glass materials) can also vary with incident viewing direction (the direction of the incoming ray).

Lambertian (diffuse) reflectance can either always scatter and attenuate light according to its

ray that isn't scattered is just absorbed into the material). It could also be a mixture of both

those strategies. We will choose to always scatter, so implementing Lambertian materials becomes a

simple task:

The reflected ray direction in red is just $\mathbf{v} + 2\mathbf{b}$. In our design, $\mathbf{n}$

is a unit vector (length one), but $\mathbf{v}$ may not be. To get the vector $\mathbf{b}$, we scale

the normal vector by the length of the projection of $\mathbf{v}$ onto $\mathbf{n}$, which is given

points _into_ the surface, and we want $\mathbf{b}$ to point _out_ of the surface, we need to negate

this projection length.