When a group of photons arrives at a material surface, some of the photons might be immediately turned away (i.e., reflected) while others might penetrate into the material (i.e., refracted). The refracted portion is scattered further inside the material, giving rise to subsurface scattering, which we will discuss in the next chapter. This chapter is concerned with surface scattering—the apparent reflection at the surface. There are two properties we care about in surface scattering: the directions of the reflection and the energy along each direction. These two properties are captured by what is known as the BRDF, which is the protagonist of this chapter. We will then derive a few very useful equations and properties based on BRDF.