Fog is simply the alpha part of the color specified in the specular member of the D3DTLVERTEX structure. Another way of thinking about this is that specular color is really RGBF color, where "F" is "fog."
In monochromatic lighting mode, fog works properly only when the fog color is black or when there is no lighting, in which case any fog color has the same effect.
There are three fog modes: linear, exponential, and exponential squared. Only the linear fog mode is currently supported.
When you use linear fog, you specify a start and end point for the fog effect. The fog effect begins at the specified starting point and increases linearly until it reaches its maximum density at the specified end point.
The exponential fog modes begin with a barely visible fog effect and increase to the maximum density along an exponential curve. The following is the formula for the exponential fog mode:
In the exponential squared fog mode, the fog effect increases more quickly than in the exponential fog mode. The following is the formula for the exponential squared fog mode:
In these formulas, e is the base of the natural logarithms; its value is approximately 2.71828. Note that fog can be considered as a measure of visibility—the lower the fog value, the less visible an object is.
For example, if an application used the exponential fog mode and a fog density of 0.5, the fog value at a distance from the camera of 0.8 would be 0.6703, as shown in the following example:
