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

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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
This commit is contained in:
Lukas Stockner
2016-06-23 22:56:43 +02:00
parent 2af4c80be6
commit 23c276832b
29 changed files with 1045 additions and 56 deletions
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4
intern/cycles/kernel/kernel_shadow.h
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@@ -117,7 +117,7 @@ ccl_device_inline bool shadow_blocked(KernelGlobals *kg, ShaderData *shadow_sd,
/* attenuation from transparent surface */
if(!(shadow_sd->flag & SD_HAS_ONLY_VOLUME)) {
path_state_modify_bounce(state, true);
shader_eval_surface(kg, shadow_sd, state, 0.0f, PATH_RAY_SHADOW, SHADER_CONTEXT_SHADOW);
shader_eval_surface(kg, shadow_sd, NULL, state, 0.0f, PATH_RAY_SHADOW, SHADER_CONTEXT_SHADOW);
path_state_modify_bounce(state, false);
throughput *= shader_bsdf_transparency(kg, shadow_sd);
@@ -252,7 +252,7 @@ ccl_device_noinline bool shadow_blocked(KernelGlobals *kg,
/* attenuation from transparent surface */
if(!(ccl_fetch(shadow_sd, flag) & SD_HAS_ONLY_VOLUME)) {
path_state_modify_bounce(state, true);
shader_eval_surface(kg, shadow_sd, state, 0.0f, PATH_RAY_SHADOW, SHADER_CONTEXT_SHADOW);
shader_eval_surface(kg, shadow_sd, NULL, state, 0.0f, PATH_RAY_SHADOW, SHADER_CONTEXT_SHADOW);
path_state_modify_bounce(state, false);
throughput *= shader_bsdf_transparency(kg, shadow_sd);