Cycles: initial subsurface multiple scattering support. It's not working as

well as I would like, but it works, just add a subsurface scattering node and
you can use it like any other BSDF.

It is using fully raytraced sampling compatible with progressive rendering
and other more advanced rendering algorithms we might used in the future, and
it uses no extra memory so it's suitable for complex scenes.

Disadvantage is that it can be quite noisy and slow. Two limitations that will
be solved are that it does not work with bump mapping yet, and that the falloff
function used is a simple cubic function, it's not using the real BSSRDF
falloff function yet.

The node has a color input, along with a scattering radius for each RGB color
channel along with an overall scale factor for the radii.

There is also no GPU support yet, will test if I can get that working later.

Node Documentation:
http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Nodes/Shaders#BSSRDF

Implementation notes:
http://wiki.blender.org/index.php/Dev:2.6/Source/Render/Cycles/Subsurface_Scattering

intern/cycles/kernel/kernel_globals.h

@@ -88,5 +88,39 @@ typedef struct KernelGlobals {
 
 #endif
 
+/* Interpolated lookup table access */
+
+__device float lookup_table_read(KernelGlobals *kg, float x, int offset, int size)
+{
+	x = clamp(x, 0.0f, 1.0f)*(size-1);
+
+	int index = min((int)x, size-1);
+	int nindex = min(index+1, size-1);
+	float t = x - index;
+
+	float data0 = kernel_tex_fetch(__lookup_table, index + offset);
+	if(t == 0.0f)
+		return data0;
+
+	float data1 = kernel_tex_fetch(__lookup_table, nindex + offset);
+	return (1.0f - t)*data0 + t*data1;
+}
+
+__device float lookup_table_read_2D(KernelGlobals *kg, float x, float y, int offset, int xsize, int ysize)
+{
+	y = clamp(y, 0.0f, 1.0f)*(ysize-1);
+
+	int index = min((int)y, ysize-1);
+	int nindex = min(index+1, ysize-1);
+	float t = y - index;
+
+	float data0 = lookup_table_read(kg, x, offset + xsize*index, xsize);
+	if(t == 0.0f)
+		return data0;
+
+	float data1 = lookup_table_read(kg, x, offset + xsize*nindex, xsize);
+	return (1.0f - t)*data0 + t*data1;
+}
+
 CCL_NAMESPACE_END