Deprecate negative offset scale during composition

Cummulative negative layer offset scale on a composed layer doesn't make
sense, as it reverses the direction of time, and can lead to incorrect
and non-intuitive results specially with various spline evaluation
behaviors and with time samples with soon to be implemented preValue
support.

Introduced PCP_ALLOW_NEGATIVE_LAYER_OFFSET_SCALE environment variable,
which currently defaults to true, allowing the use of negative layer offset,
however will issue a warning. A false value, will result in a coding error.

(Internal change: 2357711)

pxr/usd/pcp/CMakeLists.txt

@@ -119,6 +119,7 @@ pxr_test_scripts(
     testenv/testPcpResolvedPathChange.py
     testenv/testPcpStreamingLayerReload.py
     testenv/testPcpCompositionResults.py
+    testenv/testPcpNegativeOffsetScale.py
 )
 
 pxr_build_test_shared_lib(TestPcpDynamicFileFormatPlugin
@@ -2162,3 +2163,17 @@ pxr_register_test(testPcpLayerRelocatesEditBuilder
     COMMAND "${CMAKE_INSTALL_PREFIX}/tests/testPcpLayerRelocatesEditBuilder"
     EXPECTED_RETURN_CODE 0
 )
+
+pxr_register_test(testPcpNegativeOffsetScaleAllowed
+    PYTHON
+    COMMAND "${CMAKE_INSTALL_PREFIX}/tests/testPcpNegativeOffsetScale"
+    EXPECTED_RETURN_CODE 0
+)
+
+pxr_register_test(testPcpNegativeOffsetScale
+    PYTHON
+    COMMAND "${CMAKE_INSTALL_PREFIX}/tests/testPcpNegativeOffsetScale"
+    EXPECTED_RETURN_CODE 0
+    ENV 
+        PCP_ALLOW_NEGATIVE_LAYER_OFFSET_SCALE=0
+)

pxr/usd/pcp/layerStack.cpp

@@ -52,6 +52,15 @@ TF_DEFINE_ENV_SETTING(
     "non-USD caches/layer stacks; the legacy behavior cannot be enabled in USD "
     "mode");
 
+TF_DEFINE_ENV_SETTING(
+    PCP_ALLOW_NEGATIVE_LAYER_OFFSET_SCALE, true,
+    "Enables the use of negative layer offset scale. This behavior is "
+    "deprecated and a warning will be issued if this setting is enabled, "
+    "otherwise a coding error will be issued. Negative layer offset scale on a "
+    "composed property doesn't make sense, as it reverses the direction of "
+    "time, and can lead to incorrect and non intuitive results.");
+
+
 struct Pcp_SublayerInfo {
     Pcp_SublayerInfo() = default;
     Pcp_SublayerInfo(const SdfLayerRefPtr& layer_, const SdfLayerOffset& offset_,
@@ -1649,6 +1658,14 @@ PcpLayerStack::_Compute(const std::string &fileFormatTarget,
     }
 }
 
+bool
+PcpNegativeLayerOffsetScaleAllowed()
+{
+    static bool allowed = 
+        TfGetEnvSetting(PCP_ALLOW_NEGATIVE_LAYER_OFFSET_SCALE);
+    return allowed;
+}
+
 SdfLayerTreeHandle
 PcpLayerStack::_BuildLayerStack(
     const SdfLayerHandle & layer,
@@ -1786,8 +1803,20 @@ PcpLayerStack::_BuildLayerStack(
 
         // Check sublayer offset.
         SdfLayerOffset sublayerOffset = sublayerOffsets[i];
-        if (!sublayerOffset.IsValid()
-            || !sublayerOffset.GetInverse().IsValid()) {
+
+        const bool isNegativeScale = sublayerOffset.GetScale() < 0.0;
+        const bool negativeScaleAllowed = PcpNegativeLayerOffsetScaleAllowed();
+
+        if (isNegativeScale && negativeScaleAllowed) {
+            // Report warning.
+            TF_WARN("Layer @%s@ has a negative offset scale. Negative scale "
+                    "offsets are deprecated.",
+                    layer->GetIdentifier().c_str());
+        }
+
+        if ((isNegativeScale && !negativeScaleAllowed) ||
+            !sublayerOffset.IsValid() || 
+            !sublayerOffset.GetInverse().IsValid()) {
             // Report error, but continue with an identity layer offset.
             PcpErrorInvalidSublayerOffsetPtr err =
                 PcpErrorInvalidSublayerOffset::New();

pxr/usd/pcp/layerStack.h

@@ -338,6 +338,16 @@ std::ostream& operator<<(std::ostream&, const PcpLayerStackPtr&);
 PCP_API
 std::ostream& operator<<(std::ostream&, const PcpLayerStackRefPtr&);
 
+/// Returns true if negative layer offsets and scales are allowed.
+///
+/// Negative layer offset scales are deprecated and a warning will be issued
+/// when commulative scale during composition is negative with 
+/// PCP_ALLOW_NEGATIVE_LAYER_OFFSET_SCALE is set to true (default right now).
+/// If PCP_ALLOW_NEGATIVE_LAYER_OFFSET_SCALE is set to false, a coding error
+/// will be issued when a negative scale is encountered.
+PCP_API
+bool PcpNegativeLayerOffsetScaleAllowed();
+
 /// Checks if the source and target paths constitute a valid relocates. This
 /// validation is not context specific, i.e. if this returns false, the 
 /// combination of source and target paths is always invalid for any attempted

pxr/usd/pcp/primIndex.cpp

@@ -2123,8 +2123,20 @@ _EvalRefOrPayloadArcs(PcpNodeRef node,
             fail = true;
         }
 
+        const bool isNegativeScale = layerOffset.GetScale() < 0.0;
+        const bool negativeScaleAllowed = PcpNegativeLayerOffsetScaleAllowed();
+
+        if (isNegativeScale && negativeScaleAllowed) {
+            TF_WARN("Found negative scale in layer offset for %s to @%s@<%s>. "
+                    "Negative offset scale is deprecated.",
+                    ARC_TYPE == PcpArcTypePayload ? "payload" : "reference",
+                    info.authoredAssetPath.c_str(), 
+                    refOrPayload.GetPrimPath().GetText());
+        }
+
         // Validate layer offset in original reference or payload.
-        if (!layerOffset.IsValid() ||
+        if ((isNegativeScale && !negativeScaleAllowed) ||
+            !layerOffset.IsValid() ||
             !layerOffset.GetInverse().IsValid()) {
             PcpErrorInvalidReferenceOffsetPtr err =
                 PcpErrorInvalidReferenceOffset::New();

pxr/usd/pcp/testenv/testPcpNegativeOffsetScale.py

@@ -0,0 +1,66 @@
+#!/pxrpythonsubst
+#
+# Copyright 2025 Pixar
+#
+# Licensed under the terms set forth in the LICENSE.txt file available at
+
+import unittest
+
+from pxr import Pcp, Sdf, Tf
+
+def _ComposeLayersWithNegativeOffsetScale():
+    refLayer = Sdf.Layer.CreateAnonymous("ref")
+    refLayer.ImportFromString('''
+    #sdf 1.4.32
+
+    def "prim" {
+        double attr.timeSamples = {
+            0: 1.0,
+            1: 2.0,
+        }
+    }
+    '''.strip())
+
+    subLayer = Sdf.Layer.CreateAnonymous("sub")
+    subLayer.ImportFromString(f'''
+    #sdf 1.4.32
+    (
+        subLayers = [
+            @{refLayer.identifier}@ (scale = -1)
+        ]
+    )
+    '''.strip())
+
+    rootLayer = Sdf.Layer.CreateAnonymous()
+    rootLayer.ImportFromString(f'''
+    #sdf 1.4.32
+
+    def "prim" (
+        references = @{subLayer.identifier}@</prim> (scale = -1)
+    )
+    {{
+    }}
+    '''.strip())
+
+    pcpCache = Pcp.Cache(Pcp.LayerStackIdentifier(rootLayer))
+    _, errs = pcpCache.ComputePrimIndex("/prim")
+    return errs
+
+class TestPcpNegativeLayerOffsetScale(unittest.TestCase):
+
+    # Following will not result in a composition error
+    @unittest.skipIf(not Tf.GetEnvSetting("PCP_ALLOW_NEGATIVE_LAYER_OFFSET_SCALE"),
+                     "Allow negative layer offset scale, no composition error")
+    def test_NegativeLayerOffsetScaleAllowed(self):
+        errs = _ComposeLayersWithNegativeOffsetScale()
+        self.assertEqual(len(errs), 0)
+
+    # Following will result in a composition error
+    @unittest.skipIf(Tf.GetEnvSetting("PCP_ALLOW_NEGATIVE_LAYER_OFFSET_SCALE"),
+                     "Do not allow negative layer offset scale, composition error")
+    def test_NegativeLayerOffsetScaleNotAllowed(self):
+        errs = _ComposeLayersWithNegativeOffsetScale()
+        self.assertEqual(len(errs), 2)
+
+if __name__ == "__main__":
+    unittest.main()