struct GltfMaterial
{
  int shadingModel;  // 0: metallic-roughness, 1: specular-glossiness

  // PbrMetallicRoughness
  vec4  pbrBaseColorFactor;
  int   pbrBaseColorTexture;
  float pbrMetallicFactor;
  float pbrRoughnessFactor;
  int   pbrMetallicRoughnessTexture;

  // KHR_materials_pbrSpecularGlossiness
  vec4  khrDiffuseFactor;
  int   khrDiffuseTexture;
  vec3  khrSpecularFactor;
  float khrGlossinessFactor;
  int   khrSpecularGlossinessTexture;

  int   emissiveTexture;
  vec3  emissiveFactor;
  int   alphaMode;
  float alphaCutoff;
  bool  doubleSided;

  int   normalTexture;
  float normalTextureScale;
  int   occlusionTexture;
  float occlusionTextureStrength;
};

struct PrimMeshInfo
{
  uint indexOffset;
  uint vertexOffset;
  int  materialIndex;
};


vec3 computeDiffuse(GltfMaterial mat, vec3 lightDir, vec3 normal)
{
  // Lambertian
  float dotNL = max(dot(normal, lightDir), 0.0);
  return mat.pbrBaseColorFactor.xyz * dotNL;
}

vec3 computeSpecular(GltfMaterial mat, vec3 viewDir, vec3 lightDir, vec3 normal)
{
  // Compute specular only if not in shadow
  const float kPi        = 3.14159265;
  const float kShininess = 60.0;

  // Specular
  const float kEnergyConservation = (2.0 + kShininess) / (2.0 * kPi);
  vec3        V                   = normalize(-viewDir);
  vec3        R                   = reflect(-lightDir, normal);
  float       specular            = kEnergyConservation * pow(max(dot(V, R), 0.0), kShininess);

  return vec3(specular);
}