#pragma glslify: beckmannDistribution = require(glsl-specular-beckmann/distribution)

float cookTorranceSpecular(
  vec3 lightDirection,
  vec3 viewDirection,
  vec3 surfaceNormal,
  float roughness,
  float fresnel) {

  float VdotN = max(dot(viewDirection, surfaceNormal), 0.0);
  float LdotN = max(dot(lightDirection, surfaceNormal), 0.0);

  //Half angle vector
  vec3 H = normalize(lightDirection + viewDirection);

  //Geometric term
  float NdotH = max(dot(surfaceNormal, H), 0.0);
  float VdotH = max(dot(viewDirection, H), 0.0001);
  float LdotH = max(dot(lightDirection, H), 0.0001);
  float G1 = (2.0 * NdotH * VdotN) / VdotH;
  float G2 = (2.0 * NdotH * LdotN) / LdotH;
  float G = min(1.0, min(G1, G2));
  
  //Distribution term
  float D = beckmannDistribution(NdotH, roughness);

  //Fresnel term
  float F = pow(1.0 - VdotN, fresnel);

  //Multiply terms and done
  return  G * F * D / (3.14159265 * VdotN);
}

#pragma glslify: export(cookTorranceSpecular)