23c276832b
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
60 lines
1.8 KiB
Plaintext
60 lines
1.8 KiB
Plaintext
/*
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* Copyright 2011-2013 Blender Foundation
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include "stdosl.h"
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shader node_anisotropic_bsdf(
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color Color = 0.0,
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string distribution = "GGX",
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float Roughness = 0.0,
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float Anisotropy = 0.0,
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float Rotation = 0.0,
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normal Normal = N,
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normal Tangent = normalize(dPdu),
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output closure color BSDF = 0)
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{
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/* rotate tangent around normal */
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vector T = Tangent;
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if (Rotation != 0.0)
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T = rotate(T, Rotation * M_2PI, point(0.0, 0.0, 0.0), Normal);
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/* compute roughness */
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float RoughnessU, RoughnessV;
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float aniso = clamp(Anisotropy, -0.99, 0.99);
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if (aniso < 0.0) {
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RoughnessU = Roughness / (1.0 + aniso);
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RoughnessV = Roughness * (1.0 + aniso);
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}
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else {
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RoughnessU = Roughness * (1.0 - aniso);
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RoughnessV = Roughness / (1.0 - aniso);
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}
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if (distribution == "sharp")
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BSDF = Color * reflection(Normal);
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else if (distribution == "beckmann")
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BSDF = Color * microfacet_beckmann_aniso(Normal, T, RoughnessU, RoughnessV);
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else if (distribution == "GGX")
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BSDF = Color * microfacet_ggx_aniso(Normal, T, RoughnessU, RoughnessV);
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else if (distribution == "Multiscatter GGX")
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BSDF = Color * microfacet_multi_ggx_aniso(Normal, T, RoughnessU, RoughnessV, Color);
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else
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BSDF = Color * ashikhmin_shirley(Normal, T, RoughnessU, RoughnessV);
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}
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