ShaderNetwork : Allow escaped string substitutions to work correctly

Changes

@@ -1,7 +1,15 @@
 10.6.x.x (relative to 10.6.1.0)
 ========
 
+Improvements
+------------
+
+- ShaderNetwork : Optimised applySubstitutions() for cases where not all shaders need them.
+
+Fixes
+-----
 
+ - ShaderNetwork::hashSubstitutions : Fixed shader string substitutions that are escaped with backslashes ( ie. `myText\<substitutionHandledByArnold\>` ).
 
 10.6.1.0 (relative to 10.6.0.2)
 ========

include/IECoreScene/ShaderNetwork.h

@@ -197,7 +197,7 @@ class IECORESCENE_API ShaderNetwork : public IECore::BlindDataHolder
 		///
 		/// We support special syntax that allows you to substitute string attributes
 		/// into the values of string parameters on shaders.
-		/// 
+		///
 		/// If a string parameter, or string vector parameter, contains the token
 		/// <attr:PARAMETER_NAME>, then it will be subsituted with the value of a
 		/// string attribute named PARAMETER_NAME.  If there is no attribute named
@@ -208,7 +208,7 @@ class IECORESCENE_API ShaderNetwork : public IECore::BlindDataHolder
 		/// you can escape the angle brackets with backslashes, like
 		/// "\<attr:PARAMETER_NAME\>"
 
-		/// Appends all attributes used by `applySubstitutions()` into the hash.
+		/// Appends to the hash to reflect all changes made by applySubstitutions( attributes ).
 		void hashSubstitutions( const IECore::CompoundObject *attributes, IECore::MurmurHash &h ) const;
 
 		/// Apply substitutions to all string and string vector parameters in the network,

src/IECoreScene/ShaderNetwork.cpp

@@ -447,6 +447,13 @@ class ShaderNetwork::Implementation
 					h.append( 0 );
 				}
 			}
+
+			if( m_parmsNeedingSubstitution.size() && !m_neededSubstitutions.size() )
+			{
+				// We don't depend on any attributes, but some parameters have escaped substitutions.
+				// Modify hash to reflect the fact that applySubstitutions() will remove the escaping.
+				h.append( true );
+			}
 		}
 
 		void applySubstitutions( const CompoundObject *attributes )
@@ -745,7 +752,10 @@ class ShaderNetwork::Implementation
 						}
 					}
 
-					m_parmsNeedingSubstitution[ node.handle ] = parmsNeedingSub;
+					if( parmsNeedingSub.size() )
+					{
+						m_parmsNeedingSubstitution[ node.handle ] = parmsNeedingSub;
+					}
 				}
 
 				m_hash.append( m_output.shader );

test/IECoreScene/ShaderNetworkTest.py

@@ -591,6 +591,8 @@ def testSubstitutions( self ) :
 		} ) )
 		( h6, sSubst6 ) = self.__hashAndSubstitution( s, {} )
 		( h7, sSubst7 ) = self.__hashAndSubstitution( s, allAttributes )
+		self.assertNotEqual( h6, IECore.MurmurHash() )
+		self.assertNotEqual( h7, IECore.MurmurHash() )
 		self.assertEqual( h6, h7 )
 		self.assertEqual( sSubst6, sSubst7 )
 		self.assertEqual( sSubst6.parameters["a"].value, "pre<attr:fred>post" )