Function imitate a fog in the plot. Fog start from relative distance dz from view point and its density growths exponentially in depth. So that the fog influence is determined by law ~ 1-exp(-d*z). Here z is normalized to 1 depth of the plot. If value d=
0 then the fog is absent. Note, that fog was applied at stage of image creation, not at stage of drawing. See Adding fog, for sample code and picture.