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/**
* @file irradianceGenF.glsl
*
* $LicenseInfo:firstyear=2022&license=viewerlgpl$
* Second Life Viewer Source Code
* Copyright (C) 2022, Linden Research, Inc.
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation;
* version 2.1 of the License only.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*
* Linden Research, Inc., 945 Battery Street, San Francisco, CA 94111 USA
* $/LicenseInfo$
*/
/*[EXTRA_CODE_HERE]*/
out vec4 frag_color;
uniform samplerCubeArray reflectionProbes;
uniform int sourceIdx;
uniform float max_probe_lod;
in vec3 vary_dir;
// Code below is derived from the Khronos GLTF Sample viewer:
// https://github.com/KhronosGroup/glTF-Sample-Viewer/blob/master/source/shaders/ibl_filtering.frag
#define MATH_PI 3.1415926535897932384626433832795
float u_roughness = 1.0;
int u_sampleCount = 32;
float u_lodBias = 2.0;
int u_width = 64;
// Hammersley Points on the Hemisphere
// CC BY 3.0 (Holger Dammertz)
// http://holger.dammertz.org/stuff/notes_HammersleyOnHemisphere.html
// with adapted interface
float radicalInverse_VdC(uint bits)
{
bits = (bits << 16u) | (bits >> 16u);
bits = ((bits & 0x55555555u) << 1u) | ((bits & 0xAAAAAAAAu) >> 1u);
bits = ((bits & 0x33333333u) << 2u) | ((bits & 0xCCCCCCCCu) >> 2u);
bits = ((bits & 0x0F0F0F0Fu) << 4u) | ((bits & 0xF0F0F0F0u) >> 4u);
bits = ((bits & 0x00FF00FFu) << 8u) | ((bits & 0xFF00FF00u) >> 8u);
return float(bits) * 2.3283064365386963e-10; // / 0x100000000
}
// hammersley2d describes a sequence of points in the 2d unit square [0,1)^2
// that can be used for quasi Monte Carlo integration
vec2 hammersley2d(int i, int N) {
return vec2(float(i)/float(N), radicalInverse_VdC(uint(i)));
}
// Hemisphere Sample
// TBN generates a tangent bitangent normal coordinate frame from the normal
// (the normal must be normalized)
mat3 generateTBN(vec3 normal)
{
vec3 bitangent = vec3(0.0, 1.0, 0.0);
float NdotUp = dot(normal, vec3(0.0, 1.0, 0.0));
float epsilon = 0.0000001;
/*if (1.0 - abs(NdotUp) <= epsilon)
{
// Sampling +Y or -Y, so we need a more robust bitangent.
if (NdotUp > 0.0)
{
bitangent = vec3(0.0, 0.0, 1.0);
}
else
{
bitangent = vec3(0.0, 0.0, -1.0);
}
}*/
vec3 tangent = normalize(cross(bitangent, normal));
bitangent = cross(normal, tangent);
return mat3(tangent, bitangent, normal);
}
struct MicrofacetDistributionSample
{
float pdf;
float cosTheta;
float sinTheta;
float phi;
};
MicrofacetDistributionSample Lambertian(vec2 xi, float roughness)
{
MicrofacetDistributionSample lambertian;
// Cosine weighted hemisphere sampling
// http://www.pbr-book.org/3ed-2018/Monte_Carlo_Integration/2D_Sampling_with_Multidimensional_Transformations.html#Cosine-WeightedHemisphereSampling
lambertian.cosTheta = sqrt(1.0 - xi.y);
lambertian.sinTheta = sqrt(xi.y); // equivalent to `sqrt(1.0 - cosTheta*cosTheta)`;
lambertian.phi = 2.0 * MATH_PI * xi.x;
lambertian.pdf = lambertian.cosTheta / MATH_PI; // evaluation for solid angle, therefore drop the sinTheta
return lambertian;
}
// getImportanceSample returns an importance sample direction with pdf in the .w component
vec4 getImportanceSample(int sampleIndex, vec3 N, float roughness)
{
// generate a quasi monte carlo point in the unit square [0.1)^2
vec2 xi = hammersley2d(sampleIndex, u_sampleCount);
MicrofacetDistributionSample importanceSample;
// generate the points on the hemisphere with a fitting mapping for
// the distribution (e.g. lambertian uses a cosine importance)
importanceSample = Lambertian(xi, roughness);
// transform the hemisphere sample to the normal coordinate frame
// i.e. rotate the hemisphere to the normal direction
vec3 localSpaceDirection = normalize(vec3(
importanceSample.sinTheta * cos(importanceSample.phi),
importanceSample.sinTheta * sin(importanceSample.phi),
importanceSample.cosTheta
));
mat3 TBN = generateTBN(N);
vec3 direction = TBN * localSpaceDirection;
return vec4(direction, importanceSample.pdf);
}
// Mipmap Filtered Samples (GPU Gems 3, 20.4)
// https://developer.nvidia.com/gpugems/gpugems3/part-iii-rendering/chapter-20-gpu-based-importance-sampling
// https://cgg.mff.cuni.cz/~jaroslav/papers/2007-sketch-fis/Final_sap_0073.pdf
float computeLod(float pdf)
{
// // Solid angle of current sample -- bigger for less likely samples
// float omegaS = 1.0 / (float(u_sampleCount) * pdf);
// // Solid angle of texel
// // note: the factor of 4.0 * MATH_PI
// float omegaP = 4.0 * MATH_PI / (6.0 * float(u_width) * float(u_width));
// // Mip level is determined by the ratio of our sample's solid angle to a texel's solid angle
// // note that 0.5 * log2 is equivalent to log4
// float lod = 0.5 * log2(omegaS / omegaP);
// babylon introduces a factor of K (=4) to the solid angle ratio
// this helps to avoid undersampling the environment map
// this does not appear in the original formulation by Jaroslav Krivanek and Mark Colbert
// log4(4) == 1
// lod += 1.0;
// We achieved good results by using the original formulation from Krivanek & Colbert adapted to cubemaps
// https://cgg.mff.cuni.cz/~jaroslav/papers/2007-sketch-fis/Final_sap_0073.pdf
float lod = 0.5 * log2( 6.0 * float(u_width) * float(u_width) / (float(u_sampleCount) * pdf));
return lod;
}
vec4 filterColor(vec3 N)
{
vec4 color = vec4(0.f);
for(int i = 0; i < u_sampleCount; ++i)
{
vec4 importanceSample = getImportanceSample(i, N, 1.0);
vec3 H = vec3(importanceSample.xyz);
float pdf = importanceSample.w;
// mipmap filtered samples (GPU Gems 3, 20.4)
float lod = computeLod(pdf);
// apply the bias to the lod
lod += u_lodBias;
lod = clamp(lod, 0, max_probe_lod);
// sample lambertian at a lower resolution to avoid fireflies
vec4 lambertian = textureLod(reflectionProbes, vec4(H, sourceIdx), lod);
color += lambertian;
}
color /= float(u_sampleCount);
return color;
}
// entry point
void main()
{
vec4 color = vec4(0);
color = filterColor(vary_dir);
frag_color = max(color, vec4(0));
}
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