Shadows provide the visual clues for the real world interaction between light and the object. The physical world is full of light and shadow; without it, models appear artificial. This could be the desired effect, however, so shadowing is a subjective choice based on the application. More computation time is required to add shadows, but the benefit is in realism. They can be achieved through ray-tracing, radiosity, using a shadow Z-buffer or shadow maps. Shadows are important in facial animation, for example, in the look of heavy, dark eyelids for realism.