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

@@ -397,6 +397,11 @@ void USD_ensure_plugin_path_registered()
   blender::io::usd::ensure_usd_plugin_path_registered();
 }
 
+void USD_setup_usd_mtlx_environment()
+{
+  blender::io::usd::setup_usd_mtlx_environment();
+}
+
 bool USD_import(struct bContext *C,
                 const char *filepath,
                 const USDImportParams *params,

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

@@ -6,6 +6,9 @@
 #include <pxr/base/plug/registry.h>
 
 #include "BKE_appdir.h"
+#include "BLI_string.h"
+#include "BLI_fileops.h"
+#include "BLI_path_util.h"
 
 namespace blender::io::usd {
 
@@ -32,4 +35,43 @@ void ensure_usd_plugin_path_registered()
 #endif
 }
 
+void setup_usd_mtlx_environment()
+{
+  /* PXR_MTLX_STDLIB_SEARCH_PATHS - path to the MaterialX standard library
+   * USD's MaterialX code expects to be set. */
+
+  static bool mtlx_env_set = false;
+  if (mtlx_env_set) {
+    return;
+  }
+  mtlx_env_set = true;
+
+  char stdlib_path[FILE_MAX] = "";
+  size_t stdlib_path_len = BLI_path_join(
+      stdlib_path, sizeof(stdlib_path), BKE_appdir_program_dir(), "materialx", "libraries");
+  if (stdlib_path_len == 0 || !BLI_exists(stdlib_path)) {
+    return;
+  }
+
+#ifdef _WIN32
+#  define PATH_SEP ";"
+#else
+#  define PATH_SEP ":"
+#endif
+
+  const char *env_var_name = "PXR_MTLX_STDLIB_SEARCH_PATHS";
+  const char *env_var = BLI_getenv(env_var_name);
+  if (!env_var) {
+    BLI_setenv(env_var_name, stdlib_path);
+  }
+  else {
+    size_t env_var_len = strlen(env_var);
+    size_t combined_len = stdlib_path_len + 1 + env_var_len + 1;
+    char *combined = (char *)malloc(sizeof(char) * combined_len);
+    BLI_snprintf(combined, combined_len, "%s" PATH_SEP "%s", stdlib_path, env_var);
+    BLI_setenv(env_var_name, combined);
+    free(combined);
+  }
+}
+
 }  // namespace blender::io::usd

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

@@ -5,5 +5,6 @@
 namespace blender::io::usd {
 
 void ensure_usd_plugin_path_registered();
+void setup_usd_mtlx_environment();
 
 }  // namespace blender::io::usd

source/blender/io/usd/usd.h

@@ -138,6 +138,7 @@ struct CacheReader *CacheReader_open_usd_object(struct CacheArchiveHandle *handl
 void USD_CacheReader_incref(struct CacheReader *reader);
 void USD_CacheReader_free(struct CacheReader *reader);
 void USD_ensure_plugin_path_registered(void);
+void USD_setup_usd_mtlx_environment(void);
 #ifdef __cplusplus
 }
 #endif

source/creator/creator.c

@@ -477,6 +477,7 @@ int main(int argc,
 
 #ifdef WITH_USD
   USD_ensure_plugin_path_registered();
+  USD_setup_usd_mtlx_environment();
 #endif
 
 #ifndef WITH_PYTHON_MODULE