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Fog effects can give a 3-D scene greater realism. You can use fog effects for more than simulating fog. They can also decrease the clarity of a scene with distance. This mirrors what happens in the real world; as objects become more distant from the user, their detail is less distinct.
For more information about using fog in your application, see Fog.
An application controls fog through device rendering states. The D3DMRENDERSTATETYPEenumerated type includes states to control whether pixel (table) or vertex fog is used, what color it is, the fog formula the system applies, and the parameters of the formula.
You enable fog by setting the D3DRS_FOGENABLE render state to TRUE. The fog color can be set to any color value by using the D3DRS_FOGCOLOR render state; the alpha component of the fog color is ignored, .
The D3DMRS_FOGTABLEMODE and D3DMRS_FOGVERTEXMODE render states control the fog formula applied for fog calculations, and they indirectly control which type of fog is applied. Both render states can be set to a member of the D3DMFOGMODEenumerated type. Setting either render state to D3DFOG_NONE disables pixel or vertex fog, respectively. If both render states are set to valid modes, the system applies only pixel fog effects.
The D3DMRS_FOGSTART and D3DMRS_FOGEND render states control fog formula parameters for the D3DFOG_LINEAR mode. The D3DMRS_FOGDENSITY render state controls fog density in the exponential fog modes.