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, 10 insertions, 5 deletions

diff --git a/intern/cycles/kernel/kernel_bake.h b/intern/cycles/kernel/kernel_bake.h
index 8d05befe1d4..9ee0b09529e 100644
--- a/intern/cycles/kernel/kernel_bake.h
+++ b/intern/cycles/kernel/kernel_bake.h
@@ -48,7 +48,7 @@ ccl_device void compute_light_pass(KernelGlobals *kg, ShaderData *sd, PathRadian
 
 	/* evaluate surface shader */
 	float rbsdf = path_state_rng_1D(kg, &rng, &state, PRNG_BSDF);
-	shader_eval_surface(kg, sd, &state, rbsdf, state.flag, SHADER_CONTEXT_MAIN);
+	shader_eval_surface(kg, sd, &rng, &state, rbsdf, state.flag, SHADER_CONTEXT_MAIN);
 
 	/* TODO, disable the closures we won't need */
 
@@ -220,6 +220,7 @@ ccl_device_inline float3 kernel_bake_shader_bsdf(KernelGlobals *kg,
 
 ccl_device float3 kernel_bake_evaluate_direct_indirect(KernelGlobals *kg,
                                                        ShaderData *sd,
+                                                       RNG *rng,
                                                        PathState *state,
                                                        float3 direct,
                                                        float3 indirect,
@@ -239,12 +240,12 @@ ccl_device float3 kernel_bake_evaluate_direct_indirect(KernelGlobals *kg,
 		}
 		else {
 			/* surface color of the pass only */
-			shader_eval_surface(kg, sd, state, 0.0f, 0, SHADER_CONTEXT_MAIN);
+			shader_eval_surface(kg, sd, rng, state, 0.0f, 0, SHADER_CONTEXT_MAIN);
 			return kernel_bake_shader_bsdf(kg, sd, type);
 		}
 	}
 	else {
-		shader_eval_surface(kg, sd, state, 0.0f, 0, SHADER_CONTEXT_MAIN);
+		shader_eval_surface(kg, sd, rng, state, 0.0f, 0, SHADER_CONTEXT_MAIN);
 		color = kernel_bake_shader_bsdf(kg, sd, type);
 	}
 
@@ -336,7 +337,7 @@ ccl_device void kernel_bake_evaluate(KernelGlobals *kg, ccl_global uint4 *input,
 		case SHADER_EVAL_NORMAL:
 		{
 			if((sd.flag & SD_HAS_BUMP)) {
-				shader_eval_surface(kg, &sd, &state, 0.f, 0, SHADER_CONTEXT_MAIN);
+				shader_eval_surface(kg, &sd, &rng, &state, 0.f, 0, SHADER_CONTEXT_MAIN);
 			}
 
 			/* compression: normal = (2 * color) - 1 */
@@ -350,7 +351,7 @@ ccl_device void kernel_bake_evaluate(KernelGlobals *kg, ccl_global uint4 *input,
 		}
 		case SHADER_EVAL_EMISSION:
 		{
-			shader_eval_surface(kg, &sd, &state, 0.f, 0, SHADER_CONTEXT_EMISSION);
+			shader_eval_surface(kg, &sd, &rng, &state, 0.f, 0, SHADER_CONTEXT_EMISSION);
 			out = shader_emissive_eval(kg, &sd);
 			break;
 		}
@@ -403,6 +404,7 @@ ccl_device void kernel_bake_evaluate(KernelGlobals *kg, ccl_global uint4 *input,
 		{
 			out = kernel_bake_evaluate_direct_indirect(kg,
 			                                           &sd,
+			                                           &rng,
 			                                           &state,
 			                                           L.direct_diffuse,
 			                                           L.indirect_diffuse,
@@ -414,6 +416,7 @@ ccl_device void kernel_bake_evaluate(KernelGlobals *kg, ccl_global uint4 *input,
 		{
 			out = kernel_bake_evaluate_direct_indirect(kg,
 			                                           &sd,
+			                                           &rng,
 			                                           &state,
 			                                           L.direct_glossy,
 			                                           L.indirect_glossy,
@@ -425,6 +428,7 @@ ccl_device void kernel_bake_evaluate(KernelGlobals *kg, ccl_global uint4 *input,
 		{
 			out = kernel_bake_evaluate_direct_indirect(kg,
 			                                           &sd,
+			                                           &rng,
 			                                           &state,
 			                                           L.direct_transmission,
 			                                           L.indirect_transmission,
@@ -437,6 +441,7 @@ ccl_device void kernel_bake_evaluate(KernelGlobals *kg, ccl_global uint4 *input,
 #ifdef __SUBSURFACE__
 			out = kernel_bake_evaluate_direct_indirect(kg,
 			                                           &sd,
+			                                           &rng,
 			                                           &state,
 			                                           L.direct_subsurface,
 			                                           L.indirect_subsurface,