mjcPhysics MjcTendon path now has an indices attribute to allow repeated sites in tendon path.

Since relationships in USD can only contain a single instance of a path, it was previously impossible to have a tendon path that visits the same wrap target more than once. This is a fairly common occurence when dealing with tendons that have pulleys.

With this change, if mjc:path:indices is not defined then the behaviour is as it was and the mjc:path is considered to be ordered. If it is authored then the parser will use the indices into mjc:path as the order. This mirrors other similar concepts in USD such as face vertex indices.

PiperOrigin-RevId: 856339887
Change-Id: I6b9503973de377acdb0689be168c16e4aa19b70b

include/mujoco/experimental/usd/mjcPhysics/tendon.h

@@ -158,6 +158,31 @@ class MjcPhysicsTendon : public UsdTyped {
   UsdAttribute CreateTypeAttr(VtValue const& defaultValue = VtValue(),
                               bool writeSparsely = false) const;
 
+ public:
+  // --------------------------------------------------------------------- //
+  // MJCPATHINDICES
+  // --------------------------------------------------------------------- //
+  /// This list represents the order in which the tendon wraps the sites in
+  /// mjc:path.
+  ///
+  /// | ||
+  /// | -- | -- |
+  /// | Declaration | `uniform int[] mjc:path:indices = []` |
+  /// | C++ Type | VtArray<int> |
+  /// | \ref Usd_Datatypes "Usd Type" | SdfValueTypeNames->IntArray |
+  /// | \ref SdfVariability "Variability" | SdfVariabilityUniform |
+  MJCPHYSICS_API
+  UsdAttribute GetMjcPathIndicesAttr() const;
+
+  /// See GetMjcPathIndicesAttr(), and also
+  /// \ref Usd_Create_Or_Get_Property for when to use Get vs Create.
+  /// If specified, author \p defaultValue as the attribute's default,
+  /// sparsely (when it makes sense to do so) if \p writeSparsely is \c true -
+  /// the default for \p writeSparsely is \c false.
+  MJCPHYSICS_API
+  UsdAttribute CreateMjcPathIndicesAttr(VtValue const& defaultValue = VtValue(),
+                                        bool writeSparsely = false) const;
+
  public:
   // --------------------------------------------------------------------- //
   // MJCSIDESITESINDICES
@@ -771,8 +796,8 @@ class MjcPhysicsTendon : public UsdTyped {
   // --------------------------------------------------------------------- //
   // MJCPATH
   // --------------------------------------------------------------------- //
-  /// For spatial tendons, this describes a list of sites and geoms the tendon
-  /// wraps. For fixed tendons, this is instead a list of joints.
+  /// For spatial tendons, this describes a list of unique of sites and geoms
+  /// the tendon wraps. For fixed tendons, this is instead a list of joints.
   ///
   MJCPHYSICS_API
   UsdRelationship GetMjcPathRel() const;

include/mujoco/experimental/usd/mjcPhysics/tokens.h

@@ -572,6 +572,10 @@ struct MjcPhysicsTokensType {
   ///
   /// MjcPhysicsTendon
   const TfToken mjcPathDivisors;
+  /// \brief "mjc:path:indices"
+  ///
+  /// MjcPhysicsTendon
+  const TfToken mjcPathIndices;
   /// \brief "mjc:path:segments"
   ///
   /// MjcPhysicsTendon