Use the last triangle vertex for flat shading on OpenGL.

src/OpenGL/libGL/Device.cpp

@@ -29,7 +29,7 @@ namespace gl
 {
 	using namespace sw;
 
-	Device::Device(Context *context) : Renderer(context, true, true, true, true, true), context(context)
+	Device::Device(Context *context) : Renderer(context, OpenGL, true), context(context)
 	{
 		depthStencil = 0;
 		renderTarget = 0;

src/OpenGL/libGLES_CM/Device.cpp

@@ -29,7 +29,7 @@ namespace es1
 {
 	using namespace sw;
 
-	Device::Device(Context *context) : Renderer(context, true, true, true, true, true), context(context)
+	Device::Device(Context *context) : Renderer(context, OpenGL, true), context(context)
 	{
 		depthStencil = 0;
 		renderTarget = 0;

src/OpenGL/libGLESv2/Device.cpp

@@ -29,7 +29,7 @@ namespace es2
 {
 	using namespace sw;
 
-	Device::Device(Context *context) : Renderer(context, true, true, true, true, true), context(context)
+	Device::Device(Context *context) : Renderer(context, OpenGL, true), context(context)
 	{
 		depthStencil = 0;
 		renderTarget = 0;

src/Radiance/libRAD/Device.cpp

@@ -29,7 +29,7 @@ namespace es2
 {
 	using namespace sw;
 
-	Device::Device(Context *context) : Renderer(context, true, true, true, true, true), context(context)
+	Device::Device(Context *context) : Renderer(context, OpenGL, true), context(context)
 	{
 		depthStencil = 0;
 		renderTarget = 0;

src/Renderer/Context.cpp

@@ -30,6 +30,7 @@ namespace sw
 	bool symmetricNormalizedDepth = false;   // [-1, 1] instead of [0, 1]
 	bool booleanFaceRegister = false;
 	bool fullPixelPositionRegister = false;
+	bool leadingVertexFirst = false;         // Flat shading uses first vertex, else last
 
 	bool forceWindowed = false;
 	bool quadLayoutEnabled = false;

src/Renderer/Renderer.cpp

@@ -50,6 +50,7 @@ namespace sw
 	extern bool complementaryDepthBuffer;
 	extern bool postBlendSRGB;
 	extern bool exactColorRounding;
+	extern bool leadingVertexFirst;
 	extern TransparencyAntialiasing transparencyAntialiasing;
 	extern bool forceClearRegisters;
 
@@ -99,12 +100,13 @@ namespace sw
 		deallocate(data);
 	}
 
-	Renderer::Renderer(Context *context, bool halfIntegerCoordinates, bool symmetricNormalizedDepth, bool booleanFaceRegister, bool fullPixelPositionRegister, bool exactColorRounding) : context(context), VertexProcessor(context), PixelProcessor(context), SetupProcessor(context), viewport()
+	Renderer::Renderer(Context *context, Conventions conventions, bool exactColorRounding) : context(context), VertexProcessor(context), PixelProcessor(context), SetupProcessor(context), viewport()
 	{
-		sw::halfIntegerCoordinates = halfIntegerCoordinates;
-		sw::symmetricNormalizedDepth = symmetricNormalizedDepth;
-		sw::booleanFaceRegister = booleanFaceRegister;
-		sw::fullPixelPositionRegister = fullPixelPositionRegister;
+		sw::halfIntegerCoordinates = conventions.halfIntegerCoordinates;
+		sw::symmetricNormalizedDepth = conventions.symmetricNormalizedDepth;
+		sw::booleanFaceRegister = conventions.booleanFaceRegister;
+		sw::fullPixelPositionRegister = conventions.fullPixelPositionRegister;
+		sw::leadingVertexFirst = conventions.leadingVertexFirst;
 		sw::exactColorRounding = exactColorRounding;
 
 		setRenderTarget(0, 0);

src/Renderer/Renderer.hpp

@@ -53,6 +53,24 @@ namespace sw
 	extern TranscendentalPrecision rsqPrecision;
 	extern bool perspectiveCorrection;
 
+	struct Conventions
+	{
+		bool halfIntegerCoordinates;
+		bool symmetricNormalizedDepth;
+		bool booleanFaceRegister;
+		bool fullPixelPositionRegister;
+		bool leadingVertexFirst;
+	};
+
+	static const Conventions OpenGL =
+	{
+		true,   // halfIntegerCoordinates
+		true,   // symmetricNormalizedDepth
+		true,   // booleanFaceRegister
+		true,   // fullPixelPositionRegister
+		false   // leadingVertexFirst
+	};
+
 	struct Query
 	{
 		Query()
@@ -268,7 +286,7 @@ namespace sw
 		};
 
 	public:
-		Renderer(Context *context, bool halfIntegerCoordinates, bool symmetricNormalizedDepth, bool booleanFaceRegister, bool fullPixelPositionRegister, bool exactColorRounding);
+		Renderer(Context *context, Conventions conventions, bool exactColorRounding);
 
 		virtual ~Renderer();
 

src/Shader/SetupRoutine.cpp

@@ -21,6 +21,7 @@ namespace sw
 {
 	extern bool complementaryDepthBuffer;
 	extern TranscendentalPrecision logPrecision;
+	extern bool leadingVertexFirst;
 
 	SetupRoutine::SetupRoutine(const SetupProcessor::State &state) : state(state)
 	{
@@ -546,7 +547,8 @@ namespace sw
 		}
 		else
 		{
-			Float C = *Pointer<Float>(triangle + OFFSET(Triangle,v0) + attribute);
+			int leadingVertex = leadingVertexFirst ? OFFSET(Triangle,v0) : OFFSET(Triangle,v2);
+			Float C = *Pointer<Float>(triangle + leadingVertex + attribute);
 
 			*Pointer<Float4>(primitive + planeEquation + 0, 16) = Float4(0, 0, 0, 0);
 			*Pointer<Float4>(primitive + planeEquation + 16, 16) = Float4(0, 0, 0, 0);