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

source/blender/blenkernel/BKE_node.h

@@ -752,6 +752,7 @@ struct ShadeResult;
 #define SH_NODE_TANGENT					174
 #define SH_NODE_NORMAL_MAP				175
 #define SH_NODE_HAIR_INFO				176
+#define SH_NODE_SUBSURFACE_SCATTERING	177
 
 /* custom defines options for Material node */
 #define SH_NODE_MAT_DIFF   1

source/blender/blenkernel/intern/node.c

@@ -3438,6 +3438,7 @@ static void registerShaderNodes(void)
 	register_node_type_sh_holdout();
 	//register_node_type_sh_volume_transparent();
 	//register_node_type_sh_volume_isotropic();
+	register_node_type_sh_subsurface_scattering();
 	register_node_type_sh_mix_shader();
 	register_node_type_sh_add_shader();
 

source/blender/gpu/shaders/gpu_shader_material.glsl

@@ -2074,6 +2074,11 @@ void node_bsdf_velvet(vec4 color, float sigma, vec3 N, out vec4 result)
 	node_bsdf_diffuse(color, 0.0, N, result);
 }
 
+void node_subsurface_scattering(vec4 color, float roughness, vec3 N, out vec4 result)
+{
+	node_bsdf_diffuse(color, 0.0, N, result);
+}
+
 /* emission */
 
 void node_emission(vec4 color, float strength, vec3 N, out vec4 result)

source/blender/nodes/CMakeLists.txt

@@ -175,6 +175,7 @@ set(SRC
 	shader/nodes/node_shader_output_world.c
 	shader/nodes/node_shader_particle_info.c
 	shader/nodes/node_shader_script.c
+	shader/nodes/node_shader_subsurface_scattering.c
 	shader/nodes/node_shader_tangent.c
 	shader/nodes/node_shader_tex_brick.c
 	shader/nodes/node_shader_tex_checker.c

source/blender/nodes/NOD_shader.h

@@ -101,6 +101,7 @@ void register_node_type_sh_emission(void);
 void register_node_type_sh_holdout(void);
 void register_node_type_sh_volume_transparent(void);
 void register_node_type_sh_volume_isotropic(void);
+void register_node_type_sh_subsurface_scattering(void);
 void register_node_type_sh_mix_shader(void);
 void register_node_type_sh_add_shader(void);
 

source/blender/nodes/NOD_static_types.h

@@ -84,6 +84,7 @@ DefNode( ShaderNode,     SH_NODE_BSDF_REFRACTION,    def_glossy,             "BS
 DefNode( ShaderNode,     SH_NODE_BSDF_TRANSLUCENT,   0,                      "BSDF_TRANSLUCENT",   BsdfTranslucent,  "Translucent BSDF",  ""       )
 DefNode( ShaderNode,     SH_NODE_BSDF_TRANSPARENT,   0,                      "BSDF_TRANSPARENT",   BsdfTransparent,  "Transparent BSDF",  ""       )
 DefNode( ShaderNode,     SH_NODE_BSDF_VELVET,        0,                      "BSDF_VELVET",        BsdfVelvet,       "Velvet BSDF",       ""       )
+DefNode( ShaderNode,     SH_NODE_SUBSURFACE_SCATTERING, 0,                   "SUBSURFACE_SCATTERING",SubsurfaceScattering,"Subsurface Scattering","")
 DefNode( ShaderNode,     SH_NODE_VOLUME_TRANSPARENT, 0,                      "VOLUME_TRANSPARENT", VolumeTransparent,"Transparent Volume",""       )
 DefNode( ShaderNode,     SH_NODE_VOLUME_ISOTROPIC,   0,                      "VOLUME_ISOTROPIC",   VolumeIsotropic,  "Isotropic Volume",  ""       )
 DefNode( ShaderNode,     SH_NODE_EMISSION,           0,                      "EMISSION",           Emission,         "Emission",          ""       )

source/blender/nodes/shader/nodes/node_shader_subsurface_scattering.c

@@ -0,0 +1,69 @@
+/*
+ * ***** BEGIN GPL LICENSE BLOCK *****
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version. 
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * The Original Code is Copyright (C) 2005 Blender Foundation.
+ * All rights reserved.
+ *
+ * The Original Code is: all of this file.
+ *
+ * Contributor(s): none yet.
+ *
+ * ***** END GPL LICENSE BLOCK *****
+ */
+
+#include "../node_shader_util.h"
+
+/* **************** OUTPUT ******************** */
+
+static bNodeSocketTemplate sh_node_subsurface_scattering_in[] = {
+	{	SOCK_RGBA, 1, N_("Color"),		0.8f, 0.8f, 0.8f, 1.0f, 0.0f, 1.0f},
+	{	SOCK_FLOAT, 1, N_("Scale"),		1.0, 0.0f, 0.0f, 0.0f, 0.0f, 1000000.0f},
+	{	SOCK_VECTOR, 1, N_("Radius"),	1.0f, 1.0f, 1.0f, 0.0f, 0.0f, 100.0f},
+	//{	SOCK_FLOAT, 1, N_("IOR"),		1.3f, 0.0f, 0.0f, 0.0f, 1.0f, 1000.0f},
+	{	SOCK_VECTOR, 1, N_("Normal"),	0.0f, 0.0f, 0.0f, 1.0f, -1.0f, 1.0f, PROP_NONE, SOCK_HIDE_VALUE},
+	{	-1, 0, ""	}
+};
+
+static bNodeSocketTemplate sh_node_subsurface_scattering_out[] = {
+	{	SOCK_SHADER, 0, N_("BSSRDF")},
+	{	-1, 0, ""	}
+};
+
+static int node_shader_gpu_subsurface_scattering(GPUMaterial *mat, bNode *UNUSED(node), bNodeExecData *UNUSED(execdata), GPUNodeStack *in, GPUNodeStack *out)
+{
+	if (!in[1].link)
+		in[1].link = GPU_builtin(GPU_VIEW_NORMAL);
+
+	return GPU_stack_link(mat, "node_subsurface_scattering", in, out);
+}
+
+/* node type definition */
+void register_node_type_sh_subsurface_scattering(void)
+{
+	static bNodeType ntype;
+
+	sh_node_type_base(&ntype, SH_NODE_SUBSURFACE_SCATTERING, "Subsurface Scattering", NODE_CLASS_SHADER, 0);
+	node_type_compatibility(&ntype, NODE_NEW_SHADING);
+	node_type_socket_templates(&ntype, sh_node_subsurface_scattering_in, sh_node_subsurface_scattering_out);
+	node_type_size(&ntype, 150, 60, 200);
+	node_type_init(&ntype, NULL);
+	node_type_storage(&ntype, "", NULL, NULL);
+	node_type_gpu(&ntype, node_shader_gpu_subsurface_scattering);
+
+	nodeRegisterType(&ntype);
+}
+