Cycles: add Principled Hair BSDF.

This is a physically-based, easy-to-use shader for rendering hair and fur,
with controls for melanin, roughness and randomization.

Based on the paper "A Practical and Controllable Hair and Fur Model for
Production Path Tracing".

Implemented by Leonardo E. Segovia and Lukas Stockner, part of Google
Summer of Code 2018.

1 files changed, 105 insertions, 0 deletions

diff --git a/intern/cycles/kernel/shaders/node_principled_hair_bsdf.osl b/intern/cycles/kernel/shaders/node_principled_hair_bsdf.osl
new file mode 100644
index 00000000000..757a88f8ece
--- /dev/null
+++ b/intern/cycles/kernel/shaders/node_principled_hair_bsdf.osl
@@ -0,0 +1,105 @@
+/*
+ * Copyright 2018 Blender Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "stdosl.h"
+
+color log3(color a)
+{
+	return color(log(a[0]), log(a[1]), log(a[2]));
+}
+
+color sigma_from_concentration(float eumelanin, float pheomelanin)
+{
+	return eumelanin*color(0.506, 0.841, 1.653) + pheomelanin*color(0.343, 0.733, 1.924);
+}
+
+color sigma_from_reflectance(color c, float azimuthal_roughness)
+{
+	float x = azimuthal_roughness;
+	float roughness_fac = (((((0.245*x) + 5.574)*x - 10.73)*x + 2.532)*x - 0.215)*x + 5.969;
+	color sigma = log3(c) / roughness_fac;
+	return sigma * sigma;
+}
+
+shader node_principled_hair_bsdf(
+	color Color = color(0.017513, 0.005763, 0.002059),
+	float Melanin = 0.8,
+	float MelaninRedness = 1.0,
+	float RandomColor = 0.0,
+	color Tint = 1.0,
+	color AbsorptionCoefficient = color(0.245531, 0.52, 1.365),
+	normal Normal = Ng,
+	string parametrization = "Absorption coefficient",
+	float Offset = radians(2),
+	float Roughness = 0.3,
+	float RadialRoughness = 0.3,
+	float RandomRoughness = 0.0,
+	float Coat = 0.0,
+	float IOR = 1.55,
+	string AttrRandom = "geom:curve_random",
+	float Random = 0.0,
+
+	output closure color BSDF = 0)
+{
+	/* Get random value from curve in none is specified. */
+	float random_value = 0.0;
+
+	if (isconnected(Random)) {
+		random_value = Random;
+	}
+	else {
+		getattribute(AttrRandom, random_value);
+	}
+
+	/* Compute roughness. */
+	float factor_random_roughness = 1.0 + 2.0*(random_value - 0.5)*RandomRoughness;
+	float m0_roughness = 1.0 - clamp(Coat, 0.0, 1.0);
+	float roughness = Roughness*factor_random_roughness;
+	float radial_roughness = RadialRoughness*factor_random_roughness;
+
+	/* Compute absorption. */
+	color sigma;
+
+	if (parametrization == "Absorption coefficient") {
+		sigma = AbsorptionCoefficient;
+	}
+	else if (parametrization == "Melanin concentration") {
+		/* Randomize melanin. */
+		float factor_random_color = 1.0 + 2.0*(random_value - 0.5) * RandomColor;
+		float melanin = Melanin * factor_random_color;
+
+		/* Map melanin 0..inf from more perceptually linear 0..1. */
+		melanin = -log(max(1.0 - melanin, 0.0001));
+
+		/* Benedikt Bitterli's melanin ratio remapping. */
+		float eumelanin = melanin * (1.0 - MelaninRedness);
+		float pheomelanin = melanin * MelaninRedness;
+		color melanin_sigma = sigma_from_concentration(eumelanin, pheomelanin);
+
+		/* Optional tint. */
+		color tint_sigma = sigma_from_reflectance(Tint, radial_roughness);
+		sigma = melanin_sigma + tint_sigma;
+	}
+	else if (parametrization == "Direct coloring"){
+		sigma = sigma_from_reflectance(Color, radial_roughness);
+	}
+	else {
+		/* Fallback to brownish hair, same as defaults for melanin. */
+		sigma = sigma_from_concentration(0.0, 0.8054375);
+	}
+
+	BSDF = principled_hair(Normal, sigma, roughness, radial_roughness, m0_roughness, Offset, IOR);
+}