Add USD content type support in MJCF.

PiperOrigin-RevId: 788075561
Change-Id: If37cc254cab0411454de7663e0b3ff9f2b263281

include/mujoco/experimental/usd/usd.h

@@ -18,11 +18,21 @@
 #include <mujoco/mujoco.h>
 #include <pxr/usd/usd/common.h>
 
-// Given a USD file parse to a spec, then compile and return the low-level model.
+// Given a USD file parse to a spec, then compile and return the low-level
+// model.
 //
 // Particular care is taken for physics data to be lossless but visual
 // data such as materials may be lossy.
-MJAPI mjModel* mj_loadUSD(const char* filename, const mjVFS* vfs, char* error, int error_sz);
+MJAPI mjModel* mj_loadUSD(const char* filename, const mjVFS* vfs, char* error,
+                          int error_sz);
+
+// Given a USD layer identifier, this function will do a best effort conversion
+// from the composed stage to mjSpec.
+//
+// Particular care is taken for physics data to be lossless but visual
+// data such as materials may be lossy.
+MJAPI mjSpec* mj_parseUSD(const char* identifier, const mjVFS* vfs, char* error,
+                          int error_sz);
 
 // Given a USD stage, this function will do a best effort conversion to
 // mjSpec.

src/experimental/usd/usd_to_mjspec.cc

@@ -1744,3 +1744,10 @@ mjSpec* mj_parseUSDStage(const pxr::UsdStageRefPtr stage) {
 
   return spec;
 }
+
+MJAPI mjSpec* mj_parseUSD(const char* identifier, const mjVFS* vfs, char* error,
+                          int error_sz) {
+  auto stage = pxr::UsdStage::Open(identifier);
+  return mj_parseUSDStage(stage);
+}
+

src/xml/xml_native_reader.cc

@@ -45,6 +45,9 @@
 #include "xml/xml_base.h"
 #include "xml/xml_util.h"
 #include "tinyxml2.h"
+#ifdef mjUSEUSD
+#include <mujoco/experimental/usd/usd.h>
+#endif  // mjUSEUSD
 
 namespace {
 using std::string;
@@ -3332,6 +3335,10 @@ void mjXReader::Asset(XMLElement* section, const mjVFS* vfs) {
 
       if (content_type == "text/xml") {
         child = mj_parseXML(filename.c_str(), vfs, error.data(), error.size());
+#ifdef mjUSEUSD
+      } else if (content_type == "text/usd") {
+        child = mj_parseUSD(filename.c_str(), vfs, error.data(), error.size());
+#endif  // mjUSEUSD
       } else {
         throw mjXError(elem, "unsupported content_type: %s", content_type.c_str());
       }