Question

The color velvet and the moon are poorly approximated by diffuse or Phong shading. Why?

Answer 1

Answer:

Because diffuse and Phong shading are not particularly good representations of the rich variety of interactions with light. In particular, the full moon is more than twice as bright as the first or third quarter moon, even though the area is exactly twice as large. This effect is called "opposition surge". Velvet has a complex surface reflectance as well, since it is made up of many small hairs, each of which have a specular response, and which can display some anisotropy based upon how the velvet is brushed. If you'd like to read more, you should look for information on BRDFs (bidirectional reflectance distribution functions). There are many different proposed models, each with their own drawbacks and strengths.