[citation needed] Phong's methods have proven popular due to their generally efficient use of computation time per rendered pixel.
Phong shading assumes a smoothly varying surface normal vector.
The most serious problem with Gouraud shading occurs when specular highlights are found in the middle of a large polygon.
In modern graphics hardware, variants of this algorithm are implemented using pixel or fragment shaders.
The reflection model also includes an ambient term to account for the small amount of light that is scattered about the entire scene.