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

1 files changed, 12 insertions, 5 deletions

diff --git a/intern/cycles/kernel/kernel_shader.h b/intern/cycles/kernel/kernel_shader.h
index a0b56118ab7..3a4770f82f1 100644
--- a/intern/cycles/kernel/kernel_shader.h
+++ b/intern/cycles/kernel/kernel_shader.h
@@ -468,6 +468,9 @@ ccl_device void shader_merge_closures(ShaderData *sd)
 					continue;
 			}
 
+			if((sd->flag & SD_BSDF_HAS_CUSTOM) && !(sci->custom1 == scj->custom1 && sci->custom2 == scj->custom2 && sci->custom3 == scj->custom3))
+				continue;
+
 			sci->weight += scj->weight;
 			sci->sample_weight += scj->sample_weight;
 
@@ -488,7 +491,7 @@ ccl_device void shader_merge_closures(ShaderData *sd)
 
 /* BSDF */
 
-ccl_device_inline void _shader_bsdf_multi_eval(KernelGlobals *kg, const ShaderData *sd, const float3 omega_in, float *pdf,
+ccl_device_inline void _shader_bsdf_multi_eval(KernelGlobals *kg, ShaderData *sd, const float3 omega_in, float *pdf,
 	int skip_bsdf, BsdfEval *result_eval, float sum_pdf, float sum_sample_weight)
 {
 	/* this is the veach one-sample model with balance heuristic, some pdf
@@ -517,7 +520,7 @@ ccl_device_inline void _shader_bsdf_multi_eval(KernelGlobals *kg, const ShaderDa
 
 #ifdef __BRANCHED_PATH__
 ccl_device_inline void _shader_bsdf_multi_eval_branched(KernelGlobals *kg,
-                                                        const ShaderData *sd,
+                                                        ShaderData *sd,
                                                         const float3 omega_in,
                                                         BsdfEval *result_eval,
                                                         float light_pdf,
@@ -563,7 +566,7 @@ ccl_device void shader_bsdf_eval(KernelGlobals *kg,
 	}
 }
 
-ccl_device int shader_bsdf_sample(KernelGlobals *kg, const ShaderData *sd,
+ccl_device int shader_bsdf_sample(KernelGlobals *kg, ShaderData *sd,
 	float randu, float randv, BsdfEval *bsdf_eval,
 	float3 *omega_in, differential3 *domega_in, float *pdf)
 {
@@ -620,7 +623,7 @@ ccl_device int shader_bsdf_sample(KernelGlobals *kg, const ShaderData *sd,
 	return label;
 }
 
-ccl_device int shader_bsdf_sample_closure(KernelGlobals *kg, const ShaderData *sd,
+ccl_device int shader_bsdf_sample_closure(KernelGlobals *kg, ShaderData *sd,
 	const ShaderClosure *sc, float randu, float randv, BsdfEval *bsdf_eval,
 	float3 *omega_in, differential3 *domega_in, float *pdf)
 {
@@ -824,7 +827,7 @@ ccl_device float3 shader_holdout_eval(KernelGlobals *kg, ShaderData *sd)
 
 /* Surface Evaluation */
 
-ccl_device void shader_eval_surface(KernelGlobals *kg, ShaderData *sd,
+ccl_device void shader_eval_surface(KernelGlobals *kg, ShaderData *sd, RNG *rng,
 	ccl_addr_space PathState *state, float randb, int path_flag, ShaderContext ctx)
 {
 	ccl_fetch(sd, num_closure) = 0;
@@ -846,6 +849,10 @@ ccl_device void shader_eval_surface(KernelGlobals *kg, ShaderData *sd,
 		ccl_fetch(sd, flag) |= bsdf_diffuse_setup(ccl_fetch_array(sd, closure, 0));
 #endif
 	}
+
+	if(rng && (ccl_fetch(sd, flag) & SD_BSDF_NEEDS_LCG)) {
+		ccl_fetch(sd, lcg_state) = lcg_state_init(rng, state, 0xb4bc3953);
+	}
 }
 
 /* Background Evaluation */