OSL ShadingEngine : add 'shading:index' special attr to getattribute

python/GafferOSLTest/ShadingEngineTest.py

@@ -138,6 +138,15 @@ def testUserDataViaGetAttribute( self ) :
 		for i, c in enumerate( p["Ci"] ) :
 			self.assertEqual( c, rp["colorUserData"][i] )
 
+
+		e = GafferOSL.ShadingEngine( IECore.ObjectVector( [
+			IECore.Shader( shader, "surface", { "name" : "shading:index" } ),
+		] ) )
+		p = e.shade( rp )
+
+		for i, c in enumerate( p["Ci"] ) :
+			self.assertEqual( c, IECore.Color3f(i) )
+
 	def testStructs( self ) :
 
 		shader = self.compileShader( os.path.dirname( __file__ ) + "/shaders/structs.osl" )

src/GafferOSL/ShadingEngine.cpp

@@ -34,6 +34,8 @@
 //
 //////////////////////////////////////////////////////////////////////////
 
+#include <limits>
+
 #include "tbb/spin_mutex.h"
 
 #include "boost/algorithm/string/split.hpp"
@@ -154,6 +156,8 @@ DataPtr dataFromTypeDesc( TypeDesc type, void *&basePointer )
 namespace
 {
 
+OIIO::ustring gIndex( "shading:index" );
+
 class RenderState
 {
 
@@ -191,6 +195,22 @@ class RenderState
 
 		bool userData( ustring name, TypeDesc type, void *value ) const
 		{
+			if( name == gIndex )
+			{
+				// if a 4 byte type has been requested then ensure we fit and cast to narrower type
+				// this way f32 reads of shading:index will succeed.
+				if( type.size() == sizeof( int ) && m_pointIndex <= ( (size_t) std::numeric_limits<int>::max() ) )
+				{
+					int v = ( int ) m_pointIndex;
+					return ShadingSystem::convert_value( value, type, &v, OIIO::TypeDesc( OIIO::TypeDesc::INT32 ) );
+				}
+				else
+				{
+					// OSL language doesn't define UINT64 type so we'll probably never enter this branch.
+					return ShadingSystem::convert_value( value, type, &m_pointIndex, OIIO::TypeDesc( OIIO::TypeDesc::UINT64 ) );
+				}
+			}
+
 			vector<UserData>::const_iterator it = lower_bound(
 				m_userData.begin(),
 				m_userData.end(),