ToyEngine/Demo/Engine/Shaders/brdf_lut.comp

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#version 450 core
layout(local_size_x = 8, local_size_y = 8) in;
layout(rg16f, binding = 0) uniform image2D brdfLUT;
const float PI = 3.14159265359;
// ----------------------------------------------------------------------------
// http://holger.dammertz.org/stuff/notes_HammersleyOnHemisphere.html
// efficient VanDerCorpus calculation.
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
}
// ----------------------------------------------------------------------------
vec2 Hammersley(uint i, uint N)
{
return vec2(float(i)/float(N), RadicalInverse_VdC(i));
}
// ----------------------------------------------------------------------------
vec3 ImportanceSampleGGX(vec2 Xi, vec3 N, float roughness)
{
float a = roughness*roughness;
float phi = 2.0 * PI * Xi.x;
float cosTheta = sqrt((1.0 - Xi.y) / (1.0 + (a*a - 1.0) * Xi.y));
float sinTheta = sqrt(1.0 - cosTheta*cosTheta);
// from spherical coordinates to cartesian coordinates - halfway vector
vec3 H;
H.x = cos(phi) * sinTheta;
H.y = sin(phi) * sinTheta;
H.z = cosTheta;
// from tangent-space H vector to world-space sample vector
vec3 up = abs(N.z) < 0.999 ? vec3(0.0, 0.0, 1.0) : vec3(1.0, 0.0, 0.0);
vec3 tangent = normalize(cross(up, N));
vec3 bitangent = cross(N, tangent);
vec3 sampleVec = tangent * H.x + bitangent * H.y + N * H.z;
return normalize(sampleVec);
}
// ----------------------------------------------------------------------------
float GeometrySchlickGGX(float NdotV, float roughness)
{
// note that we use a different k for IBL
float a = roughness;
float k = (a * a) / 2.0;
float nom = NdotV;
float denom = NdotV * (1.0 - k) + k;
return nom / denom;
}
// ----------------------------------------------------------------------------
float GeometrySmith(vec3 N, vec3 V, vec3 L, float roughness)
{
float NdotV = max(dot(N, V), 0.0);
float NdotL = max(dot(N, L), 0.0);
float ggx2 = GeometrySchlickGGX(NdotV, roughness);
float ggx1 = GeometrySchlickGGX(NdotL, roughness);
return ggx1 * ggx2;
}
// ----------------------------------------------------------------------------
vec2 IntegrateBRDF(float NdotV, float roughness)
{
vec3 V;
V.x = sqrt(1.0 - NdotV*NdotV);
V.y = 0.0;
V.z = NdotV;
float A = 0.0;
float B = 0.0;
vec3 N = vec3(0.0, 0.0, 1.0);
const uint SAMPLE_COUNT = 1024u;
for(uint i = 0u; i < SAMPLE_COUNT; ++i)
{
// generates a sample vector that's biased towards the
// preferred alignment direction (importance sampling).
vec2 Xi = Hammersley(i, SAMPLE_COUNT);
vec3 H = ImportanceSampleGGX(Xi, N, roughness);
vec3 L = normalize(2.0 * dot(V, H) * H - V);
float NdotL = max(L.z, 0.0);
float NdotH = max(H.z, 0.0);
float VdotH = max(dot(V, H), 0.0);
if(NdotL > 0.0)
{
float G = GeometrySmith(N, V, L, roughness);
float G_Vis = (G * VdotH) / (NdotH * NdotV);
float Fc = pow(1.0 - VdotH, 5.0);
A += (1.0 - Fc) * G_Vis;
B += Fc * G_Vis;
}
}
A /= float(SAMPLE_COUNT);
B /= float(SAMPLE_COUNT);
return vec2(A, B);
}
// ----------------------------------------------------------------------------
void main()
{
uvec2 pixelLocation = gl_GlobalInvocationID.xy;
vec2 texCoords = (vec2(pixelLocation)+vec2(0.5))/vec2(imageSize(brdfLUT));
vec2 integratedBRDF = IntegrateBRDF(texCoords.x, texCoords.y);
imageStore(brdfLUT, ivec2(pixelLocation), vec4(integratedBRDF,0,0));
}