The Phong model sums ambient, diffuse, and specular components to shade 3D surfaces

The Phong reflection model approximates surface lighting by adding three terms. Ambient is a constant base color simulating indirect light. Diffuse (Lambertian) scales with the dot product of the surface normal and light direction. Specular produces the glossy highlight: it depends on the reflected light direction and view direction dot product raised to a shininess exponent. High shininess gives a tight bright spot; low gives a broad dull sheen. The final color is ambient + diffuse + specular. Each term is scaled by a material constant and a light color. The model is physically approximate but cheap and widely used in real-time shader code.

Examples

vec3 phong(vec3 lightDir, vec3 normal, vec3 rd, Material mat) {
  vec3 ambient  = mat.ambientColor;
  vec3 diffuse  = mat.diffuseColor * clamp(dot(lightDir, normal), 0., 1.);
  vec3 R        = reflect(lightDir, normal);
  vec3 specular = mat.specularColor * pow(clamp(dot(R, -rd), 0., 1.), mat.alpha);
  return ambient + diffuse + specular;
}

Assessment

Add a Phong shader to a raymarched sphere. Separately zero out each term and describe the visual effect of each in isolation.