MeshAlgo : Anchor the first vertex in MeshAlgo::reverseWinding()

Changes

@@ -1,7 +1,15 @@
 10.7.x.x (relative to 10.7.1.3)
 ========
 
+Improvements
+------------
+
+- MeshAlgo : Changed `reverseWinding()` so that the first vertex of each face remains unchanged. This matches the algorithm used in other DCCs.
+
+Breaking Changes
+----------------
 
+- MeshAlgo : Changed the output of `reverseWinding()`.
 
 10.7.1.3 (relative to 10.7.1.2)
 ========

contrib/IECoreAlembic/test/IECoreAlembic/AlembicSceneTest.py

@@ -294,10 +294,10 @@ def testConvertArbGeomParams( self ) :
 		self.assertEqual(
 			m["colorSet1"].expandedData(),
 			IECore.Color4fVectorData( [
+				imath.Color4f( 0, 1, 0, 1 ),
 				imath.Color4f( 1, 0, 0, 1 ),
 				imath.Color4f( 0, 0, 0, 1 ),
 				imath.Color4f( 0, 0, 1, 1 ),
-				imath.Color4f( 0, 1, 0, 1 ),
 			] )
 		)
 

src/IECoreScene/MeshAlgoWinding.cpp

@@ -56,7 +56,14 @@ void reverseWinding( MeshPrimitive *mesh, T &values, const Canceller *canceller
 	for( PolygonIterator it = mesh->faceBegin(), eIt = mesh->faceEnd(); it != eIt; ++it )
 	{
 		Canceller::check( canceller );
-		std::reverse( it.faceVaryingBegin( values.begin() ), it.faceVaryingEnd( values.begin() ) );
+		auto b = it.faceVaryingBegin( values.begin() );
+		auto e = it.faceVaryingEnd( values.begin() );
+		if( b == e )
+		{
+			continue;
+		}
+		++b;
+		std::reverse( b, e );
 	}
 }
 

test/IECoreScene/MeshAlgoWindingTest.py

@@ -78,8 +78,8 @@ def testSingleTriangle( self ) :
 
 		# Except for vertex ids, and facevarying data
 
-		self.assertEqual( list( meshReversed.vertexIds ), list( reversed( mesh.vertexIds ) ) )
-		self.assertEqual( list( meshReversed["uv"].data ), list( reversed( mesh["uv"].data ) ) )
+		self.assertEqual( list( meshReversed.vertexIds ), list( mesh.vertexIds[:1] ) + list( reversed( mesh.vertexIds[1:] ) ) )
+		self.assertEqual( list( meshReversed["uv"].data ), list( mesh["uv"].data[:1] ) + list( reversed( mesh["uv"].data[1:] ) ) )
 
 	def testPlane( self ) :
 
@@ -142,7 +142,7 @@ def testUVIndices( self ) :
 		# UV indices should change, but UV data doesn't need to
 
 		self.assertEqual( meshReversed["uv"].data, mesh["uv"].data )
-		self.assertEqual( list( meshReversed["uv"].indices ), list( reversed( mesh["uv"].indices ) ) )
+		self.assertEqual( list( meshReversed["uv"].indices ), list( mesh["uv"].indices[:1] ) + list( reversed( mesh["uv"].indices[1:] ) ) )
 
 	def testReferencedData( self ) :