ValueMightBeTimeVarying now takes splines into account

- Updated UsdStage::_ValueMightBeTimeVaryingFromResolveInfo, which
  determines if the resolveInfo's source is UsdResolveInfoSourceSpline,
  in which case it returns true.
  Do note that we do not inspect the spline itself here to determine if
  the spline represents a constant in time function, as doing so will be
  expensive. Our aim is to give the clients a hint that the attribute
  you are dealing with, "might" be time varying. This is similar to what
  is done with TimeSamples, where we don't go and inspect every time
  samples on an attribute to determine if its constant across time or
  not.

- Note that UsdAttributeQuery's ValueMightBeTimeVarying will
  automatically work since it relies on the
  UsdStage::_ValueMightBeTimeVaryingFromResolveInfo itself.

- Updated testUsdSplines* to test the modified API.

(Internal change: 2357151)

pxr/usd/usd/attribute.h

@@ -35,7 +35,7 @@ typedef std::vector<UsdAttribute> UsdAttributeVector;
 /// \class UsdAttribute
 ///
 /// Scenegraph object for authoring and retrieving numeric, string, and array
-/// valued data, sampled over time.
+/// valued data, sampled over time, or animated by a spline
 ///
 /// The allowed value types for UsdAttribute are dictated by the Sdf
 /// ("Scene Description Foundations") core's data model, which we summarize in
@@ -46,15 +46,15 @@ typedef std::vector<UsdAttribute> UsdAttributeVector;
 /// In addition to its value type, an Attribute has two other defining
 /// qualities:
 /// \li <b>Variability</b> Expresses whether an attribute is intended to
-/// have time samples (GetVariability() == \c SdfVariabilityVarying), or only
-/// a default (GetVariability() == \c SdfVariabilityUniform).  For more on
-/// reasoning about time samples, 
+/// have time samples or a spline (GetVariability() == \c SdfVariabilityVarying), 
+/// or only a default (GetVariability() == \c SdfVariabilityUniform).  For more 
+/// on reasoning about time samples, 
 /// see \ref Usd_AttributeValueMethods "Value & Time-Sample Accessors".
 ///
 /// \li <b>Custom</b> Determines whether an attribute belongs to a
 /// schema (IsCustom() == \c false), or is a user-defined, custom attribute.
 /// schema attributes will always be defined on a prim of the schema type,
-/// ans may possess fallback values from the schema, whereas custom 
+/// and may possess fallback values from the schema, whereas custom 
 /// attributes must always first be authored in order to be defined.  Note
 /// that \em custom is actually an aspect of UsdProperty, as UsdRelationship
 /// can also be custom or provided by a schema.
@@ -90,6 +90,10 @@ typedef std::vector<UsdAttribute> UsdAttributeVector;
 /// The desired behavior may be specified via UsdStage::SetInterpolationType.
 /// That behavior will be used for all calls to UsdAttribute::Get.
 ///
+/// Note that for attributes with spline value sources, the interpolation
+/// behavior is determined by the spline itself, and the interpolation type
+/// set on the stage is ignored.
+///
 /// The supported interpolation types are:
 ///
 /// \li <b>Held</b> Attribute values are held constant between authored
@@ -169,8 +173,8 @@ class UsdAttribute : public UsdProperty {
     /// @{
 
     /// An attribute's variability expresses whether it is intended to have
-    /// time-samples (\c SdfVariabilityVarying), or only a single default 
-    /// value (\c SdfVariabilityUniform).
+    /// time-samples or splines (\c SdfVariabilityVarying), or only a single 
+    /// default value (\c SdfVariabilityUniform).
     ///
     /// Variability is required meta-data of all attributes, and its fallback
     /// value is SdfVariabilityVarying.
@@ -374,9 +378,10 @@ class UsdAttribute : public UsdProperty {
     /// If this function returns false, it is certain that this attribute's
     /// value remains constant over time.
     ///
-    /// This function is equivalent to checking if GetNumTimeSamples() > 1,
-    /// but may be more efficient since it does not actually need to get a
-    /// full count of all time samples.
+    /// This function checks if the attribute either has more than 1 time
+    /// samples or is spline valued. Which is more efficient than actually
+    /// counting the time samples or evaluating the spline, both of which
+    /// are potentially expensive operations.
     USD_API
     bool ValueMightBeTimeVarying() const;
 

pxr/usd/usd/stage.cpp

@@ -9397,6 +9397,13 @@ bool
 UsdStage::_ValueMightBeTimeVaryingFromResolveInfo(const UsdResolveInfo &info,
                                                   const UsdAttribute &attr) const
 {
+    if (info._source == UsdResolveInfoSourceSpline) {
+        // Although a spline could represent a constant function, determining
+        // this would require analyzing the spline, which is potentially 
+        // expensive. Hence, all splines are deemed as possibly time varying.
+        return true;
+    }
+
     if (info._source == UsdResolveInfoSourceValueClips) {
         // Do a specialized check for value clips instead of falling through
         // to calling _GetNumTimeSamplesFromResolveInfo, which requires opening

pxr/usd/usd/testenv/testUsdSplines.py

@@ -53,8 +53,10 @@ def _DoSerializationTest(
             prim = stage.DefinePrim(Sdf.Path("/MyPrim"))
             attr = prim.CreateAttribute("myAttr", attrType)
             self.assertFalse(attr.HasSpline())
+            self.assertFalse(attr.ValueMightBeTimeVarying())
             attr.SetSpline(spline)
             self.assertTrue(attr.HasSpline())
+            self.assertTrue(attr.ValueMightBeTimeVarying())
             print(f"Original spline, {case}, {format}:")
             print(spline)
 
@@ -230,7 +232,11 @@ def _DoLayerOffsetTest(self, case, attrType, timeValued, scale):
 
         prim = stage.DefinePrim(Sdf.Path("/MyPrim"))
         attr = prim.CreateAttribute("myAttr", attrType)
+        self.assertFalse(attr.HasSpline())
+        self.assertFalse(attr.ValueMightBeTimeVarying())
         attr.SetSpline(spline)
+        self.assertTrue(attr.HasSpline())
+        self.assertTrue(attr.ValueMightBeTimeVarying())
 
         sdfAttr = deepLayer.GetAttributeAtPath("/MyPrim.myAttr")
         sdfSpline = sdfAttr.GetInfo("spline")
@@ -283,6 +289,8 @@ def test_InvalidType(self):
         spline = self._GetTestSpline()
 
         gotException = False
+        self.assertFalse(attr.HasSpline())
+        self.assertFalse(attr.ValueMightBeTimeVarying())
         try:
             attr.SetSpline(spline)
         except Tf.ErrorException as e:
@@ -291,8 +299,9 @@ def test_InvalidType(self):
             print(e)
         except:
             pass
-
         self.assertTrue(gotException)
+        self.assertFalse(attr.HasSpline())
+        self.assertFalse(attr.ValueMightBeTimeVarying())
 
 
 if __name__ == "__main__":

pxr/usd/usd/testenv/testUsdSplinesCpp.cpp

@@ -114,8 +114,11 @@ _DoSerializationTest(
         const UsdPrim prim = stage->DefinePrim(SdfPath("/MyPrim"));
         UsdAttribute attr = prim.CreateAttribute(TfToken("myAttr"), attrType);
         TF_AXIOM(!attr.HasSpline());
+        TF_AXIOM(!attr.ValueMightBeTimeVarying());
         attr.SetSpline(spline);
         TF_AXIOM(attr.HasSpline());
+        // Having a spline makes this attr might be time varying.
+        TF_AXIOM(attr.ValueMightBeTimeVarying());
 
         stage->Save();
         stage->GetRootLayer()->Export(filename2);
@@ -158,7 +161,11 @@ _DoLayerOffsetTest(
 
     const UsdPrim prim = stage->DefinePrim(SdfPath("/MyPrim"));
     UsdAttribute attr = prim.CreateAttribute(TfToken("myAttr"), attrType);
+    TF_AXIOM(!attr.HasSpline());
+    TF_AXIOM(!attr.ValueMightBeTimeVarying());
     attr.SetSpline(spline);
+    TF_AXIOM(attr.HasSpline());
+    TF_AXIOM(attr.ValueMightBeTimeVarying());
 
     _TestSplineAndAttr(spline, attr);
 
@@ -362,10 +369,14 @@ TestInvalidType()
     const TsSpline spline = _GetTestSpline();
 
     TfErrorMark m;
+    TF_AXIOM(!attr.HasSpline());
     attr.SetSpline(spline);
+
     // A coding error should have been posted as String value splines are not
     // allowed. Only double, float or GfHalf!
     TF_AXIOM(!m.IsClean());
+    TF_AXIOM(!attr.HasSpline());
+    TF_AXIOM(!attr.ValueMightBeTimeVarying());
 }
 
 int main()