USD Preview Surface import as a fallback.

Added logic to fall back on importing existing
USD Preview Surface shaders if importing MDL
is selected as an option but the material has
no MDL shaders.

source/blender/editors/io/io_usd.c

@@ -85,7 +85,9 @@ const EnumPropertyItem rna_enum_usd_import_shaders_mode_items[] = {
      "USD MDL",
      0,
      "MDL",
-     "Convert MDL shaders to Blender materials"},
+     "Convert MDL shaders to Blender materials; if no MDL shaders "
+     "exist on the material, log a warning and import existing USD "
+     "Preview Surface shaders instead"},
     {0, NULL, 0, NULL, NULL},
 };
 

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

@@ -33,6 +33,8 @@
 
 #include "DNA_material_types.h"
 
+#include "WM_api.h"
+
 #include <pxr/base/gf/vec3f.h>
 #include <pxr/usd/ar/resolver.h>
 #include <pxr/usd/usdShade/material.h>
@@ -347,18 +349,29 @@ Material *USDMaterialReader::add_material(const pxr::UsdShadeMaterial &usd_mater
    * if there is one. */
   pxr::UsdShadeShader usd_preview;
   if (get_usd_preview_surface(usd_material, usd_preview)) {
+    /* Always set the viewport material properties from the USD
+     * Preview Surface settings. */
     set_viewport_material_props(mtl, usd_preview);
-
-    /* Optionally, create shader nodes to represent a UsdPreviewSurface. */
-    if (params_.import_shaders_mode == USD_IMPORT_USD_PREVIEW_SURFACE) {
-      import_usd_preview(mtl, usd_preview);
-    }
   }
 
-  if (params_.import_shaders_mode == USD_IMPORT_MDL) {
+  if (params_.import_shaders_mode == USD_IMPORT_USD_PREVIEW_SURFACE
+    && usd_preview) {
+    /* Create shader nodes to represent a UsdPreviewSurface. */
+    import_usd_preview(mtl, usd_preview);
+  }
+  else if (params_.import_shaders_mode == USD_IMPORT_MDL) {
+    bool imported_mdl = false;
 #ifdef WITH_PYTHON
-    umm_import_material(mtl, usd_material);
+    /* Invoke UMM to convert to MDL. */
+    imported_mdl = umm_import_material(mtl, usd_material, true /* Verbose */);
 #endif
+    if (!imported_mdl && usd_preview) {
+      /* We failed to import an MDL, so fall back on importing UsdPreviewSuface. */
+      std::string message = "Couldn't import MDL shaders for material "
+        + mtl_name + ", importing USD Preview Surface shaders instead";
+      WM_reportf(RPT_INFO, message.c_str());
+      import_usd_preview(mtl, usd_preview);
+    }
   }
 
   return mtl;

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

@@ -577,7 +577,11 @@ static bool import_material(Material *mtl,
   if (ret) {
     std::cout << "result:\n";
     print_obj(ret);
-    report_notification(ret);
+    if (report_notification(ret)) {
+      /* The function returned a notification object,
+       * indicating a failure. */
+      success = false;
+    }
     Py_DECREF(ret);
   }
 
@@ -712,7 +716,7 @@ bool umm_module_loaded()
   return loaded;
 }
 
-bool umm_import_material(Material *mtl, const pxr::UsdShadeMaterial &usd_material)
+bool umm_import_material(Material *mtl, const pxr::UsdShadeMaterial &usd_material, bool verbose)
 {
   if (!(mtl && usd_material)) {
     return false;
@@ -725,13 +729,17 @@ bool umm_import_material(Material *mtl, const pxr::UsdShadeMaterial &usd_materia
     /* Check if we have an mdl source asset. */
     pxr::SdfAssetPath source_asset;
     if (!surf_shader.GetSourceAsset(&source_asset, usdtokens::mdl)) {
-      std::cout << "No mdl source asset for shader " << surf_shader.GetPath() << std::endl;
+      if (verbose) {
+        std::cout << "No mdl source asset for shader " << surf_shader.GetPath() << std::endl;
+      }
       return false;
     }
     pxr::TfToken source_asset_sub_identifier;
     if (!surf_shader.GetSourceAssetSubIdentifier(&source_asset_sub_identifier, usdtokens::mdl)) {
-      std::cout << "No mdl source asset sub identifier for shader " << surf_shader.GetPath()
-                << std::endl;
+      if (verbose) {
+        std::cout << "No mdl source asset sub identifier for shader " << surf_shader.GetPath()
+          << std::endl;
+      }
       return false;
     }
 

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

@@ -31,7 +31,7 @@ struct USDExporterContext;
 
 bool umm_module_loaded();
 
-bool umm_import_material(Material *mtl, const pxr::UsdShadeMaterial &usd_material);
+bool umm_import_material(Material *mtl, const pxr::UsdShadeMaterial &usd_material, bool verbose = false);
 
 bool umm_export_material(const USDExporterContext &usd_export_context,
                          const Material *mtl,