Add mujoco/experimental and move USD work there.

PiperOrigin-RevId: 740730698
Change-Id: If9c68798cda502d6d7632906a034437e2557802e

src/experimental/usd/plugins/mjcf/mjcf_file_format.cc

@@ -0,0 +1,201 @@
+// Copyright 2025 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "mjcf/mjcf_file_format.h"
+
+#include <array>
+#include <memory>
+#include <string>
+#include <unordered_set>
+
+#include <mujoco/mujoco.h>
+#include "mjcf/mujoco_to_usd.h"
+#include <pxr/base/tf/diagnostic.h>
+#include <pxr/base/tf/enum.h>
+#include <pxr/base/tf/pathUtils.h>
+#include <pxr/base/tf/registryManager.h>
+#include <pxr/base/tf/staticTokens.h>
+#include <pxr/base/tf/type.h>
+#include <pxr/base/work/loops.h>
+#include <pxr/pxr.h>
+#include <pxr/usd/ar/asset.h>
+#include <pxr/usd/ar/resolvedPath.h>
+#include <pxr/usd/ar/resolver.h>
+#include <pxr/usd/sdf/declareHandles.h>
+#include <pxr/usd/sdf/fileFormat.h>
+#include <pxr/usd/sdf/layer.h>
+#include <pxr/usd/usd/usdaFileFormat.h>
+#include "tinyxml2.h"
+
+PXR_NAMESPACE_OPEN_SCOPE
+
+TF_DEFINE_PUBLIC_TOKENS(UsdMjcfFileFormatTokens, USD_MJCF_FILE_FORMAT_TOKENS);
+
+TF_REGISTRY_FUNCTION(TfType) {
+  SDF_DEFINE_FILE_FORMAT(UsdMjcfFileFormat, SdfFileFormat);
+}
+
+enum ErrorCodes { XmlParsingError };
+TF_REGISTRY_FUNCTION(TfEnum) {
+  TF_ADD_ENUM_NAME(XmlParsingError, "Error when parsing XML.");
+};
+
+namespace {
+
+void ResolveMjcfDependencies(const std::string &xml_string,
+                             const std::string &resolved_path);
+
+void AccumulateFilesRecursive(std::unordered_set<std::string> &files,
+                              tinyxml2::XMLElement *elem,
+                              const std::string &resolved_path) {
+  // get filename
+  const char *file = elem->Attribute("file");
+
+  if (file != nullptr) {
+    auto identifier = pxr::ArGetResolver().CreateIdentifier(
+        std::string(file), pxr::ArResolvedPath(resolved_path));
+    if (!strcasecmp(elem->Value(), "include") ||
+        !strcasecmp(elem->Value(), "model")) {
+      auto include_resolved_path = pxr::ArGetResolver().Resolve(identifier);
+      auto asset = pxr::ArGetResolver().OpenAsset(include_resolved_path);
+      ResolveMjcfDependencies(asset->GetBuffer().get(), include_resolved_path);
+
+      // Neither of these elements should have children.
+      return;
+    }
+
+    files.insert(identifier);
+  }
+
+  if (!strcasecmp(elem->Value(), "texture")) {
+    static const char *attributes[] = {"fileright", "fileup",    "fileleft",
+                                       "filedown",  "filefront", "fileback"};
+    for (const auto &attribute : attributes) {
+      const char *file = elem->Attribute(attribute);
+      if (file != nullptr) {
+        auto identifier = pxr::ArGetResolver().CreateIdentifier(
+            std::string(file), pxr::ArResolvedPath(resolved_path));
+        files.insert(identifier);
+      }
+    }
+  }
+
+  tinyxml2::XMLElement *child = elem->FirstChildElement();
+  for (; child; child = child->NextSiblingElement()) {
+    AccumulateFilesRecursive(files, child, resolved_path);
+  }
+}
+
+void ResolveMjcfDependencies(const std::string &xml_string,
+                             const std::string &resolved_path) {
+  // load XML file or parse string
+  tinyxml2::XMLDocument doc;
+  doc.Parse(xml_string.c_str());
+
+  // error checking
+  if (doc.Error()) {
+    TF_ERROR(XmlParsingError, "%d:\n%s\n", doc.ErrorID(), doc.ErrorStr());
+    return;
+  }
+
+  // get top-level element
+  tinyxml2::XMLElement *root = doc.RootElement();
+  if (!root) {
+    TF_ERROR(XmlParsingError, "XML root element not found");
+    return;
+  }
+
+  // Accumulate file dependencies.
+  std::unordered_set<std::string> files = {};
+  AccumulateFilesRecursive(files, root, resolved_path);
+
+  auto open_asset = [resolved_path](const std::string &identifier) {
+    pxr::ArGetResolver().OpenAsset(pxr::ArGetResolver().Resolve(identifier));
+  };
+  // Open all assets in parallel.
+  pxr::WorkParallelForEach(files.begin(), files.end(), open_asset);
+}
+}  // namespace
+
+UsdMjcfFileFormat::UsdMjcfFileFormat()
+    : SdfFileFormat(
+          UsdMjcfFileFormatTokens->Id, UsdMjcfFileFormatTokens->Version,
+          UsdMjcfFileFormatTokens->Target, UsdMjcfFileFormatTokens->Id) {}
+
+UsdMjcfFileFormat::~UsdMjcfFileFormat() {}
+
+bool UsdMjcfFileFormat::CanRead(const std::string &filePath) const {
+  auto extension = pxr::TfGetExtension(filePath);
+  if (extension.empty()) {
+    return false;
+  }
+
+  return extension == this->GetFormatId();
+}
+
+bool UsdMjcfFileFormat::ReadImpl(pxr::SdfLayer *layer, mjSpec *spec) const {
+  auto data = InitData(layer->GetFileFormatArguments());
+
+  auto success = mujoco::usd::WriteSpecToData(spec, data);
+  mj_deleteSpec(spec);
+  if (!success) {
+    return false;
+  }
+
+  _SetLayerData(layer, data);
+
+  return true;
+}
+
+bool UsdMjcfFileFormat::ReadFromString(pxr::SdfLayer *layer,
+                                       const std::string &str) const {
+  std::array<char, 1024> error;
+  mjSpec *spec =
+      mj_parseXMLString(str.c_str(), nullptr, error.data(), error.size());
+  if (spec == nullptr) {
+    TF_WARN(XmlParsingError, "%s", error.data());
+    return false;
+  }
+
+  return ReadImpl(layer, spec);
+}
+
+bool UsdMjcfFileFormat::Read(pxr::SdfLayer *layer,
+                             const std::string &resolved_path,
+                             bool metadata_only) const {
+  // Resolved all dependencies so that they are accessible when parsing
+  // the XML.
+  std::shared_ptr<pxr::ArAsset> asset =
+      pxr::ArGetResolver().OpenAsset(pxr::ArResolvedPath(resolved_path));
+  auto buffer = asset->GetBuffer();
+  ResolveMjcfDependencies(buffer.get(), resolved_path);
+
+  // Parse to USD.
+  std::array<char, 1024> error;
+  mjSpec *spec =
+      mj_parseXML(resolved_path.c_str(), nullptr, error.data(), error.size());
+  if (spec == nullptr) {
+    TF_WARN(XmlParsingError, "%s", error.data());
+    return false;
+  }
+  return ReadImpl(layer, spec);
+}
+
+bool UsdMjcfFileFormat::WriteToString(const SdfLayer &layer, std::string *str,
+                                      const std::string &comment) const {
+  return SdfFileFormat::FindById(pxr::UsdUsdaFileFormatTokens->Id)
+      ->WriteToString(layer, str, comment);
+}
+
+PXR_NAMESPACE_CLOSE_SCOPE

src/experimental/usd/plugins/mjcf/mjcf_file_format.h

@@ -0,0 +1,85 @@
+// Copyright 2025 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_SRC_EXPERIMENTAL_USD_PLUGINS_MJCF_MJCF_FILE_FORMAT_H_
+#define MUJOCO_SRC_EXPERIMENTAL_USD_PLUGINS_MJCF_MJCF_FILE_FORMAT_H_
+
+#include <string>
+
+#include <mujoco/mujoco.h>
+#include <pxr/base/tf/declarePtrs.h>
+#include <pxr/base/tf/staticTokens.h>
+#include <pxr/pxr.h>
+#include <pxr/usd/sdf/declareHandles.h>
+#include <pxr/usd/sdf/fileFormat.h>
+#include <pxr/usd/usd/api.h>
+
+PXR_NAMESPACE_OPEN_SCOPE
+
+// The Id should realistically be mjcf, but the id and extension need to match.
+// So near term it just assumes the only .xml file we would import is MJCF.
+#define USD_MJCF_FILE_FORMAT_TOKENS \
+  ((Id, "xml"))((Version, "1.0"))((Target, "usd"))
+
+TF_DECLARE_PUBLIC_TOKENS(UsdMjcfFileFormatTokens, USD_MJCF_FILE_FORMAT_TOKENS);
+
+TF_DECLARE_WEAK_AND_REF_PTRS(UsdMjcfFileFormat);
+
+class UsdMjcfFileFormat : public SdfFileFormat {
+ public:
+  using SdfFileFormat::FileFormatArguments;
+
+  // Returns true if 'file' can be read by this format plugin.
+  USD_API
+  bool CanRead(const std::string &file) const override;
+
+  // Reads scene description from the asset specified by resolved_path into
+  // 'layer'.
+  //
+  // metadataOnly is a flag that asks for only the layer metadata to be read in,
+  // which can be much faster if that is all that is required but currently we
+  // ignore it.
+  //
+  // Returns true if the asset is successfully read into layer, false otherwise.
+  USD_API
+  bool Read(pxr::SdfLayer *layer, const std::string &resolved_path,
+            bool metadata_only) const override;
+
+  // Reads data in the string 'str' into 'layer'.
+  //
+  // If the file is successfully read, this method returns true. Otherwise,
+  // false is returned and errors are posted.
+  USD_API
+  bool ReadFromString(SdfLayer *layer, const std::string &str) const override;
+
+  // Writes the contents in 'layer' to 'str'. This just forwards to the usda
+  // implementation.
+  USD_API
+  bool WriteToString(const SdfLayer &layer, std::string *str,
+                     const std::string &comment) const override;
+
+ protected:
+  SDF_FILE_FORMAT_FACTORY_ACCESS;
+
+  UsdMjcfFileFormat();
+  virtual ~UsdMjcfFileFormat();
+
+ private:
+  // Function delegated to by Read and ReadFromString.
+  bool ReadImpl(SdfLayer *layer, mjSpec *spec) const;
+};
+
+PXR_NAMESPACE_CLOSE_SCOPE
+
+#endif  // MUJOCO_SRC_EXPERIMENTAL_USD_PLUGINS_MJCF_MJCF_FILE_FORMAT_H_

src/experimental/usd/plugins/mjcf/mujoco_to_usd.h

@@ -0,0 +1,32 @@
+// Copyright 2025 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_SRC_EXPERIMENTAL_USD_PLUGINS_MJCF_MUJOCO_TO_USD_H_
+#define MUJOCO_SRC_EXPERIMENTAL_USD_PLUGINS_MJCF_MUJOCO_TO_USD_H_
+
+#include <mujoco/mujoco.h>
+#include <pxr/usd/sdf/abstractData.h>
+
+namespace mujoco {
+namespace usd {
+// Given an mjSpec, write it to a SdfAbstractData.
+//
+// Args:
+//   spec: mjSpec built programmatically or via parsed XML.
+//   data: SdfAbstractDataRefPtr that will be written to.
+bool WriteSpecToData(mjSpec* spec, pxr::SdfAbstractDataRefPtr& data);
+}  // namespace usd
+}  // namespace mujoco
+
+#endif  // MUJOCO_SRC_EXPERIMENTAL_USD_PLUGINS_MJCF_MUJOCO_TO_USD_H_

src/experimental/usd/plugins/mjcf/plugInfo.json

@@ -0,0 +1,25 @@
+{
+  "Plugins": [
+    {
+      "Info": {
+        "Types": {
+          "UsdMjcfFileFormat": {
+            "bases": ["SdfFileFormat"],
+            "displayName": "MJCF USD File Format",
+            "extensions": ["xml"],
+            "formatId": "xml",
+            "primary": true,
+            "supportsReading": true,
+            "supportsWriting": false,
+            "target": "usd"
+          }
+        }
+      },
+      "LibraryPath": "",
+      "Name": "usdMjcf",
+      "ResourcePath": "",
+      "Root": ".",
+      "Type": "library"
+    }
+  ]
+}

src/experimental/usd/plugins/mjcf/utils.cc

@@ -0,0 +1,199 @@
+// Copyright 2025 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "mjcf/utils.h"
+
+#include <string>
+#include <vector>
+
+#include <pxr/base/tf/token.h>
+#include <pxr/usd/sdf/abstractData.h>
+#include <pxr/usd/sdf/listOp.h>
+#include <pxr/usd/sdf/path.h>
+#include <pxr/usd/sdf/reference.h>
+#include <pxr/usd/sdf/schema.h>
+#include <pxr/usd/sdf/types.h>
+#include <pxr/usd/sdf/valueTypeName.h>
+#include <pxr/usd/usd/tokens.h>
+namespace {
+
+template <typename T>
+void AppendChild(pxr::SdfAbstractDataRefPtr& data, const pxr::SdfPath& specPath,
+                 const pxr::TfToken& childKey, const T& child) {
+  // Get existing children.
+  std::vector<T> children;
+  pxr::SdfAbstractDataTypedValue<std::vector<T>> getter(&children);
+  data->Has(specPath, childKey, &getter);
+
+  children.push_back(child);
+  data->Set(specPath, childKey,
+            pxr::SdfAbstractDataConstTypedValue<std::vector<T>>(&children));
+}
+
+template <typename T>
+void AppendListOp(pxr::SdfAbstractDataRefPtr& data,
+                  const pxr::SdfPath& spec_path, const pxr::TfToken& field,
+                  const T& item) {
+  // Get existing list op.
+  pxr::SdfListOp<T> list_op;
+  pxr::SdfAbstractDataTypedValue<pxr::SdfListOp<T>> getter(&list_op);
+  data->Has(spec_path, field, &getter);
+
+  auto items = list_op.GetExplicitItems();
+  items.push_back(item);
+  list_op.SetExplicitItems(items);
+  data->Set(spec_path, field,
+            pxr::SdfAbstractDataConstTypedValue<pxr::SdfListOp<T>>(&list_op));
+}
+
+template <typename T>
+void PrependListOp(pxr::SdfAbstractDataRefPtr& data,
+                   const pxr::SdfPath& spec_path, const pxr::TfToken& field,
+                   const T& item) {
+  // Get existing list op.
+  pxr::SdfListOp<T> listOp;
+  pxr::SdfAbstractDataTypedValue<pxr::SdfListOp<T>> getter(&listOp);
+  data->Has(spec_path, field, &getter);
+
+  auto prependedItems = listOp.GetPrependedItems();
+  prependedItems.insert(prependedItems.begin(), item);
+  listOp.SetPrependedItems(prependedItems);
+  data->Set(spec_path, field,
+            pxr::SdfAbstractDataConstTypedValue<pxr::SdfListOp<T>>(&listOp));
+}
+}  // namespace
+
+namespace mujoco {
+namespace usd {
+
+pxr::SdfPath CreatePrimSpec(pxr::SdfAbstractDataRefPtr& data,
+                            const pxr::SdfPath& parent_path,
+                            const pxr::TfToken& name, const pxr::TfToken& type,
+                            pxr::SdfSpecifier specifier) {
+  const pxr::SdfPath prim_path = parent_path.AppendChild(name);
+  data->CreateSpec(prim_path, pxr::SdfSpecTypePrim);
+  data->Set(prim_path, pxr::SdfFieldKeys->Specifier,
+            pxr::SdfAbstractDataConstTypedValue<pxr::SdfSpecifier>(&specifier));
+  if (!type.IsEmpty()) {
+    data->Set(prim_path, pxr::SdfFieldKeys->TypeName,
+              pxr::SdfAbstractDataConstTypedValue<const pxr::TfToken>(&type));
+  }
+
+  AppendChild(data, parent_path, pxr::SdfChildrenKeys->PrimChildren, name);
+
+  return prim_path;
+}
+
+pxr::SdfPath CreateAttributeSpec(pxr::SdfAbstractDataRefPtr& data,
+                                 const pxr::SdfPath& prim_path,
+                                 const pxr::TfToken& name,
+                                 const pxr::SdfValueTypeName& type_name,
+                                 pxr::SdfVariability variability) {
+  const pxr::SdfPath propertyPath = prim_path.AppendProperty(name);
+  data->CreateSpec(propertyPath, pxr::SdfSpecTypeAttribute);
+
+  pxr::TfToken typeNameToken = type_name.GetAsToken();
+  data->Set(propertyPath, pxr::SdfFieldKeys->TypeName,
+            pxr::SdfAbstractDataConstTypedValue<pxr::TfToken>(&typeNameToken));
+  if (variability != pxr::SdfVariabilityVarying) {
+    data->Set(
+        propertyPath, pxr::SdfFieldKeys->Variability,
+        pxr::SdfAbstractDataConstTypedValue<pxr::SdfVariability>(&variability));
+  }
+
+  AppendChild(data, prim_path, pxr::SdfChildrenKeys->PropertyChildren, name);
+
+  return propertyPath;
+}
+
+pxr::SdfPath CreateRelationshipSpec(pxr::SdfAbstractDataRefPtr& data,
+                                    const pxr::SdfPath& prim_path,
+                                    const pxr::TfToken& relationship_name,
+                                    const pxr::SdfPath& relationship_path,
+                                    pxr::SdfVariability variability) {
+  pxr::SdfPath prop_path = prim_path.AppendProperty(relationship_name);
+  data->CreateSpec(prop_path, pxr::SdfSpecTypeRelationship);
+  if (variability != pxr::SdfVariabilityVarying) {
+    data->Set(
+        prop_path, pxr::SdfFieldKeys->Variability,
+        pxr::SdfAbstractDataConstTypedValue<pxr::SdfVariability>(&variability));
+  }
+
+  AppendChild(data, prim_path, pxr::SdfChildrenKeys->PropertyChildren,
+              relationship_name);
+
+  AppendChild(data, prop_path, pxr::SdfChildrenKeys->RelationshipTargetChildren,
+              relationship_path);
+  AppendListOp(data, prop_path, pxr::SdfFieldKeys->TargetPaths,
+               relationship_path);
+
+  pxr::SdfPath target_path = prop_path.AppendTarget(relationship_path);
+  data->CreateSpec(target_path, pxr::SdfSpecTypeRelationshipTarget);
+
+  return prop_path;
+}
+
+pxr::SdfPath CreateClassSpec(pxr::SdfAbstractDataRefPtr& data,
+                             const pxr::SdfPath& prim_path,
+                             const pxr::TfToken& class_name) {
+  pxr::SdfPath class_path = prim_path.AppendChild(class_name);
+  pxr::SdfSpecifier class_specifier = pxr::SdfSpecifier::SdfSpecifierClass;
+  data->CreateSpec(class_path, pxr::SdfSpecTypePrim);
+  data->Set(
+      class_path, pxr::SdfFieldKeys->Specifier,
+      pxr::SdfAbstractDataConstTypedValue<pxr::SdfSpecifier>(&class_specifier));
+
+  AppendChild(data, prim_path, pxr::SdfChildrenKeys->PrimChildren, class_name);
+
+  return class_path;
+}
+
+void AddAttributeConnection(pxr::SdfAbstractDataRefPtr& data,
+                            const pxr::SdfPath& attribute_path,
+                            const pxr::SdfPath& target_attribute_path) {
+  AppendChild(data, attribute_path, pxr::SdfChildrenKeys->ConnectionChildren,
+              target_attribute_path);
+  AppendListOp(data, attribute_path, pxr::SdfFieldKeys->ConnectionPaths,
+               target_attribute_path);
+
+  data->CreateSpec(attribute_path.AppendTarget(target_attribute_path),
+                   pxr::SdfSpecTypeConnection);
+}
+
+void AddPrimReference(pxr::SdfAbstractDataRefPtr& data,
+                      const pxr::SdfPath& prim_path,
+                      const pxr::SdfPath& referenced_prim_path) {
+  PrependListOp(data, prim_path, pxr::SdfFieldKeys->References,
+                pxr::SdfReference("", referenced_prim_path));
+}
+
+void AddPrimInherit(pxr::SdfAbstractDataRefPtr& data,
+                    const pxr::SdfPath& prim_path,
+                    const pxr::SdfPath& class_path) {
+  PrependListOp(data, prim_path, pxr::SdfFieldKeys->InheritPaths, class_path);
+}
+
+void ApplyApiSchema(pxr::SdfAbstractDataRefPtr& data,
+                    const pxr::SdfPath& prim_path,
+                    const pxr::TfToken& schema_name) {
+  PrependListOp(data, prim_path, pxr::UsdTokens->apiSchemas, schema_name);
+}
+
+void SetPrimKind(pxr::SdfAbstractDataRefPtr& data,
+                 const pxr::SdfPath& prim_path, pxr::TfToken kind) {
+  SetPrimMetadata(data, prim_path, pxr::TfToken("kind"), kind);
+}
+
+}  // namespace usd
+}  // namespace mujoco

src/experimental/usd/plugins/mjcf/utils.h

@@ -0,0 +1,127 @@
+// Copyright 2025 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_SRC_EXPERIMENTAL_USD_PLUGINS_MJCF_UTILS_H_
+#define MUJOCO_SRC_EXPERIMENTAL_USD_PLUGINS_MJCF_UTILS_H_
+
+#include <type_traits>
+
+#include <pxr/base/tf/token.h>
+#include <pxr/imaging/hd/primTypeIndex.h>
+#include <pxr/usd/sdf/abstractData.h>
+#include <pxr/usd/sdf/path.h>
+#include <pxr/usd/sdf/schema.h>
+#include <pxr/usd/sdf/types.h>
+#include <pxr/usd/sdf/valueTypeName.h>
+
+namespace mujoco {
+namespace usd {
+
+// Create a prim spec and append it as a child of parent_path.
+pxr::SdfPath CreatePrimSpec(
+    pxr::SdfAbstractDataRefPtr& data, const pxr::SdfPath& parent_path,
+    const pxr::TfToken& name, const pxr::TfToken& type = pxr::TfToken(),
+    pxr::SdfSpecifier specifier = pxr::SdfSpecifier::SdfSpecifierDef);
+
+// Create an attribute spec and append it as a child of parent_path.
+// By default the attribute will be varying.
+pxr::SdfPath CreateAttributeSpec(
+    pxr::SdfAbstractDataRefPtr& data, const pxr::SdfPath& prim_path,
+    const pxr::TfToken& name, const pxr::SdfValueTypeName& type_name,
+    pxr::SdfVariability variability = pxr::SdfVariabilityVarying);
+
+// Create a relationship spec and append it as a child of prim_path.
+pxr::SdfPath CreateRelationshipSpec(
+    pxr::SdfAbstractDataRefPtr& data, const pxr::SdfPath& prim_path,
+    const pxr::TfToken& relationship_name,
+    const pxr::SdfPath& relationship_path,
+    pxr::SdfVariability variability = pxr::SdfVariabilityVarying);
+
+pxr::SdfPath CreateClassSpec(pxr::SdfAbstractDataRefPtr& data,
+                             const pxr::SdfPath& prim_path,
+                             const pxr::TfToken& class_name);
+
+void AddAttributeConnection(pxr::SdfAbstractDataRefPtr& data,
+                            const pxr::SdfPath& attribute_path,
+                            const pxr::SdfPath& target_attribute_path);
+
+void AddPrimReference(pxr::SdfAbstractDataRefPtr& data,
+                      const pxr::SdfPath& prim_path,
+                      const pxr::SdfPath& referenced_prim_path);
+
+void AddPrimInherit(pxr::SdfAbstractDataRefPtr& data,
+                    const pxr::SdfPath& prim_path,
+                    const pxr::SdfPath& class_path);
+
+void ApplyApiSchema(pxr::SdfAbstractDataRefPtr& data,
+                    const pxr::SdfPath& prim_path,
+                    const pxr::TfToken& schema_name);
+
+void SetPrimKind(pxr::SdfAbstractDataRefPtr& data,
+                 const pxr::SdfPath& prim_path, pxr::TfToken kind);
+
+// Set the value specified by key on any field at field_path.
+template <typename T>
+void SetField(pxr::SdfAbstractDataRefPtr& data, const pxr::SdfPath& field_path,
+              const pxr::TfToken key, T&& value) {
+  using Deduced = typename std::remove_reference_t<T>;
+  const auto typed_val = pxr::SdfAbstractDataConstTypedValue<Deduced>(&value);
+  const pxr::SdfAbstractDataConstValue& untyped_val = typed_val;
+
+  data->Set(field_path, key, untyped_val);
+}
+
+// Set the value specified by key on an attribute spec at attribute_path.
+template <typename T>
+void SetAttribute(pxr::SdfAbstractDataRefPtr& data,
+                  const pxr::SdfPath& attribute_path, const pxr::TfToken key,
+                  T&& value) {
+  SetField(data, attribute_path, key, value);
+}
+
+// Set the value specified by key on a prim spec at prim_path.
+template <typename T>
+void SetPrimMetadata(pxr::SdfAbstractDataRefPtr& data,
+                     const pxr::SdfPath& prim_path, const pxr::TfToken key,
+                     T&& value) {
+  SetAttribute(data, prim_path, key, value);
+}
+
+// Set the value specified by key on an attribute spec at attribute_path.
+template <typename T>
+void SetAttributeMetadata(pxr::SdfAbstractDataRefPtr& data,
+                          const pxr::SdfPath& attribute_path,
+                          const pxr::TfToken key, T&& value) {
+  SetAttribute(data, attribute_path, key, value);
+}
+
+// Set the default value on an attribute spec at attribute_path.
+template <typename T>
+void SetAttributeDefault(pxr::SdfAbstractDataRefPtr& data,
+                         const pxr::SdfPath& attribute_path,
+                         T&& default_value) {
+  SetAttribute(data, attribute_path, pxr::SdfFieldKeys->Default, default_value);
+}
+
+// Set the value specified by key on the root layer.
+template <typename T>
+void SetLayerMetadata(pxr::SdfAbstractDataRefPtr& data, const pxr::TfToken& key,
+                      T&& value) {
+  SetAttribute(data, pxr::SdfPath::AbsoluteRootPath(), key, value);
+}
+
+}  // namespace usd
+}  // namespace mujoco
+
+#endif  // MUJOCO_SRC_EXPERIMENTAL_USD_PLUGINS_MJCF_UTILS_H_