primvar.h

Go to the documentation of this file.

//
// Copyright 2016 Pixar
//
// Licensed under the Apache License, Version 2.0 (the "Apache License")
// with the following modification; you may not use this file except in
// compliance with the Apache License and the following modification to it:
// Section 6. Trademarks. is deleted and replaced with:
//
// 6. Trademarks. This License does not grant permission to use the trade
//    names, trademarks, service marks, or product names of the Licensor
//    and its affiliates, except as required to comply with Section 4(c) of
//    the License and to reproduce the content of the NOTICE file.
//
// You may obtain a copy of the Apache License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the Apache License with the above modification is
// distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the Apache License for the specific
// language governing permissions and limitations under the Apache License.
//
#ifndef PXR_USD_USD_GEOM_PRIMVAR_H
#define PXR_USD_USD_GEOM_PRIMVAR_H

#include "pxr/pxr.h"
#include "pxr/usd/usdGeom/api.h"
#include "pxr/usd/usd/attribute.h"
#include "pxr/usd/usdGeom/tokens.h"

#include <atomic>
#include <string>
#include <vector>

PXR_NAMESPACE_OPEN_SCOPE


/// \class UsdGeomPrimvar
///
/// Schema wrapper for UsdAttribute for authoring and introspecting attributes
/// that are primvars.
///
/// UsdGeomPrimvar provides API for authoring and retrieving the
/// additional data required to encode an attribute as a "Primvar",
/// which is a convenient contraction of RenderMan's "Primitive Variable" 
/// concept, which is represented in Alembic as 
/// "arbitrary geometry parameters" (arbGeomParams).
///
/// This includes the attribute's \ref GetInterpolation() "interpolation"
/// across the primitive (which RenderMan refers to as its 
/// \ref Usd_InterpolationVals "class specifier"
/// and Alembic as its <A HREF="https://github.com/alembic/alembic/blob/master/lib/Alembic/AbcGeom/GeometryScope.h#L47"> "geometry scope"</A>);
/// it also includes the attribute's \ref GetElementSize() "elementSize",
/// which states how many values in the value array must be aggregated for
/// each element on the primitive.  An attribute's \ref
/// UsdAttribute::GetTypeName() "TypeName" also factors into the encoding of 
/// Primvar.
///
/// \section Usd_What_Is_Primvar What is the Purpose of a Primvar?
///
/// There are three key aspects of Primvar identity:
/// \li Primvars define a value that can vary across the primitive on which
///     they are defined, via prescribed interpolation rules
/// \li Taken collectively on a prim, its Primvars describe the "per-primitive
///     overrides" to the material to which the prim is bound.  Different
///     renderers may communicate the variables to the shaders using different
///     mechanisms over which Usd has no control; Primvars simply provide the
///     classification that any renderer should use to locate potential
///     overrides.  Do please note that primvars override parameters on
///     UsdShadeShader objects, \em not 
///     \ref UsdShadeNodeGraph_Interfaces "Interface Attributes" on UsdShadeMaterial
///     prims.
/// \li *Primvars inherit down scene namespace.*  Regular USD attributes only
///     apply to the prim on which they are specified, but primvars implicitly
///     also apply to any child prims, unless those child prims have their
///     own opinions about those primvars.  This capability necessarily
///     entails added cost to check for inherited values, but the benefit
///     is that it allows concise encoding of certain opinions that broadly
///     affect large amounts of geometry.  See
///     UsdGeomImageable::FindInheritedPrimvars().
///
/// \section Usd_Creating_and_Accessing_Primvars Creating and Accessing Primvars
/// 
/// The UsdGeomPrimvarsAPI schema provides a complete interface for creating
/// and querying prims for primvars.
///
/// The <b>only</b> way to create a new Primvar in scene description is by
/// calling UsdGeomPrimvarsAPI::CreatePrimvar().  One cannot "enhance" or
/// "promote" an already existing attribute into a Primvar, because doing so
/// may require a namespace edit to rename the attribute, which cannot, in
/// general, be done within a single UsdEditContext.  Instead, create a new
/// UsdGeomPrimvar of the desired name using
/// UsdGeomPrimvarsAPI::CreatePrimvar(), and then copy the existing attribute
/// onto the new UsdGeomPrimvar.
///
/// Primvar names can contain arbitrary sub-namespaces. The behavior of
/// UsdGeomImageable::GetPrimvar(TfToken const &name) is to prepend "primvars:"
/// onto 'name' if it is not already a prefix, and return the result, which
/// means we do not have any ambiguity between the primvars
/// "primvars:nsA:foo" and "primvars:nsB:foo".  <b>There are reserved keywords
/// that may not be used as the base names of primvars,</b> and attempting to
/// create Primvars of these names will result in a coding error.  The
/// reserved keywords are tokens the Primvar uses internally to encode various
/// features, such as the "indices" keyword used by
/// \ref UsdGeomPrimvar_Indexed_primvars "Indexed Primvars".
///
/// \anchor UsdGeomPrimvar_Using_Primvar
/// If a client wishes to access an already-extant attribute as a Primvar,
/// (which may or may not actually be valid Primvar), they can use the
/// speculative constructor; typically, a primvar is only "interesting" if it
/// additionally provides a value.  This might look like:
/// \code
/// UsdGeomPrimvar primvar = UsdGeomPrimvar(usdAttr);
/// if (primvar.HasValue()) {
///     VtValue values;
///     primvar.Get(&values, timeCode);
///     TfToken interpolation = primvar.GetInterpolation();
///     int     elementSize = primvar.GetElementSize();
///     ...
/// }
/// \endcode
///
/// or, because Get() returns `true` if and only if it found a value:
/// \code
/// UsdGeomPrimvar primvar = UsdGeomPrimvar(usdAttr);
/// VtValue values;
/// if (primvar.Get(&values, timeCode)) {
///     TfToken interpolation = primvar.GetInterpolation();
///     int     elementSize = primvar.GetElementSize();
///     ...
/// }
/// \endcode
///
/// \subsection Usd_Handling_Indexed_Primvars Proper Client Handling of "Indexed" Primvars
///
/// As discussed in greater detail in 
/// \ref UsdGeomPrimvar_Indexed_primvars "Indexed Primvars", primvars can 
/// optionally contain a (possibly time-varying) indexing attribute that 
/// establishes a sharing topology for elements of the primvar.  Consumers
/// can always chose to ignore the possibility of indexed data by exclusively 
/// using the ComputeFlattened() API.  If a client wishes to preserve indexing
/// in their processing of a primvar, we suggest a pattern like the following,
/// which accounts for the fact that a stronger layer can 
/// \ref UsdAttribute::Block() "block" a primvar's indexing from a weaker
/// layer, via UsdGeomPrimvar::BlockIndices():
/// \code
/// VtValue values;
/// VtIntArray indices;
///
/// if (primvar.Get(&values, timeCode)){
///     if (primvar.GetIndices(&indices, timeCode)){
///         // primvar is indexed: validate/process values and indices together
///     }
///     else {
///         // primvar is not indexed: validate/process values as flat array
///     }
/// }
/// \endcode
///
/// \subsection Usd_Primvar_As_Attribute UsdGeomPrimvar and UsdAttribute API
/// 
/// UsdGeomPrimvar presents a small slice of the UsdAttribute API - enough to 
/// extract the data that comprises the "Declaration info", and get/set of
/// the attribute value.  A UsdGeomPrimvar also auto-converts to UsdAttribute, 
/// so you can pass a UsdGeomPrimvar to any function that accepts a UsdAttribute
/// or const-ref thereto.
///
/// \section Usd_Primvar_Types Primvar Allowed Scene Description Types and Plurality
/// There are no limitations imposed on the allowable scene description types
/// for Primvars; it is the responsibility of each consuming client to perform
/// renderer-specific conversions, if need be (the USD distribution will include
/// reference RenderMan conversion utilities).
///
/// A note about type plurality of Primvars: It is legitimate for a Primvar
/// to be of scalar or array type, and again, consuming clients must be
/// prepared to accommodate both.  However, while it is not possible, in all
/// cases, for USD to \em prevent one from \em changing the type of an attribute
/// in different layers or variants of an asset, it is never a good idea to
/// do so.  This is relevant because, except in a few special cases, it is
/// not possible to encode an \em interpolation of any value greater than 
/// \em constant without providing multiple (i.e. array) data values. Therefore,
/// if there is any possibility that downstream clients might need to change
/// a Primvar's interpolation, the Primvar-creator should encode it as an
/// array rather than a scalar.
///
/// Why allow scalar values at all, then?  First, sometimes it brings clarity
/// to (use of) a shader's API to acknowledge that some parameters are meant
/// to be single-valued over a shaded primitive.  Second, many DCC's provide
/// far richer affordances for editing scalars than they do array values, and
/// we feel it is safer to let the content creator make the decision/tradeoff
/// of which kind of flexibility is more relevant, rather than leaving it to
/// an importer/exporter pair to interpret.
///
/// Also, like all attributes, Primvars can be time-sampled, and values can
/// be authored and consumed just as any other attribute.  There is currently
/// no validation that the length of value arrays matches to the size 
/// required by a gprim's topology, interpolation, and elementSize.
///
/// For consumer convenience, we provide GetDeclarationInfo(), which returns
/// all the type information (other than topology) needed to compute the
/// required array size, which is also all the information required to 
/// prepare the Primvar's value for consumption by a renderer.
///
/// \section Usd_UsdGeomPrimvar_Lifetime Lifetime Management and Primvar Validity
///
/// UsdGeomPrimvar has an explicit bool operator that validates that
/// the attribute IsDefined() and thus valid for querying and authoring
/// values and metadata.  This is a fairly expensive query that we do 
/// <b>not</b> cache, so if client code retains UsdGeomPrimvar objects, it should 
/// manage its object validity closely, for performance.  An ideal pattern
/// is to listen for UsdNotice::StageContentsChanged notifications, and 
/// revalidate/refetch its retained UsdGeomPrimvar s only then, and otherwise use
/// them without validity checking.
///
/// \section Usd_InterpolationVals Interpolation of Geometric Primitive Variables
/// In the following explanation of the meaning of the various kinds/levels
/// of Primvar interpolation, each bolded bullet gives the name of the token
/// in \ref UsdGeomTokens that provides the value.  So to set a Primvar's
/// interpolation to "varying", one would:
/// \code
/// primvar.SetInterpolation(UsdGeomTokens->varying);
/// \endcode
/// 
/// Reprinted and adapted from <a HREF="http://renderman.pixar.com/resources/current/rps/appnote.22.html#classSpecifiers">
/// the RPS documentation</a>, which contains further details, \em interpolation
/// describes how the Primvar will be interpolated over the uv parameter
/// space of a surface primitive (or curve or pointcloud).  The possible
/// values are:
/// \li <b>constant</b> One value remains constant over the entire surface 
///     primitive.
/// \li <b>uniform</b> One value remains constant for each uv patch segment of 
///     the surface primitive (which is a \em face for meshes). 
/// \li <b>varying</b> Four values are interpolated over each uv patch segment 
///     of the surface. Bilinear interpolation is used for interpolation 
///     between the four values.
/// \li <b>vertex</b> Values are interpolated between each vertex in the 
///     surface primitive. The basis function of the surface is used for 
///     interpolation between vertices.
/// \li <b>faceVarying</b> For polygons and subdivision surfaces, four values 
///     are interpolated over each face of the mesh. Bilinear interpolation 
///     is used for interpolation between the four values.
///
/// \section Usd_Extending_UsdObject_Classes UsdGeomPrimvar As Example of Attribute Schema
///
/// Just as UsdSchemaBase and its subclasses provide the pattern for how to
/// layer schema onto the generic UsdPrim object, UsdGeomPrimvar provides an
/// example of how to layer schema onto a generic UsdAttribute object.  In both
/// cases, the schema object wraps and contains the UsdObject.
///
/// \section Usd_UsdGeomPrimvar_Inheritance Primvar Namespace Inheritance
///
/// Constant interpolation primvar values can be inherited down namespace.
/// That is, a primvar value set on a prim will also apply to any child
/// prims, unless those children have their own opinions about those named
/// primvars. For complete details on how primvars inherit, see
/// \ref usdGeom_PrimvarInheritance .
///
/// \sa UsdGeomImageable::FindInheritablePrimvars().
///
class UsdGeomPrimvar
{
public:

    // Default constructor returns an invalid Primvar.  Exists for 
    // container classes
    UsdGeomPrimvar()
    {
        /* NOTHING */
    }

    /// Copy construct.
    USDGEOM_API
    UsdGeomPrimvar(const UsdGeomPrimvar &other);

    /// Copy assign.
    USDGEOM_API
    UsdGeomPrimvar &operator=(const UsdGeomPrimvar &other);

    /// Speculative constructor that will produce a valid UsdGeomPrimvar when
    /// \p attr already represents an attribute that is Primvar, and
    /// produces an \em invalid Primvar otherwise (i.e. 
    /// \ref UsdGeomPrimvar_bool "operator bool()" will return false).
    ///
    /// Calling \c UsdGeomPrimvar::IsPrimvar(attr) will return the same truth
    /// value as this constructor, but if you plan to subsequently use the
    /// Primvar anyways, just use this constructor, as demonstrated in the 
    /// \ref UsdGeomPrimvar_Using_Primvar "class documentation".
    USDGEOM_API
    explicit UsdGeomPrimvar(const UsdAttribute &attr);

    /// Return the Primvar's interpolation, which is 
    /// \ref Usd_InterpolationVals "UsdGeomTokens->constant" if unauthored
    ///
    /// Interpolation determines how the Primvar interpolates over
    /// a geometric primitive.  See \ref Usd_InterpolationVals
    USDGEOM_API
    TfToken GetInterpolation() const;

    /// Set the Primvar's interpolation.
    ///
    /// Errors and returns false if \p interpolation is out of range as
    /// defined by IsValidInterpolation().  No attempt is made to validate
    /// that the Primvar's value contains the right number of elements
    /// to match its interpolation to its topology.
    ///
    /// \sa GetInterpolation(), \ref Usd_InterpolationVals
    USDGEOM_API
    bool SetInterpolation(const TfToken &interpolation);
    
    /// Has interpolation been explicitly authored on this Primvar?
    ///
    /// \sa GetInterpolationSize()
    USDGEOM_API
    bool HasAuthoredInterpolation() const;

    /// Return the "element size" for this Primvar, which is 1 if
    /// unauthored.  If this Primvar's type is \em not an array type,
    /// (e.g. "Vec3f[]"), then elementSize is irrelevant.
    ///
    /// ElementSize does \em not generally encode the length of an array-type
    /// primvar, and rarely needs to be authored.  ElementSize can be thought
    /// of as a way to create an "aggregate interpolatable type", by
    /// dictating how many consecutive elements in the value array should be
    /// taken as an atomic element to be interpolated over a gprim. 
    ///
    /// For example, spherical harmonics are often represented as a
    /// collection of nine floating-point coefficients, and the coefficients
    /// need to be sampled across a gprim's surface: a perfect case for
    /// primvars.  However, USD has no <tt>float9</tt> datatype.  But we can
    /// communicate the aggregation of nine floats successfully to renderers
    /// by declaring a simple float-array valued primvar, and setting its
    /// \em elementSize to 9.  To author a \em uniform spherical harmonic
    /// primvar on a Mesh of 42 faces, the primvar's array value would contain
    /// 9*42 = 378 float elements.
    USDGEOM_API
    int GetElementSize() const;
    
    /// Set the elementSize for this Primvar.
    ///
    /// Errors and returns false if \p eltSize less than 1.
    ///
    /// \sa GetElementSize()
    USDGEOM_API
    bool SetElementSize(int eltSize);
    
    /// Has elementSize been explicitly authored on this Primvar?
    ///
    /// \sa GetElementSize()
    USDGEOM_API
    bool HasAuthoredElementSize() const;
    

    /// Test whether a given UsdAttribute represents valid Primvar, which
    /// implies that creating a UsdGeomPrimvar from the attribute will succeed.
    ///
    /// Success implies that \c attr.IsDefined() is true.
    USDGEOM_API
    static bool IsPrimvar(const UsdAttribute &attr);


    /// Test whether a given \p name represents a valid name of a primvar, 
    /// which implies that creating a UsdGeomPrimvar with the given name will 
    /// succeed.
    /// 
    USDGEOM_API
    static bool IsValidPrimvarName(const TfToken& name);

    /// Returns the \p name, devoid of the "primvars:" token if present,
    /// otherwise returns the \p name unchanged
    USDGEOM_API
    static TfToken StripPrimvarsName(const TfToken& name);

    /// Validate that the provided \p interpolation is a valid setting for 
    /// interpolation as defined by \ref Usd_InterpolationVals.  
    USDGEOM_API
    static bool IsValidInterpolation(const TfToken &interpolation);

    /// Convenience function for fetching all information required to 
    /// properly declare this Primvar.  The \p name returned is the
    /// "client name", stripped of the "primvars" namespace, i.e. equivalent to
    /// GetPrimvarName()
    ///
    /// May also be more efficient than querying key individually.
    USDGEOM_API
    void GetDeclarationInfo(TfToken *name, SdfValueTypeName *typeName,
                            TfToken *interpolation, int *elementSize) const;

    // ---------------------------------------------------------------
    /// \name UsdAttribute API
    // ---------------------------------------------------------------
    /// @{

    /// Allow UsdGeomPrimvar to auto-convert to UsdAttribute, so you can
    /// pass a UsdGeomPrimvar to any function that accepts a UsdAttribute or
    /// const-ref thereto.
    operator UsdAttribute const& () const { return _attr; }

    /// Explicit UsdAttribute extractor
    UsdAttribute const &GetAttr() const { return _attr; }
    
    /// Return true if the underlying UsdAttribute::IsDefined(), and in
    /// addition the attribute is identified as a Primvar.  Does not imply
    /// that the primvar provides a value
    bool IsDefined() const { return IsPrimvar(_attr); }

    /// Return true if the underlying attribute has a value, either from
    /// authored scene description or a fallback.
    bool HasValue() const { return _attr.HasValue(); }

    /// Return true if the underlying attribute has an unblocked, authored
    /// value.
    bool HasAuthoredValue() const { return _attr.HasAuthoredValue(); }

    /// \anchor UsdGeomPrimvar_bool
    /// Return true if this Primvar is valid for querying and authoring
    /// values and metadata, which is identically equivalent to IsDefined().
    explicit operator bool() const {
        return IsDefined() ? &UsdGeomPrimvar::_attr : 0;
    }

    /// \sa UsdAttribute::GetName()
    TfToken const &GetName() const { return _attr.GetName(); }

    /// Returns the primvar's name, devoid of the "primvars:" namespace.
    /// This is the name by which clients should refer to the primvar, if
    /// not by its full attribute name - i.e. they should **not**, in general,
    /// use GetBaseName().  In the error condition in which this Primvar
    /// object is not backed by a properly namespaced UsdAttribute, return
    /// an empty TfToken.
    USDGEOM_API
    TfToken GetPrimvarName() const;

    /// Does this primvar contain any namespaces other than the "primvars:"
    /// namespace?
    ///
    /// Some clients may only wish to consume primvars that have no extra
    /// namespaces in their names, for ease of translating to other systems
    /// that do not allow namespaces.
    USDGEOM_API
    bool NameContainsNamespaces() const;

    /// \sa UsdAttribute::GetBaseName()
    TfToken GetBaseName() const { return _attr.GetBaseName(); }

    /// \sa UsdAttribute::GetNamespace()
    TfToken GetNamespace() const { return _attr.GetNamespace(); }

    /// \sa UsdAttribute::SplitName()
    std::vector<std::string> SplitName() const { return _attr.SplitName(); };

    /// \sa UsdAttribute::GetTypeName()
    SdfValueTypeName GetTypeName() const { return _attr.GetTypeName(); }

    /// Get the attribute value of the Primvar at \p time .
    ///
    /// \sa Usd_Handling_Indexed_Primvars for proper handling of 
    /// \ref Usd_Handling_Indexed_Primvars "indexed primvars"
    template <typename T>
    bool Get(T* value, UsdTimeCode time = UsdTimeCode::Default()) const {
        return _attr.Get(value, time);
    }

    /// Set the attribute value of the Primvar at \p time 
    template <typename T>
    bool Set(const T& value, UsdTimeCode time = UsdTimeCode::Default()) const {
        return _attr.Set(value, time);
    }

    /// Populates a vector with authored sample times for this primvar.
    /// Returns false on error.
    /// 
    /// This considers any timeSamples authored on the associated "indices"
    /// attribute if the primvar is indexed.
    /// 
    /// \sa UsdAttribute::GetTimeSamples
    USDGEOM_API
    bool GetTimeSamples(std::vector<double>* times) const;

    /// Populates a vector with authored sample times in \p interval. 
    /// 
    /// This considers any timeSamples authored on the associated "indices"
    /// attribute if the primvar is indexed.
    /// 
    /// \sa UsdAttribute::GetTimeSamplesInInterval
    USDGEOM_API
    bool GetTimeSamplesInInterval(const GfInterval& interval,
                                  std::vector<double>* times) const;

    /// Return true if it is possible, but not certain, that this primvar's
    /// value changes over time, false otherwise. 
    /// 
    /// This considers time-varyingness of the associated "indices" attribute 
    /// if the primvar is indexed.
    /// 
    /// \sa UsdAttribute::ValueMightBeTimeVarying
    USDGEOM_API
    bool ValueMightBeTimeVarying() const;

    /// @}

    // ---------------------------------------------------------------
    /// @{
    /// \anchor UsdGeomPrimvar_Indexed_primvars
    /// \name Indexed primvars API
    /// 
    /// For non-constant values of interpolation, it is often the case that the 
    /// same value is repeated many times in the array value of a primvar. An 
    /// indexed primvar can be used in such cases to optimize for data storage
    /// if the primvar's interpolation is uniform, varying, or vertex.
    /// For **faceVarying primvars**, however, indexing serves a higher
    /// purpose (and should be used *only* for this purpose, since renderers
    /// and OpenSubdiv will assume it) of establishing a surface topology
    /// for the primvar.  That is, faceVarying primvars use indexing to 
    /// unambiguously define discontinuities in their functions at edges
    /// and vertices.  Please see the <a href="http://graphics.pixar.com/opensubdiv/docs/subdivision_surfaces.html#face-varying-interpolation-rules">
    /// OpenSubdiv documentation on FaceVarying Primvars</a> for more 
    /// information.
    /// 
    /// To create an indexed primvar, the value of the attribute associated with 
    /// the primvar is set to an array consisting of all the unique values that 
    /// appear in the primvar array. A separate namespaced "indices" attribute 
    /// is set to an integer array containing indices into the array with all 
    /// the unique elements. The final value of the primvar is computed using 
    /// the indices array and the attribute value array.
    /// 
    /// See also \ref Usd_Handling_Indexed_Primvars

    /// Sets the indices value of the indexed primvar at \p time.
    /// 
    /// The values in the indices array must be valid indices into the authored
    /// array returned by Get(). The element numerality of the primvar's 
    /// 'interpolation' metadata applies to the "indices" array, not the attribute
    /// value array (returned by Get()).
    USDGEOM_API
    bool SetIndices(const VtIntArray &indices, 
                    UsdTimeCode time = UsdTimeCode::Default()) const;

    /// Returns the value of the indices array associated with the indexed 
    /// primvar at \p time.
    /// 
    /// \sa SetIndices(), \ref Usd_Handling_Indexed_Primvars
    USDGEOM_API
    bool GetIndices(VtIntArray *indices,
                    UsdTimeCode time = UsdTimeCode::Default()) const;

    /// Block the indices that were previously set.  This effectively makes an
    /// indexed primvar no longer indexed.  This is useful when overriding an
    /// existing primvar.
    USDGEOM_API
    void BlockIndices() const;

    /// Returns true if the primvar is indexed, i.e., if it has an associated
    /// "indices" attribute.
    ///
    /// If you are going to query the indices anyways, prefer to simply 
    /// consult the return-value of GetIndices(), which will be more efficient.
    USDGEOM_API
    bool IsIndexed() const;

    /// Returns a valid indices attribute if the primvar is indexed. Returns 
    /// an invalid attribute otherwise.
    USDGEOM_API
    UsdAttribute GetIndicesAttr() const;

    /// Returns the existing indices attribute if the primvar is indexed
    /// or creates a new one.
    USDGEOM_API
    UsdAttribute CreateIndicesAttr() const;
    
    /// Set the index that represents unauthored values in the indices array.
    /// 
    /// Some apps (like Maya) allow you to author primvars sparsely over a 
    /// surface. Since most apps can't handle sparse primvars, Maya needs to 
    /// provide a value even for the elements it didn't author. This metadatum 
    /// provides a way to recover the information in apps that do support 
    /// sparse authoring / representation of primvars. 
    /// 
    /// The fallback value of unauthoredValuesIndex is -1, which indicates that 
    /// there are no unauthored values.
    /// 
    /// \sa GetUnauthoredValuesIndex()
    USDGEOM_API
    bool SetUnauthoredValuesIndex(int unauthoredValuesIndex) const;

    /// Returns the index that represents unauthored values in the indices array.
    /// 
    /// \sa SetUnauthoredValuesIndex()
    USDGEOM_API
    int GetUnauthoredValuesIndex() const;
    
    /// Computes the flattened value of the primvar at \p time. 
    /// 
    /// If the primvar is not indexed or if the value type of this primvar is 
    /// a scalar, this returns the authored value, which is the same as 
    /// \ref Get(). Hence, it's safe to call ComputeFlattened() on non-indexed 
    /// primvars.
    template <typename ScalarType>
    bool ComputeFlattened(VtArray<ScalarType> * value, 
                          UsdTimeCode time = UsdTimeCode::Default()) const;

    /// \overload
    /// Computes the flattened value of the primvar at \p time as a VtValue. 
    /// 
    /// If the primvar is not indexed or if the value type of this primvar is 
    /// a scalar, this returns the authored value, which is the same as 
    /// \ref Get(). Hence, it's safe to call ComputeFlattened() on non-indexed 
    /// primvars.
    USDGEOM_API
    bool ComputeFlattened(VtValue *value, 
                          UsdTimeCode time=UsdTimeCode::Default()) const;

    /// Computes the flattened value of \p attrValue given \p indices.
    ///
    /// This method is a static convenience function that performs the main
    /// work of ComputeFlattened above without needing an instance of a 
    /// UsdGeomPrimvar.
    ///
    /// Returns \c false if the value contained in \p attrVal is not a supported
    /// type for flattening. Otherwise returns \c true.  The output 
    /// \p errString variable may be populated with an error string if an error
    /// is encountered during flattening.
    USDGEOM_API
    static bool ComputeFlattened(VtValue *value, const VtValue &attrVal,
                                 const VtIntArray &indices, 
                                 std::string *errString);
                          

    /// @}

    // ---------------------------------------------------------------
    /// @{
    /// \anchor UsdGeomPrimvar_Id_primvars
    /// \name Id attribute API
    ///
    /// Often there is the need to identify a prim within a scene (e.g. a mesh
    /// on which a procedural should operate, or a shader to inherit).  A
    /// string or string[] -typed primvar can be turned into an "Id Path" primvar by
    /// calling SetIdTarget() with the path of any object on the current stage.  When
    /// the primvar is subsequently queried via Get(), the returned value will
    /// be the stringified value of the targeted object's path in whatever
    /// namespace is defined by the querying stage's root layer.  In other
    /// words, authoring an Id primvar into a published model will return the
    /// path-to-target in the model, but when the model is referenced into a
    /// larger scene, it will return the complete scene path.
    /// 
    /// This works by adding a paired UsdRelationship in a ":idFrom" namespace
    /// "below" the string primvar.  Get() evaluates
    /// UsdRelationship::GetForwardedTargets() to determine the id-string.
    /// Thus, this mechanism will always produce a unique identifier for every
    /// object in a scene, regardless of how many times an asset is referenced
    /// into a scene.  Providing a mesh with a unique identifier primvar can be
    /// especially useful for renderers that allow plugins/shaders access to
    /// processed scene data based on user-provided string identifiers.
    ///
    /// If an Id primvar has both an \em authored string value and a SetIdTarget()'d
    /// target, the target path takes precedence.
    /// 
    /// Currently Id primvars can have only a single target, so the only useful
    /// interpolation is constant.
    // ---------------------------------------------------------------
    
    /// Returns true if the primvar is an Id primvar.
    ///
    /// \sa \ref UsdGeomPrimvar_Id_primvars
    USDGEOM_API
    bool IsIdTarget() const;

    /// This primvar must be of String or StringArray type for this method to
    /// succeed.  If not, a coding error is raised.
    ///
    /// \sa \ref UsdGeomPrimvar_Id_primvars
    USDGEOM_API
    bool SetIdTarget(const SdfPath& path) const;
    
    /// @}

    /// Equality comparison.  Return true if \a lhs and \a rhs represent the
    /// same UsdGeomPrimvar, false otherwise.
    friend bool operator==(const UsdGeomPrimvar &lhs, const UsdGeomPrimvar &rhs) {
        return lhs.GetAttr() == rhs.GetAttr();
    }

    /// Inequality comparison. Return false if \a lhs and \a rhs represent the
    /// same UsdPrimvar, true otherwise.
    friend bool operator!=(const UsdGeomPrimvar &lhs, const UsdGeomPrimvar &rhs) {
        return !(lhs == rhs);
    }

    /// Less-than operator. Returns true if \a lhs < \a rhs. 
    /// 
    /// This simply compares the paths of the underlyingattributes. 
    friend bool operator<(const UsdGeomPrimvar &lhs, const UsdGeomPrimvar &rhs) {
        return lhs.GetAttr().GetPath() < rhs.GetAttr().GetPath();
    }

    // Specialize TfHashAppend for TfHash
    template <typename HashState>
    friend void TfHashAppend(HashState& h, const UsdGeomPrimvar& obj) {
        h.Append(obj.GetAttr());
    }

    // hash_value overload for std/hboost hash.
    friend size_t hash_value(const UsdGeomPrimvar &obj) {
        return TfHash{}(obj);
    }


private:
    friend class UsdGeomImageable;
    friend class UsdGeomPrimvarsAPI;
    
    /// Validate that the given \p name contains the primvars namespace.
    /// Does not validate name as a legal property identifier
    static bool _IsNamespaced(const TfToken& name);

    /// Return \p name prepended with the proper primvars namespace, if
    /// it is not already prefixed.
    ///
    /// Does not validate name as a legal property identifier, but will
    /// verify that \p name contains no reserved keywords, and will return
    /// an empty TfToken if it does. If \p quiet is true, the verification
    /// will be silent
    static TfToken _MakeNamespaced(const TfToken& name, bool quiet=false);

    static TfToken const &_GetNamespacePrefix();

    /// Factory for UsdGeomImageable's use, so that we can encapsulate the
    /// logic of what discriminates Primvar in this class, while
    /// preserving the pattern that attributes can only be created
    /// via their container objects.
    ///
    /// The name of the created attribute may or may not be the specified
    /// \p attrName, due to the possible need to apply property namespacing
    /// for Primvar.
    ///
    /// The behavior with respect to the provided \p typeName
    /// is the same as for UsdAttributes::Create().
    ///
    /// \return an invalid UsdGeomPrimvar if we failed to create a valid
    /// attribute, a valid UsdGeomPrimvar otherwise.  It is not an
    /// error to create over an existing, compatible attribute.
    ///
    /// It is a failed verification for \p prim to be invalid/expired
    ///
    /// \sa UsdPrim::CreateAttribute()
    UsdGeomPrimvar(const UsdPrim& prim, const TfToken& attrName,
                   const SdfValueTypeName &typeName);

    UsdAttribute _attr;

    // Gets or creates the indices attribute corresponding to the primvar.
    UsdAttribute _GetIndicesAttr(bool create) const;

    // Helper method for computing the flattened value of an indexed primvar.
    template<typename ScalarType>
    static bool _ComputeFlattenedHelper(const VtArray<ScalarType> &authored,
                                        const VtIntArray &indices,
                                        VtArray<ScalarType> *value,
                                        std::string *errString);
    
    // Helper function to evaluate the flattened array value of a primvar given
    // the attribute value and the indices array.
    template <typename ArrayType>
    static bool _ComputeFlattenedArray(const VtValue &attrVal,
                                       const VtIntArray &indices,
                                       VtValue *value, 
                                       std::string *errString);

    // Should only be called if _idTargetRelName is set
    UsdRelationship _GetIdTargetRel(bool create) const;

    // Compute & cache whether or not this primvar can be an idtarget.  After a
    // call to this function, _idTargetStatus will be either IdTargetImpossible
    // or IdTargetPossible.  If the result is "possible" then _idTargetRelName
    // will contain the relationship name.  This function returns true if
    // _idTargetStatus was set to IdTargetPossible, else false.
    bool _ComputeIdTargetPossibility() const;

    enum _IdTargetStatus {
        IdTargetUninitialized,
        IdTargetInitializing,
        IdTargetImpossible,
        IdTargetPossible };

    mutable TfToken _idTargetRelName;
    mutable std::atomic<_IdTargetStatus> _idTargetStatus;
};

// We instantiate the following so we can check and provide the correct value
// for Id attributes.
template <>
USDGEOM_API bool UsdGeomPrimvar::Get(std::string* value, UsdTimeCode time) const;

template <>
USDGEOM_API bool UsdGeomPrimvar::Get(VtStringArray* value, UsdTimeCode time) const;

template <>
USDGEOM_API bool UsdGeomPrimvar::Get(VtValue* value, UsdTimeCode time) const;

template <typename ScalarType>
bool 
UsdGeomPrimvar::ComputeFlattened(VtArray<ScalarType> *value, UsdTimeCode time) const
{
    VtArray<ScalarType> authored;
    if (!Get(&authored, time))
        return false;

    if (!IsIndexed()) {
        *value = authored;
        return true;
    }

    VtIntArray indices;
    if (!GetIndices(&indices, time)) {
        TF_WARN("No indices authored for indexed primvar <%s>.", 
                _attr.GetPath().GetText());
        return false;
    }

    // If the authored array is empty, there's nothing to do.
    if (authored.empty())
        return false;

    std::string errString;
    bool res = _ComputeFlattenedHelper(authored, indices, value, &errString);
    if (!errString.empty()) {
        TF_WARN("For primvar %s: %s", 
                UsdDescribe(_attr).c_str(), errString.c_str());
    }
    return res;
}

template<typename ScalarType>
bool
UsdGeomPrimvar::_ComputeFlattenedHelper(const VtArray<ScalarType> &authored,
                                        const VtIntArray &indices,
                                        VtArray<ScalarType> *value,
                                        std::string *errString)
{
    value->resize(indices.size());
    bool success = true;

    std::vector<size_t> invalidIndexPositions;
    for (size_t i=0; i < indices.size(); i++) {
        int index = indices[i];
        if (index >= 0 && (size_t)index < authored.size()) {
            (*value)[i] = authored[index];
        } else {
            invalidIndexPositions.push_back(i);
            success = false;
        }
    }

    if (!invalidIndexPositions.empty()) {
        std::vector<std::string> invalidPositionsStrVec;
        // Print a maximum of 5 invalid index positions.
        size_t numElementsToPrint = std::min(invalidIndexPositions.size(), 
                                             size_t(5));
        invalidPositionsStrVec.reserve(numElementsToPrint);
        for (size_t i = 0; i < numElementsToPrint ; ++i) {
            invalidPositionsStrVec.push_back(
                    TfStringify(invalidIndexPositions[i]));
        }

        if (errString) {
            *errString = TfStringPrintf(
                "Found %ld invalid indices at positions [%s%s] that are out of "
                "range [0,%ld).", invalidIndexPositions.size(), 
                TfStringJoin(invalidPositionsStrVec, ", ").c_str(), 
                invalidIndexPositions.size() > 5 ? ", ..." : "",
                authored.size());
        }
    }

    return success;
}


PXR_NAMESPACE_CLOSE_SCOPE

#endif // USD_PRIMVAR_H