Conscience is the inner voice that warns us
that somebody is looking - H. L. Mencken

Concept 3.2 : Fog Equations
===========================

Idea:
=====
Fog blends a fog color with an incoming fragment's 
color using a fog blending factor.  This factor, 'f' 
is computed with one of three equations and then clamped
to the range [0,1]

Equations :
===========
1) f = [e ^ -(density.z)] (GL_EXP)

2) f = [e ^ ( -(density - z) ^ 2)] (GL_EXP2)

3) f = [(end - z) / (end - start)] (GL_LINEAR)

In these three equations, z is the eye coordinate
distance between the viewpoint and the fragment
center.  The values for density, start and end are
all specified with glFog*().  The f factor is used
differently depending on whether one is in RGBA 
mode or in color-index mode.  Note how density affects
only the exponential equations and the start and end
points specified affect only the linear equation.

The glFog*() function
=====================
void glFog{if}(GLenum pname, TYPE param);

Sets the parameters and function for calcuation of
fog using one of the above equations.

If pname is GL_FOG_MODE, then param is either
GL_EXP GL_EXP2 or GL_LINEAR.

If pname is GL_FOG_DENSITY, GL_FOG_START or 
GL_FOG_END, then pname is a value for density
start or end in the equations respectively.

In RGBA mode, pname can be GL_FOG_COLOR in which
case params points to four values that specify the
fog's RGBA color values.  

Once we know the fog factor f from one of the 
equations above, we plug it into another formula to 
get the final fogged color.

In RGBA mode the final fogged color is given as shown 
below :

C = fCi + (1 - f) Cf

where 
Ci = incoming fragment's RGBA values and
Cf = fog colorvalues assigned with GL_FOG_COLOR.