AttributeTweaks : Add fallback for linkedLights attribute value

This is necessary to make `{source}` substitutions useable in all situations. As noted in the comment, we should really do this for _all_ attributes using a central registry of default values. We're just doing this one on its own because it is such high value - `linkedLights` is the main thing we had in mind when adding the `{source}` token in the first place.

Note that once we have fallbacks for all standard and renderer-specific attributes, the `ignoreMissing` plug will only be relevant for custom attributes not present in the registry. I view this as a good thing. If I make a `Multiply` tweak to double `ai:AA_samples`, it's not because I want to double it only if it exists. Rather, it's because I want to double the value that is received by the renderer.

Changes.md

@@ -5,12 +5,14 @@ Improvements
 ------------
 
 - CameraTweaks : Added `ignoreMissing` plug to align behaviour with the other Tweaks nodes.
+- AttributeTweaks : The `{source}` substitution for `linkedLights` now expands to `defaultLights` if the attribute doesn't exist yet. This makes tweaks such as `({source}) - unwantedLights` reliable even if no light links have been authored yet.
 
 Breaking Changes
 ----------------
 
 - CameraTweaks : `Replace` mode now errors if the input parameter does not exist. Use `Create` mode or the new `ignoreMissing` plug instead.
 - TweakPlug : Remove deprecated `MissingMode::IgnoreOrReplace`.
+- AttributeTweaks : `Replace` mode no longer errors if the `linkedLights` attribute doesn't exist.
 
 1.4.x.x (relative to 1.4.1.0)
 =======

python/GafferSceneTest/AttributeTweaksTest.py

@@ -202,6 +202,30 @@ def testCreateMode( self ) :
 
 		self.assertEqual( tweaks["out"].attributes( "/plane" )["test:attribute"], IECore.IntData( 2 ) )
 
+	def testLinkedLightsSourceSubstitution( self ) :
+
+		plane = GafferScene.Plane()
+
+		planeFilter = GafferScene.PathFilter()
+		planeFilter["paths"].setValue( IECore.StringVectorData( [ "/plane" ] ) )
+
+		standardAttributes = GafferScene.StandardAttributes()
+		standardAttributes["in"].setInput( plane["out"] )
+		standardAttributes["filter"].setInput( planeFilter["out"] )
+
+		self.assertNotIn( "linkedLights", standardAttributes["out"].attributes( "/plane" ) )
+
+		tweaks = GafferScene.AttributeTweaks()
+		tweaks["in"].setInput( standardAttributes["out"] )
+		tweaks["filter"].setInput( planeFilter["out"] )
+
+		testTweak = Gaffer.TweakPlug( "linkedLights", "({source}) - unwantedLights" )
+		tweaks["tweaks"].addChild( testTweak )
+		self.assertEqual( tweaks["out"].attributes( "/plane" )["linkedLights"], IECore.StringData( "(defaultLights) - unwantedLights" ) )
+
+		standardAttributes["attributes"]["linkedLights"]["enabled"].setValue( True )
+		standardAttributes["attributes"]["linkedLights"]["value"].setValue( "someLights" )
+		self.assertEqual( tweaks["out"].attributes( "/plane" )["linkedLights"], IECore.StringData( "(someLights) - unwantedLights" ) )
 
 if __name__ == "__main__" :
 	unittest.main()

src/GafferScene/AttributeTweaks.cpp

@@ -43,6 +43,13 @@ using namespace IECore;
 using namespace Gaffer;
 using namespace GafferScene;
 
+namespace
+{
+
+const ConstStringDataPtr g_linkedLightsDefault = new StringData( "defaultLights" );
+
+} // namespace
+
 GAFFER_NODE_DEFINE_TYPE( AttributeTweaks );
 
 size_t AttributeTweaks::g_firstPlugIndex = 0;
@@ -147,9 +154,16 @@ IECore::ConstCompoundObjectPtr AttributeTweaks::computeProcessedAttributes( cons
 	}
 
 	tweaksPlug->applyTweaks(
-		[&source]( const std::string &valueName )
+		[&source]( const std::string &valueName ) -> const IECore::Data *
 		{
-			return source->member<Data>( valueName );
+			const Data *result = source->member<Data>( valueName );
+			if( !result && valueName == "linkedLights" )
+			{
+				/// \todo Use a registry to provide default values for
+				/// all attributes.
+				return g_linkedLightsDefault.get();
+			}
+			return result;
 		},
 		[&result]( const std::string &valueName, DataPtr newData )
 		{
@@ -164,4 +178,4 @@ IECore::ConstCompoundObjectPtr AttributeTweaks::computeProcessedAttributes( cons
 	);
 
 	return result;
-}
+}