Cycles: Add multi-scattering, energy-conserving GGX as an option to the Glossy, Anisotropic and Glass BSDFs

This commit adds a new distribution to the Glossy, Anisotropic and Glass BSDFs that implements the
multiple-scattering microfacet model described in the paper "Multiple-Scattering Microfacet BSDFs with the Smith Model".

Essentially, the improvement is that unlike classical GGX, which only models single scattering and assumes
the contribution of multiple bounces to be zero, this new model performs a random walk on the microsurface until
the ray leaves it again, which ensures perfect energy conservation.

In practise, this means that the "darkening problem" - GGX materials becoming darker with increasing
roughness - is solved in a physically correct and efficient way.

The downside of this model is that it has no (known) analytic expression for evalation. However, it can be
evaluated stochastically, and although the correct PDF isn't known either, the properties of MIS and the
balance heuristic guarantee an unbiased result at the cost of slightly higher noise.

Reviewers: dingto, #cycles, brecht

Reviewed By: dingto, #cycles, brecht

Subscribers: bliblubli, ace_dragon, gregzaal, brecht, harvester, dingto, marcog, swerner, jtheninja, Blendify, nutel

Differential Revision: https://developer.blender.org/D2002

3 files changed, 24 insertions, 2 deletions

diff --git a/source/blender/makesdna/DNA_node_types.h b/source/blender/makesdna/DNA_node_types.h
index 6cba3322135..46b30f41f5b 100644
--- a/source/blender/makesdna/DNA_node_types.h
+++ b/source/blender/makesdna/DNA_node_types.h
@@ -912,7 +912,8 @@ typedef struct NodeSunBeams {
 #define SHD_GLOSSY_BECKMANN				0
 #define SHD_GLOSSY_SHARP				1
 #define SHD_GLOSSY_GGX					2
-#define SHD_GLOSSY_ASHIKHMIN_SHIRLEY	3
+#define SHD_GLOSSY_ASHIKHMIN_SHIRLEY			3
+#define SHD_GLOSSY_MULTI_GGX				4
 
 /* vector transform */
 #define SHD_VECT_TRANSFORM_TYPE_VECTOR	0
diff --git a/source/blender/makesrna/intern/rna_nodetree.c b/source/blender/makesrna/intern/rna_nodetree.c
index bf7e4634c2f..7f2cd1e6080 100644
--- a/source/blender/makesrna/intern/rna_nodetree.c
+++ b/source/blender/makesrna/intern/rna_nodetree.c
@@ -3193,12 +3193,14 @@ static EnumPropertyItem node_glossy_items[] = {
 	{SHD_GLOSSY_BECKMANN,          "BECKMANN",          0, "Beckmann", ""},
 	{SHD_GLOSSY_GGX,               "GGX",               0, "GGX",      ""},
 	{SHD_GLOSSY_ASHIKHMIN_SHIRLEY, "ASHIKHMIN_SHIRLEY", 0, "Ashikhmin-Shirley", ""},
+	{SHD_GLOSSY_MULTI_GGX,         "MULTI_GGX",         0, "Multiscatter GGX", ""},
 	{0, NULL, 0, NULL, NULL}
 };
 
 static EnumPropertyItem node_anisotropic_items[] = {
 	{SHD_GLOSSY_BECKMANN,          "BECKMANN",          0, "Beckmann", ""},
 	{SHD_GLOSSY_GGX,               "GGX",               0, "GGX",      ""},
+	{SHD_GLOSSY_MULTI_GGX,         "MULTI_GGX",         0, "Multiscatter GGX", ""},
 	{SHD_GLOSSY_ASHIKHMIN_SHIRLEY, "ASHIKHMIN_SHIRLEY", 0, "Ashikhmin-Shirley", ""},
 	{0, NULL, 0, NULL, NULL}
 };
@@ -3207,6 +3209,14 @@ static EnumPropertyItem node_glass_items[] = {
 	{SHD_GLOSSY_SHARP,             "SHARP",             0, "Sharp",    ""},
 	{SHD_GLOSSY_BECKMANN,          "BECKMANN",          0, "Beckmann", ""},
 	{SHD_GLOSSY_GGX,               "GGX",               0, "GGX",      ""},
+	{SHD_GLOSSY_MULTI_GGX,         "MULTI_GGX",         0, "Multiscatter GGX", ""},
+	{0, NULL, 0, NULL, NULL}
+};
+
+static EnumPropertyItem node_refraction_items[] = {
+	{SHD_GLOSSY_SHARP,             "SHARP",             0, "Sharp",    ""},
+	{SHD_GLOSSY_BECKMANN,          "BECKMANN",          0, "Beckmann", ""},
+	{SHD_GLOSSY_GGX,               "GGX",               0, "GGX",      ""},
 	{0, NULL, 0, NULL, NULL}
 };
 
@@ -4159,6 +4169,17 @@ static void def_glass(StructRNA *srna)
 	RNA_def_property_update(prop, NC_NODE | NA_EDITED, "rna_Node_update");
 }
 
+static void def_refraction(StructRNA *srna)
+{
+	PropertyRNA *prop;
+	
+	prop = RNA_def_property(srna, "distribution", PROP_ENUM, PROP_NONE);
+	RNA_def_property_enum_sdna(prop, NULL, "custom1");
+	RNA_def_property_enum_items(prop, node_refraction_items);
+	RNA_def_property_ui_text(prop, "Distribution", "");
+	RNA_def_property_update(prop, NC_NODE | NA_EDITED, "rna_Node_update");
+}
+
 static void def_anisotropic(StructRNA *srna)
 {
 	PropertyRNA *prop;
diff --git a/source/blender/nodes/NOD_static_types.h b/source/blender/nodes/NOD_static_types.h
index 9e1a0c926fa..171d5313c1d 100644
--- a/source/blender/nodes/NOD_static_types.h
+++ b/source/blender/nodes/NOD_static_types.h
@@ -82,7 +82,7 @@ DefNode( ShaderNode,     SH_NODE_BSDF_ANISOTROPIC,   def_anisotropic,        "BS
 DefNode( ShaderNode,     SH_NODE_BSDF_DIFFUSE,       0,                      "BSDF_DIFFUSE",       BsdfDiffuse,      "Diffuse BSDF",      ""       )
 DefNode( ShaderNode,     SH_NODE_BSDF_GLOSSY,        def_glossy,             "BSDF_GLOSSY",        BsdfGlossy,       "Glossy BSDF",       ""       )
 DefNode( ShaderNode,     SH_NODE_BSDF_GLASS,         def_glass,              "BSDF_GLASS",         BsdfGlass,        "Glass BSDF",        ""       )
-DefNode( ShaderNode,     SH_NODE_BSDF_REFRACTION,    def_glass,              "BSDF_REFRACTION",    BsdfRefraction,   "Refraction BSDF",   ""       )
+DefNode( ShaderNode,     SH_NODE_BSDF_REFRACTION,    def_refraction,         "BSDF_REFRACTION",    BsdfRefraction,   "Refraction BSDF",   ""       )
 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",       ""       )