Added missing brdf and tonemapping files to viewer

viewer/brdf.hxx

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+// Adapted from https://github.com/KhronosGroup/glTF-Sample-Viewer/blob/master/src/shaders/brdf.glsl
+
+#pragma once
+#include <cmath>
+
+//
+// Fresnel
+//
+// http://graphicrants.blogspot.com/2013/08/specular-brdf-reference.html
+// https://github.com/wdas/brdf/tree/master/src/brdfs
+// https://google.github.io/filament/Filament.md.html
+inline vec3 F_None(vec3 f0, vec3 f90, float VdotH) {
+  return f0;
+}
+
+// The following equation models the Fresnel reflectance term of the spec equation (aka F())
+// Implementation of fresnel from [4], Equation 15
+inline vec3 F_Schlick(vec3 f0, vec3 f90, float VdotH) {
+  return f0 + (f90 - f0) * pow(clamp(1 - VdotH, 0.0f, 1.0f), 5);
+}
+
+inline vec3 F_CookTorrance(vec3 f0, vec3 f90, float VdotH) {
+  vec3 f0_sqrt = sqrt(f0);
+  vec3 ior = (1 + f0_sqrt) / (1 - f0_sqrt);
+  vec3 c = vec3(VdotH);
+  vec3 g = sqrt(ior * ior + c*c - 1);
+  return 0.5f * pow(g-c, vec3(2)) / 
+    pow(g+c, vec3(2)) * (1 + pow(c*(g+c) - 1, 2) / pow(c*(g-c) + 1, 2));
+}
+
+// Smith Joint GGX
+// Note: Vis = G / (4 * NdotL * NdotV)
+// see Eric Heitz. 2014. Understanding the Masking-Shadowing Function in Microfacet-Based BRDFs. Journal of Computer Graphics Techniques, 3
+// see Real-Time Rendering. Page 331 to 336.
+// see https://google.github.io/filament/Filament.md.html#materialsystem/specularbrdf/geometricshadowing(specularg)
+inline float V_GGX(float NdotL, float NdotV, float alphaRoughness) {
+  float r2 = alphaRoughness * alphaRoughness;
+
+  float GGXV = NdotL * sqrt(NdotV * NdotV * (1 - r2) + r2);
+  float GGXL = NdotV * sqrt(NdotL * NdotL * (1 - r2) + r2);
+
+  float GGX = GGXV + GGXL;
+  return (GGX > 0) ? 0.5f / GGX : 0;
+}
+
+// Anisotropic GGX visibility function, with height correlation.
+// T: Tanget, B: Bi-tanget
+inline float V_GGX_anisotropic(float NdotL, float NdotV, float BdotV, 
+  float TdotV, float TdotL, float BdotL, float anisotropy, float at, float ab) {
+  float GGXV = NdotL * length(vec3(at * TdotV, ab * BdotV, NdotV));
+  float GGXL = NdotV * length(vec3(at * TdotL, ab * BdotL, NdotL));
+  float v = 0.5f / (GGXV + GGXL);
+  return clamp(v, 0.f, 1.f);
+}
+
+// https://github.com/google/filament/blob/master/shaders/src/brdf.fs#L136
+// https://github.com/google/filament/blob/master/libs/ibl/src/CubemapIBL.cpp#L179
+// Note: Google call it V_Ashikhmin and V_Neubelt
+inline float V_Ashikhmin(float NdotL, float NdotV) {
+  return clamp(1 / (4 * (NdotL + NdotV - NdotL * NdotV)), 0.f, 1.f);
+}
+
+// https://github.com/google/filament/blob/master/shaders/src/brdf.fs#L131
+inline float V_Kelemen(float LdotH) {
+  // Kelemen 2001, "A Microfacet Based Coupled Specular-Matte BRDF Model with Importance Sampling"
+  return 0.25f / (LdotH * LdotH);
+}
+
+// The following equation(s) model the distribution of microfacet normals across the area being drawn (aka D())
+// Implementation from "Average Irregularity Representation of a Roughened Surface for Ray Reflection" by T. S. Trowbridge, and K. P. Reitz
+// Follows the distribution function recommended in the SIGGRAPH 2013 course notes from EPIC Games [1], Equation 3.
+inline float D_GGX(float NdotH, float alphaRoughness) {
+  float alphaRoughnessSq = alphaRoughness * alphaRoughness;
+  float f = (NdotH * NdotH) * (alphaRoughnessSq - 1) + 1;
+  return alphaRoughnessSq / (M_PIf32 * f * f);
+}
+
+// Anisotropic GGX NDF with a single anisotropy parameter controlling the normal orientation.
+// See https://google.github.io/filament/Filament.html#materialsystem/anisotropicmodel
+// T: Tanget, B: Bi-tanget
+inline float D_GGX_anisotropic(float NdotH, float TdotH, float BdotH, 
+  float anisotropy, float at, float ab) {
+
+  float a2 = at * ab;
+  vec3 f = vec3(ab * TdotH, at * BdotH, a2 * NdotH);
+  float w2 = a2 / dot(f, f);
+  return a2 * w2 * w2 / M_PIf32;
+}
+
+inline float D_Ashikhmin(float NdotH, float alphaRoughness) {
+  // Ashikhmin 2007, "Distribution-based BRDFs"
+  float a2 = alphaRoughness * alphaRoughness;
+  float cos2h = NdotH * NdotH;
+  float sin2h = 1.0 - cos2h;
+  float sin4h = sin2h * sin2h;
+  float cot2 = -cos2h / (a2 * sin2h);
+  return 1 / (M_PIf32 * (4 * a2 + 1) * sin4h) * (4 * exp(cot2) + sin4h);
+}
+
+//Sheen implementation-------------------------------------------------------------------------------------
+// See  https://github.com/sebavan/glTF/tree/KHR_materials_sheen/extensions/2.0/Khronos/KHR_materials_sheen
+
+// Estevez and Kulla http://www.aconty.com/pdf/s2017_pbs_imageworks_sheen.pdf
+inline float D_Charlie(float sheenRoughness, float NdotH) {
+  sheenRoughness = max(sheenRoughness, 0.000001f); //clamp (0,1]
+  float alphaG = sheenRoughness * sheenRoughness;
+  float invR = 1 / alphaG;
+  float cos2h = NdotH * NdotH;
+  float sin2h = 1 - cos2h;
+  return (2 + invR) * pow(sin2h, invR * 0.5f) / (2 * M_PIf32);
+}
+
+//https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#acknowledgments AppendixB
+inline vec3 BRDF_lambertian(vec3 f0, vec3 f90, vec3 diffuseColor, float VdotH) {
+  // see https://seblagarde.wordpress.com/2012/01/08/pi-or-not-to-pi-in-game-lighting-equation/
+  return (1 - F_Schlick(f0, f90, VdotH)) * (diffuseColor / M_PIf32);
+}
+
+//  https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#acknowledgments AppendixB
+inline vec3 BRDF_specularGGX(vec3 f0, vec3 f90, float alphaRoughness, 
+  float VdotH, float NdotL, float NdotV, float NdotH) {
+
+  vec3 F = F_Schlick(f0, f90, VdotH);
+  float Vis = V_GGX(NdotL, NdotV, alphaRoughness);
+  float D = D_GGX(NdotH, alphaRoughness);
+  return F * Vis * D;
+}
+
+inline vec3 BRDF_specularAnisotropicGGX(vec3 f0, vec3 f90, 
+  float alphaRoughness, float VdotH, float NdotL, float NdotV, float NdotH,
+  float BdotV, float TdotV, float TdotL, float BdotL, float TdotH, float BdotH,
+  float anisotropy) {
+
+  // Roughness along tangent and bitangent.
+  // Christopher Kulla and Alejandro Conty. 2017. Revisiting Physically Based Shading at Imageworks
+  float at = max(alphaRoughness * (1 + anisotropy), 0.00001f);
+  float ab = max(alphaRoughness * (1 - anisotropy), 0.00001f);
+
+  vec3 F = F_Schlick(f0, f90, VdotH);
+  float V = V_GGX_anisotropic(NdotL, NdotV, BdotV, TdotV, TdotL, BdotL, 
+    anisotropy, at, ab);
+  float D = D_GGX_anisotropic(NdotH, TdotH, BdotH, anisotropy, at, ab);
+
+  return F * V * D;
+}
+
+// f_sheen
+inline vec3 BRDF_specularSheen(vec3 sheenColor, float sheenIntensity, 
+  float sheenRoughness, float NdotL, float NdotV, float NdotH) {
+  float sheenDistribution = D_Charlie(sheenRoughness, NdotH);
+  float sheenVisibility = V_Ashikhmin(NdotL, NdotV);
+  return sheenColor * sheenIntensity * sheenDistribution * sheenVisibility;
+}

viewer/tonemapping.hxx

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+#pragma once
+
+// linear to sRGB approximation
+// see http://chilliant.blogspot.com/2012/08/srgb-approximations-for-hlsl.html
+inline vec3 linearTosRGB(vec3 color) {
+  const float GAMMA = 2.2;
+  const float INV_GAMMA = 1.0 / GAMMA;
+
+  return pow(color, vec3(INV_GAMMA));
+}
+
+// sRGB to linear approximation
+// see http://chilliant.blogspot.com/2012/08/srgb-approximations-for-hlsl.html
+inline vec3 sRGBToLinear(vec3 srgbIn) {
+  const float GAMMA = 2.2;
+  return vec3(pow(srgbIn.xyz, vec3(GAMMA)));
+}
+
+inline vec4 sRGBToLinear(vec4 srgbIn) {
+  return vec4(sRGBToLinear(srgbIn.xyz), srgbIn.w);
+}
+
+// Uncharted 2 tone map
+// see: http://filmicworlds.com/blog/filmic-tonemapping-operators/
+inline vec3 toneMapUncharted2Impl(vec3 color) {
+  const float A = 0.15;
+  const float B = 0.50;
+  const float C = 0.10;
+  const float D = 0.20;
+  const float E = 0.02;
+  const float F = 0.30;
+  return ((color*(A*color+C*B)+D*E)/(color*(A*color+B)+D*F))-E/F;
+}
+
+inline vec3 toneMapUncharted(vec3 color) {
+  const float W = 11.2;
+  color = toneMapUncharted2Impl(2 * color);
+  vec3 whiteScale = 1 / toneMapUncharted2Impl(W);
+  return linearTosRGB(color * whiteScale);
+}
+
+// Hejl Richard tone map
+// see: http://filmicworlds.com/blog/filmic-tonemapping-operators/
+inline vec3 toneMapHejlRichard(vec3 color) {
+  color = max(vec3(0.0), color - vec3(0.004));
+  return (color * (6.2f * color + .5f)) / 
+    (color * (6.2f * color + 1.7f) + 0.06f);
+}
+
+// ACES tone map
+// see: https://knarkowicz.wordpress.com/2016/01/06/aces-filmic-tone-mapping-curve/
+inline vec3 toneMapACES(vec3 color) {
+  const float A = 2.51;
+  const float B = 0.03;
+  const float C = 2.43;
+  const float D = 0.59;
+  const float E = 0.14;
+  return linearTosRGB(clamp((color * (A * color + B)) / (color * (C * color + D) + E), 0.f, 1.f));
+}
+
+inline vec3 toneMap(vec3 color, float exposure) {
+  color *= exposure;
+/*
+#ifdef TONEMAP_UNCHARTED
+    return toneMapUncharted(color);
+#endif
+
+#ifdef TONEMAP_HEJLRICHARD
+    return toneMapHejlRichard(color);
+#endif
+
+#ifdef TONEMAP_ACES
+    return toneMapACES(color);
+#endif
+*/
+  return linearTosRGB(color);
+}