gistfile1.glsl

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, float Roughness, vec3 N) {
  float a = Roughness * Roughness;

  float Phi = 2 * PI * Xi.x;
  float CosTheta = sqrt( (1 - Xi.y) / ( 1 + (a*a - 1) * Xi.y ) );
  float SinTheta = sqrt( 1 - CosTheta * CosTheta );

  vec3 H;
  H.x = SinTheta * cos( Phi );
  H.y = SinTheta * sin( Phi );
  H.z = CosTheta;
  vec3 UpVector = abs(N.z) < 0.999 ? vec3(0,0,1) : vec3(1,0,0);
  vec3 TangentX = normalize( cross( UpVector, N ) );
  vec3 TangentY = cross( N, TangentX );
  // Tangent to world space
  return TangentX * H.x + TangentY * H.y + N * H.z;
}



vec3 IBLDiffuse(vec3 DiffuseColor, vec3 N, vec3 V) {
  const uint NumSamples = 1024u;
  vec3 DiffuseLighting = vec3(0.0);
  float NoV = saturate(dot(N, V));
  for( uint i = 0u; i < NumSamples; i++ ) {
    vec2 Xi = Hammersley(i, NumSamples);
    vec3 H2 = ImportanceSampleGGX(Xi, 1.0, N);
    vec3 L2 = 2.0 * dot( V, H2 ) * H2 - V;

    float NoL2 = saturate(dot(N, L2));

    if(NoL2 > 0.0) {
      vec3 SampleColor2 = textureCube(environment_texture, L2).rgb;
      DiffuseLighting += DiffuseColor * SampleColor2 * NoL2 / PI;
    }
  }
  return DiffuseLighting / NumSamples;
}