Cycles ShaderNetworkAlgo : Add support for normal imputs for USDPreviewSurface

Changes.md

@@ -24,6 +24,7 @@ Improvements
 - CurvesPrimitive : Added `Pinned` wrap mode in addition to the existing `Periodic` and `NonPeriodic` modes. This conveniently interpolates CatmullRom
 and BSpline curves to their endpoints automatically, without manual management of duplicate endpoints or "phantom vertices".
 - SceneReader, SceneWriter : Added support for pinned UsdGeomBasisCurves.
+- Cycles ShaderNetworkAlgo : Added support for USDPreviewSurface normal inputs.
 
 Fixes
 -----

python/GafferCyclesTest/IECoreCyclesPreviewTest/ShaderNetworkAlgoTest.py

@@ -864,6 +864,41 @@ def testConvertUSDPrimvarReader( self ) :
 			self.assertEqual( len( reader.parameters ), 1 )
 			self.assertEqual( reader.parameters["attribute"].value, "test" )
 
+	def testConvertUSDNormal( self ) :
+
+		network = IECoreScene.ShaderNetwork(
+			shaders = {
+				"previewSurface" : IECoreScene.Shader(
+					"UsdPreviewSurface", "surface"
+				)
+			},
+			output = "previewSurface",
+		)
+
+		network.addShader( "texture", IECoreScene.Shader( "UsdUVTexture" ) )
+		network.addConnection( ( ( "texture", "color" ), ( "previewSurface", "normal" ) ) )
+
+		convertedNetwork = network.copy()
+		IECoreCycles.ShaderNetworkAlgo.convertUSDShaders( convertedNetwork )
+
+		self.assertEqual( len( convertedNetwork ), 4 )
+
+		normalize = convertedNetwork.getShader( "previewSurfaceNormalize" )
+		self.assertEqual( normalize.parameters["math_type"].value, "normalize" )
+
+		self.assertEqual(
+			convertedNetwork.input( ( "previewSurface", "normal" ) ),
+			( "previewSurfaceNormalize", "vector" ),
+		)
+		self.assertEqual(
+			convertedNetwork.input( ( "previewSurfaceNormalize", "value1" ) ),
+			( "previewSurfaceNormalMap", "normal" ),
+		)
+		self.assertEqual(
+			convertedNetwork.input( ( "previewSurfaceNormalMap", "color" ) ),
+			( "texture", "color" ),
+		)
+
 	def __assertShadersEqual( self, shader1, shader2, message = None ) :
 
 		self.assertEqual( shader1.name, shader2.name, message )

src/GafferCycles/IECoreCyclesPreview/ShaderNetworkAlgo.cpp

@@ -1314,9 +1314,20 @@ void IECoreCycles::ShaderNetworkAlgo::convertUSDShaders( ShaderNetwork *shaderNe
 			newShader->parameters()[g_alphaParameter] = new FloatData( opacity );
 
 			// Normal.
-			/// \todo Convert normal parameters once we have a solution for Cycles'
-			/// need for tangents to be provided for the correct use of normal maps.
-			removeInput( shaderNetwork, { handle, g_normalParameter } );
+			if( const ShaderNetwork::Parameter normalInput = shaderNetwork->input( { handle, g_normalParameter } ) )
+			{
+				ShaderPtr normalmapShader = new Shader( "normal_map", "cycles:shader" );
+				const InternedString normalmapHandle = shaderNetwork->addShader( handle.string() + "NormalMap", std::move( normalmapShader ) );
+				// ALab has perfect examples where it makes invalid normals from scale/bias tweaks,
+				// so if we don't normalise the output Cycles will hard-crash.
+				ShaderPtr normalizeShader = new Shader( "vector_math", "cycles:shader" );
+				normalizeShader->parameters()[g_mathTypeParameter] = new StringData( "normalize" );
+				const InternedString normalizeHandle = shaderNetwork->addShader( handle.string() + "Normalize", std::move( normalizeShader ) );
+				shaderNetwork->addConnection( ShaderNetwork::Connection( normalInput, { normalmapHandle, g_colorParameter } ) );
+				shaderNetwork->addConnection( ShaderNetwork::Connection( { normalmapHandle, g_normalParameter }, { normalizeHandle, g_value1Parameter } ) );
+				shaderNetwork->removeConnection( ShaderNetwork::Connection( normalInput, { handle, g_normalParameter } ) );
+				shaderNetwork->addConnection( ShaderNetwork::Connection( { normalizeHandle, g_vectorParameter }, { handle, g_normalParameter } ) );
+			}
 
 			// Remove occlusion.
 			removeInput( shaderNetwork, { handle, g_occlusionParameter } );