// Compute Phong shading to model Diffuse materials.
// Arguments:
// intersection_position - position of the intersection (vec3)
// intersection_normal - normal at the intersection (vec3)
// light_position - position of the light source (vec3)
// light_intensity - intensity of the light source (vec3)
// ray_origin - origin of the ray (vec3)
// kd - diffuse coefficient (vec3)
// ks - specular coefficient (vec3)
// s - shininess coefficient (float)
// Returns:
// vec3 - color of the intersection point
phong_shading_function = function(intersection_position, intersection_normal, light_position, light_intensity, ray_origin, kd, ks, s){
return vec3.create();
};