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precision highp float;
#define PI 3.1415926535897932384626433832795
#define NUM_SAMPLES 1024
in vec2 Texcoord;
layout (location = 0) out vec2 Color;
float radical_inverse_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
}
vec2 hammersley(uint i, uint N) {
return vec2(float(i)/float(N), radical_inverse_VdC(i));
}
vec3 importance_sample_GGX(vec2 sample_box, vec3 N, float roughness) {
float r2 = roughness * roughness;
// Spherical coordinates.
float phi = 2.0 * PI * sample_box.x;
float cos_theta = sqrt((1.0 - sample_box.y) / (1.0 + (r2*r2 - 1.0) * sample_box.y));
float sin_theta = sqrt(1.0 - cos_theta * cos_theta);
// Map spherical coordinates to Cartesian coordinates in tangent space.
vec3 H = vec3(cos(phi) * sin_theta, sin(phi) * sin_theta, cos_theta);
// Map from tangent space to world space.
//
// Tangent space:
//
// N
// |
// |
// |
// |_ _ _ _ _ B
// /
// /
// T
vec3 up = abs(N.z) < 0.999 ? vec3(0.0, 0.0, 1.0) : vec3(1.0, 0.0, 0.0);
vec3 T = normalize(cross(up,N));
vec3 B = cross(N,T);
vec3 H_ws = H.x*T + H.y*B + H.z*N;
return H_ws;
}
float geometry_schlick_GGX(float k, float NdotV) {
return NdotV / (NdotV * (1.0 - k) + k);
}
float geometry_smith(float roughness, float NdotL, float NdotV) {
float k = roughness * roughness / 2.0; // IBL
return geometry_schlick_GGX(k, NdotV) * geometry_schlick_GGX(k, NdotL);
}
vec2 integrate_brdf(float NdotV, float roughness)
{
vec3 V = vec3(sqrt(1.0 - NdotV * NdotV), 0.0, NdotV);
vec3 N = vec3(0.0, 0.0, 1.0);
float scale = 0.0;
float bias = 0.0;
for (int i = 0; i < NUM_SAMPLES; ++i) {
vec2 sample_box = hammersley(i, NUM_SAMPLES);
vec3 H = importance_sample_GGX(sample_box, N, roughness);
vec3 L = reflect(-V,H);
float NdotL = max(0.0, L.z);
if (NdotL > 0.0) {
float NdotH = max(0.0, H.z);
float VdotH = max(0.0, dot(V,H));
float G = geometry_smith(roughness, NdotL, NdotV);
float G_vis = (G * VdotH) / (NdotH * NdotV);
float Fc = pow(1.0 - VdotH, 5.0);
scale += (1.0 - Fc) * G_vis;
bias += Fc * G_vis;
}
}
scale /= float(NUM_SAMPLES);
bias /= float(NUM_SAMPLES);
return vec2(scale, bias);
}
void main()
{
Color = integrate_brdf(Texcoord.x, Texcoord.y);
}
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