There are 2 bits in the polyheader labelled discard. When both are set ie in sega rally, the polygons are totally discarded. Scud seems to set bit 1 which seems halve the opacity.

Src/Graphics/New3D/New3D.cpp

@@ -1118,6 +1118,10 @@ void CNew3D::CacheModel(Model *m, const UINT32 *data)
 		}
 
 		p.faceColour[3] = ph.Transparency() / 255.f;
+
+		if (ph.Discard1() && !ph.Discard2()) {
+			p.faceColour[3] *= 0.5f;
+		}
 				
 		// if we have flat shading, we can't re-use normals from shared vertices
 		for (i = 0; i < p.number && !ph.SmoothShading(); i++) {
@@ -1200,7 +1204,7 @@ void CNew3D::CacheModel(Model *m, const UINT32 *data)
 		}
 
 		// check if we need double up vertices for two sided lighting
-		if (ph.DoubleSided() && !ph.Disabled()) {
+		if (ph.DoubleSided() && !ph.Discard()) {
 
 			R3DPoly tempP = p;
 
@@ -1215,7 +1219,7 @@ void CNew3D::CacheModel(Model *m, const UINT32 *data)
 		}
 
 		// Copy this polygon into the model buffer
-		if (!ph.Disabled()) {
+		if (!ph.Discard()) {
 			CopyVertexData(p, currentMesh->polys);
 		}
 		

Src/Graphics/New3D/PolyHeader.cpp

@@ -91,7 +91,7 @@ int PolyHeader::PolyNumber()
 	return (header[0] & 0x000FFFC00) >> 10;				// not all programs pass this, instead they are set to 0
 }
 
-bool PolyHeader::Disabled()
+bool PolyHeader::Discard()
 {
 	if ((header[0] & 0x100) && (header[0] & 0x200)) {
 		return true;
@@ -100,6 +100,16 @@ bool PolyHeader::Disabled()
 	return false;
 }
 
+bool PolyHeader::Discard1()
+{
+	return (header[0] & 0x200) > 0;
+}
+
+bool PolyHeader::Discard2()
+{
+	return (header[0] & 0x100) > 0;
+}
+
 int	PolyHeader::NumVerts() 
 {
 	return (header[0] & 0x40) ? 4 : 3;

Src/Graphics/New3D/PolyHeader.h

@@ -10,7 +10,7 @@ Polygon Data
 xxxxxx-- -------- -------- --------		Specular
 ------x- -------- -------- --------		Clockwise winding
 -------x xxxxxxxx xxxxxx-- --------		Polygon number (not always present)
--------- -------- ------xx --------		Discard polys
+-------- -------- ------xx --------		Discard
 -------- -------- -------- x-------		Enable specular
 -------- -------- -------- -x------		0 = Triangle, 1 = Quad
 -------- -------- -------- --x-----		Poly is points
@@ -90,7 +90,9 @@ public:
 	float	SpecularValue();
 	bool	Clockwise();
 	int		PolyNumber();
-	bool	Disabled();
+	bool	Discard();				// both bits
+	bool	Discard1();				// bit 1 only
+	bool	Discard2();				// bit 2 only
 	int		NumVerts();
 	int		NumSharedVerts();
 	bool	SharedVertex(int vertex);
@@ -100,7 +102,7 @@ public:
 	float	UVScale();
 	bool	DoubleSided();
 	bool	LastPoly();
-	bool	PolyColor();	// if false uses LUT from ram
+	bool	PolyColor();			// if false uses LUT from ram
 	bool	FixedShading();
 	bool	SmoothShading();