These equations allow to define the way fog density behaves according to the distance between the camera and a given vertex .
The aim of these equations is to compute a fog factor, (the `fogFactor`) that will allow to mix the fragment color
(at the pixel shader level) with the fog color. With fixed pipeline, there are two ways to compute the fog factor:
linear or exponential.

We will focus on the exponential method, because of its more realistic fog rendering possibilities .
But keep in mind that those methods are linked to the fixed pipeline and that with the programmable pipeline
we can compute totaly different calculus which shall suit better to a given scene (as an example, for each color composant
we can set a different fog factor) .

The fog density equations are as follows :

Linear equation (corresponds to GL_LINEAR):

fogFactor = (end - z) / (end - start)

Exponential equations (correspond to GL_EXP and GL_EXP2):

fogFactor = e^{-(density * z)}

and

fogFactor = e^{-(density * z)2}

The following chart shows the fog factor evolution according to the distance :