDataLayer *layer,
+                     const char *name_override = nullptr);
   void write_vertex_colors(const Mesh *mesh,
                            pxr::UsdGeomMesh usd_mesh,
                            const CustomDataLayer *layer);