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, 143 insertions, 1 deletions

diff --git a/intern/cycles/kernel/osl/osl_closures.cpp b/intern/cycles/kernel/osl/osl_closures.cpp
index 95b8cea0922..02b1491489c 100644
--- a/intern/cycles/kernel/osl/osl_closures.cpp
+++ b/intern/cycles/kernel/osl/osl_closures.cpp
@@ -44,11 +44,13 @@
 #include "kernel_compat_cpu.h"
 #include "kernel_globals.h"
 #include "kernel_montecarlo.h"
+#include "kernel_random.h"
 
 #include "closure/bsdf_util.h"
 #include "closure/bsdf_ashikhmin_velvet.h"
 #include "closure/bsdf_diffuse.h"
 #include "closure/bsdf_microfacet.h"
+#include "closure/bsdf_microfacet_multi.h"
 #include "closure/bsdf_oren_nayar.h"
 #include "closure/bsdf_reflection.h"
 #include "closure/bsdf_refraction.h"
@@ -205,6 +207,12 @@ void OSLShader::register_closures(OSLShadingSystem *ss_)
 		bsdf_microfacet_ggx_aniso_params(), bsdf_microfacet_ggx_aniso_prepare);
 	register_closure(ss, "microfacet_ggx_refraction", id++,
 		bsdf_microfacet_ggx_refraction_params(), bsdf_microfacet_ggx_refraction_prepare);
+	register_closure(ss, "microfacet_multi_ggx", id++,
+		closure_bsdf_microfacet_multi_ggx_params(), closure_bsdf_microfacet_multi_ggx_prepare);
+	register_closure(ss, "microfacet_multi_ggx_glass", id++,
+		closure_bsdf_microfacet_multi_ggx_glass_params(), closure_bsdf_microfacet_multi_ggx_glass_prepare);
+	register_closure(ss, "microfacet_multi_ggx_aniso", id++,
+		closure_bsdf_microfacet_multi_ggx_aniso_params(), closure_bsdf_microfacet_multi_ggx_aniso_prepare);
 	register_closure(ss, "microfacet_beckmann", id++,
 		bsdf_microfacet_beckmann_params(), bsdf_microfacet_beckmann_prepare);
 	register_closure(ss, "microfacet_beckmann_aniso", id++,
@@ -250,5 +258,127 @@ void OSLShader::register_closures(OSLShadingSystem *ss_)
 		volume_absorption_params(), volume_absorption_prepare);
 }
 
+/* Multiscattering GGX closures */
+
+class MicrofacetMultiClosure : public CBSDFClosure {
+public:
+	float3 color;
+
+	/* Technically, the MultiGGX Glass closure may also transmit.
+	 * However, since this is set statically and only used for caustic flags, this is probably as good as it gets. */
+	MicrofacetMultiClosure() : CBSDFClosure(LABEL_GLOSSY|LABEL_REFLECT)
+	{
+	}
+
+	void setup()
+	{
+		sc.prim = NULL;
+		sc.custom1 = color.x;
+		sc.custom2 = color.y;
+		sc.custom3 = color.z;
+	}
+
+	void blur(float roughness)
+	{
+	}
+
+	float3 eval_reflect(const float3 &omega_out, const float3 &omega_in, float& pdf) const
+	{
+		pdf = 0.0f;
+		return make_float3(0.0f, 0.0f, 0.0f);
+	}
+
+	float3 eval_transmit(const float3 &omega_out, const float3 &omega_in, float& pdf) const
+	{
+		pdf = 0.0f;
+		return make_float3(0.0f, 0.0f, 0.0f);
+	}
+
+	int sample(const float3 &Ng,
+	           const float3 &omega_out, const float3 &domega_out_dx, const float3 &domega_out_dy,
+	           float randu, float randv,
+	           float3 &omega_in, float3 &domega_in_dx, float3 &domega_in_dy,
+	           float &pdf, float3 &eval) const
+	{
+		pdf = 0;
+		return LABEL_NONE;
+	}
+};
+
+class MicrofacetMultiGGXClosure : public MicrofacetMultiClosure {
+public:
+	MicrofacetMultiGGXClosure() : MicrofacetMultiClosure() {}
+
+	void setup()
+	{
+		MicrofacetMultiClosure::setup();
+		m_shaderdata_flag = bsdf_microfacet_multi_ggx_setup(&sc);
+	}
+};
+
+ClosureParam *closure_bsdf_microfacet_multi_ggx_params()
+{
+	static ClosureParam params[] = {
+		CLOSURE_FLOAT3_PARAM(MicrofacetMultiGGXClosure, sc.N),
+		CLOSURE_FLOAT_PARAM(MicrofacetMultiGGXClosure, sc.data0),
+		CLOSURE_FLOAT3_PARAM(MicrofacetMultiGGXClosure, color),
+		CLOSURE_STRING_KEYPARAM(MicrofacetMultiGGXClosure, label, "label"),
+		CLOSURE_FINISH_PARAM(MicrofacetMultiGGXClosure)
+	};
+	return params;
+}
+CCLOSURE_PREPARE(closure_bsdf_microfacet_multi_ggx_prepare, MicrofacetMultiGGXClosure);
+
+class MicrofacetMultiGGXAnisoClosure : public MicrofacetMultiClosure {
+public:
+	MicrofacetMultiGGXAnisoClosure() : MicrofacetMultiClosure() {}
+
+	void setup()
+	{
+		MicrofacetMultiClosure::setup();
+		m_shaderdata_flag = bsdf_microfacet_multi_ggx_aniso_setup(&sc);
+	}
+};
+
+ClosureParam *closure_bsdf_microfacet_multi_ggx_aniso_params()
+{
+	static ClosureParam params[] = {
+		CLOSURE_FLOAT3_PARAM(MicrofacetMultiGGXClosure, sc.N),
+		CLOSURE_FLOAT3_PARAM(MicrofacetMultiGGXClosure, sc.T),
+		CLOSURE_FLOAT_PARAM(MicrofacetMultiGGXClosure, sc.data0),
+		CLOSURE_FLOAT_PARAM(MicrofacetMultiGGXClosure, sc.data1),
+		CLOSURE_FLOAT3_PARAM(MicrofacetMultiGGXClosure, color),
+		CLOSURE_STRING_KEYPARAM(MicrofacetMultiGGXClosure, label, "label"),
+		CLOSURE_FINISH_PARAM(MicrofacetMultiGGXClosure)
+	};
+	return params;
+}
+CCLOSURE_PREPARE(closure_bsdf_microfacet_multi_ggx_aniso_prepare, MicrofacetMultiGGXAnisoClosure);
+
+class MicrofacetMultiGGXGlassClosure : public MicrofacetMultiClosure {
+public:
+	MicrofacetMultiGGXGlassClosure() : MicrofacetMultiClosure() {}
+
+	void setup()
+	{
+		MicrofacetMultiClosure::setup();
+		m_shaderdata_flag = bsdf_microfacet_multi_ggx_glass_setup(&sc);
+	}
+};
+
+ClosureParam *closure_bsdf_microfacet_multi_ggx_glass_params()
+{
+	static ClosureParam params[] = {
+		CLOSURE_FLOAT3_PARAM(MicrofacetMultiGGXClosure, sc.N),
+		CLOSURE_FLOAT_PARAM(MicrofacetMultiGGXClosure, sc.data0),
+		CLOSURE_FLOAT_PARAM(MicrofacetMultiGGXClosure, sc.data2),
+		CLOSURE_FLOAT3_PARAM(MicrofacetMultiGGXClosure, color),
+		CLOSURE_STRING_KEYPARAM(MicrofacetMultiGGXClosure, label, "label"),
+		CLOSURE_FINISH_PARAM(MicrofacetMultiGGXClosure)
+	};
+	return params;
+}
+CCLOSURE_PREPARE(closure_bsdf_microfacet_multi_ggx_glass_prepare, MicrofacetMultiGGXGlassClosure);
+
 CCL_NAMESPACE_END
 
diff --git a/intern/cycles/kernel/osl/osl_closures.h b/intern/cycles/kernel/osl/osl_closures.h
index 1578d06cd56..c5a1a29b6af 100644
--- a/intern/cycles/kernel/osl/osl_closures.h
+++ b/intern/cycles/kernel/osl/osl_closures.h
@@ -52,6 +52,9 @@ OSL::ClosureParam *closure_bssrdf_cubic_params();
 OSL::ClosureParam *closure_bssrdf_gaussian_params();
 OSL::ClosureParam *closure_bssrdf_burley_params();
 OSL::ClosureParam *closure_henyey_greenstein_volume_params();
+OSL::ClosureParam *closure_bsdf_microfacet_multi_ggx_params();
+OSL::ClosureParam *closure_bsdf_microfacet_multi_ggx_glass_params();
+OSL::ClosureParam *closure_bsdf_microfacet_multi_ggx_aniso_params();
 
 void closure_emission_prepare(OSL::RendererServices *, int id, void *data);
 void closure_background_prepare(OSL::RendererServices *, int id, void *data);
@@ -63,6 +66,9 @@ void closure_bssrdf_cubic_prepare(OSL::RendererServices *, int id, void *data);
 void closure_bssrdf_gaussian_prepare(OSL::RendererServices *, int id, void *data);
 void closure_bssrdf_burley_prepare(OSL::RendererServices *, int id, void *data);
 void closure_henyey_greenstein_volume_prepare(OSL::RendererServices *, int id, void *data);
+void closure_bsdf_microfacet_multi_ggx_prepare(OSL::RendererServices *, int id, void *data);
+void closure_bsdf_microfacet_multi_ggx_glass_prepare(OSL::RendererServices *, int id, void *data);
+void closure_bsdf_microfacet_multi_ggx_aniso_prepare(OSL::RendererServices *, int id, void *data);
 
 #define CCLOSURE_PREPARE(name, classname)          \
 void name(RendererServices *, int id, void *data) \
diff --git a/intern/cycles/kernel/osl/osl_shader.cpp b/intern/cycles/kernel/osl/osl_shader.cpp
index f58368e6789..6cde7419e10 100644
--- a/intern/cycles/kernel/osl/osl_shader.cpp
+++ b/intern/cycles/kernel/osl/osl_shader.cpp
@@ -177,6 +177,7 @@ static void flatten_surface_closure_tree(ShaderData *sd, int path_flag,
 					case CClosurePrimitive::BSDF: {
 						CBSDFClosure *bsdf = (CBSDFClosure *)prim;
 						int scattering = bsdf->scattering();
+						int shaderdata_flag = bsdf->shaderdata_flag();
 
 						/* caustic options */
 						if((scattering & LABEL_GLOSSY) && (path_flag & PATH_RAY_DIFFUSE)) {
@@ -201,11 +202,16 @@ static void flatten_surface_closure_tree(ShaderData *sd, int path_flag,
 						sc.data1 = bsdf->sc.data1;
 						sc.data2 = bsdf->sc.data2;
 						sc.prim = bsdf->sc.prim;
+						if(shaderdata_flag & SD_BSDF_HAS_CUSTOM) {
+							sc.custom1 = bsdf->sc.custom1;
+							sc.custom2 = bsdf->sc.custom2;
+							sc.custom3 = bsdf->sc.custom3;
+						}
 
 						/* add */
 						if(sc.sample_weight > CLOSURE_WEIGHT_CUTOFF && sd->num_closure < MAX_CLOSURE) {
 							sd->closure[sd->num_closure++] = sc;
-							sd->flag |= bsdf->shaderdata_flag();
+							sd->flag |= shaderdata_flag;
 						}
 						break;
 					}