The Effects Framework - Introduction to 3D Game Programming with DirectX 9.0 - page 354

FogStart —Marks the starting depth at which objects will begin
to be fogged
FogEnd —Marks the ending depth at which objects will stop being
fogged
Note: FogStart and FogEnd essentially define the depth interval
(from the viewpoint) that objects must be in to be fogged.
FogColor —A DWORD or D3DCOLOR value type that describes the
fog's color
FogEnable —Specify true to enable vertex fog or false to dis-
able vertex fog
Any geometry that we render using the fog.txt effect will have fog
applied to it. In this way, we can control which objects get fogged and
which ones don't. This is useful for only fogging certain areas. For
example, generally if it is foggy outside, the insides of houses are not
foggy. On the same note, certain parts of a geographic region may be
foggy, but other parts may not be. Figure 19.2 shows screen shots taken
from this section's sample program called Fog Effect.
Figure 19.2: Screen shots from the
Fog Effect sample. In this sample
we use a linear fog function, and
the fog render states are specified
inside an effect file.
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