ArnoldShader : Support camera_projection.camera parameter

This is just a case of setting the `gaffer.plugType` to "StringPlug" and letting the recently added `NodeParameter` mechanism do the rest in `ArnoldRenderer`.

Also added a nice camera browser using ScenePathPlugValueWidget. I was unsure what to do for this in ArnoldShaderUI, as the "camera" value for "widget" isn't part of the OSL standard. The other option would have been to support three new Gaffer-specific metadata entries for `gaffer.plugValueWidget.type`, `scenePathPlugValueWidget.setNames` and `scenePathPlugValueWidget:setsLabel`. But that seemed quite faffy when we don't have any other use cases for them at present, and "camera" seems like something that could reasonably be supported by other DCCs.

Changes.md

@@ -4,6 +4,7 @@
 Improvements
 ------------
 
+- ArnoldShader : Improved support for camera projections by exposing the `camera` plug on the `camera_projection` shader.
 - Instancer :
   - Improved scene generation for encapsulated instancers significantly, with some production scenes now generating 5-7x faster.
   - Added `omitDuplicateIds` plug, to determine whether points with duplicate IDs are ignored or should trigger an error.
@@ -14,6 +15,7 @@ API
 
 - Capsule : Added protected `renderOptions()` and `throwIfNoScene()` methods.
 - ScenePath : Added support for passing `nullptr` for the scene.
+- ArnoldShaderUI : Added support for `camera` widget type metadata, to add a camera browser to a string parameter.
 
 1.3.6.1 (relative to 1.3.6.0)
 =======

arnoldPlugins/gaffer.mtd

@@ -1029,7 +1029,8 @@
 	gaffer.layout.activator.coordSpaceIsPref STRING "parameters['coord_space'].getValue() == 'Pref'"
 
 	[attr camera]
-		gaffer.plugType STRING ""
+		gaffer.plugType STRING "StringPlug"
+		widget STRING "camera"
 
 	[attr pref_name]
 		gaffer.layout.activator STRING "coordSpaceIsPref"

python/GafferArnoldTest/ArnoldShaderTest.py

@@ -937,5 +937,15 @@ def testLoadQuadLight( self ) :
 		self.assertTrue( shader["parameters"]["width"].isSetToDefault() )
 		self.assertTrue( shader["parameters"]["height"].isSetToDefault() )
 
+	def testLoadCameraProjection( self ) :
+
+		shader = GafferArnold.ArnoldShader()
+		shader.loadShader( "camera_projection" )
+
+		self.assertIn( "camera", shader["parameters"] )
+		self.assertIsInstance( shader["parameters"]["camera"], Gaffer.StringPlug )
+		self.assertEqual( shader["parameters"]["camera"].getValue(), "" )
+		self.assertEqual( shader["parameters"]["camera"].defaultValue(), "" )
+
 if __name__ == "__main__":
 	unittest.main()

python/GafferArnoldUI/ArnoldShaderUI.py

@@ -287,9 +287,14 @@ def __translateNodeMetadata( nodeEntry ) :
 				"popup" : "GafferUI.PresetsPlugValueWidget",
 				"mapper" : "GafferUI.PresetsPlugValueWidget",
 				"filename" : "GafferUI.PathPlugValueWidget",
+				"camera" : "GafferSceneUI.ScenePathPlugValueWidget",
 				"null" : "",
 			}[widget]
 
+			if widget == "camera" :
+				__metadata[paramPath]["scenePathPlugValueWidget:setNames"] = IECore.StringVectorData( [ "__cameras" ] )
+				__metadata[paramPath]["scenePathPlugValueWidget:setsLabel"] = "Show only cameras"
+
 		# Layout section from OSL "page".
 
 		page = __aiMetadataGetStr( nodeEntry, paramName, "page" )

src/GafferArnold/ParameterHandler.cpp

@@ -62,6 +62,7 @@ using namespace GafferArnold;
 namespace
 {
 
+const AtString g_emptyArnoldString( "" );
 const AtString g_nameArnoldString( "name" );
 const AtString g_gafferPlugTypeArnoldString( "gaffer.plugType" );
 const AtString g_gafferDefaultArnoldString( "gaffer.default" );
@@ -70,6 +71,7 @@ const AtString g_FloatPlugArnoldString( "FloatPlug" );
 const AtString g_Color3fPlugArnoldString( "Color3fPlug" );
 const AtString g_Color4fPlugArnoldString( "Color4fPlug" );
 const AtString g_ClosurePlugArnoldString( "ClosurePlug" );
+const AtString g_StringPlugArnoldString( "StringPlug" );
 
 const AtString g_quadLightShaderName( "quad_light" );
 
@@ -403,6 +405,10 @@ Gaffer::Plug *ParameterHandler::setupPlug( const AtNodeEntry *node, const AtPara
 		{
 			parameterType = AI_TYPE_CLOSURE;
 		}
+		else if( plugTypeOverride == g_StringPlugArnoldString )
+		{
+			parameterType = AI_TYPE_STRING;
+		}
 		else
 		{
 			msg(
@@ -512,7 +518,19 @@ Gaffer::Plug *ParameterHandler::setupPlug( const AtNodeEntry *node, const AtPara
 		case AI_TYPE_STRING :
 
 			{
-				AtString defaultValue = AiParamGetDefault( parameter )->STR();
+				AtString defaultValue;
+				if( AiParamGetType( parameter ) == AI_TYPE_STRING )
+				{
+					defaultValue = AiParamGetDefault( parameter )->STR();
+				}
+				else
+				{
+					// We get here when `plugTypeOverride` causes us to treat a
+					// non-string parameter as a string. In this case, Arnold's
+					// default value won't be a valid `AtString` so we must
+					// ignore it.
+					defaultValue = g_emptyArnoldString;
+				}
 				AiMetaDataGetStr( node, name, g_gafferDefaultArnoldString, &defaultValue );
 				plug = setupTypedPlug<StringPlug>(
 					node,