beckmann

/* Get the half angle vector (microfacet normal) */
H = normalize(exitant - incident);

/* Get all the dot products. */
float NdV = abs(incident * normal);
float NdL = abs(normal * exitant);
float VdH = abs(incident * H);
float NdH = abs(normal * H);

/* Get the half angle vector's angle with the normal. */
float alpha = acos(H * normal);

/* Compute the Beckmann distribution term. */
D = exp(-pow(tan(alpha) / this->roughness, 2));

/* Normalize the Beckmann distribution. */
D = D / (PI * pow(this->roughness, 2) * pow(NdH, 4));

/* Compute the geometric attenuation term. */
float G = min(1, min(2 * NdH * NdV / VdH, 2 * NdH * NdL / VdH));

/* ... */