Microsoft DirectX 8.1 (Visual Basic)

Fog Formulas

Applications written in Microsoft® Visual Basic® can control how fog affects the color of objects in a scene by changing how Microsoft Direct3D® computes fog effects over distance. The CONST_D3DFOGMODE enumeration contains members that identify the three fog formulas. All formulas calculate a fog factor as a function of distance, given parameters that your application sets.

D3DFOG_LINEAR

where

start is the distance at which fog effects begin.
end is the distance at which fog effects no longer increase.
d represents depth, or the distance from the viewpoint. For range based fog, the value for d is the distance between the camera position and a vertex. For non-range based fog, the value for d is the absolute value of the Z coordinate in camera space.

Linear and exponential formulas are supported for both pixel fog and vertex fog.

D3DFOG_EXP

where

e is the base of natural logarithms. (approximately 2.71828)
density is an arbitrary fog density that can range from 0.0 to 1.0.
d represents depth, or the distance from the viewpoint, as stated earlier.

D3DFOG_EXP2

where

e is the base of natural logarithms as stated above.
density is an arbitrary fog density that can range from 0.0 to 1.0 as stated above.
d represents depth, or the distance from the viewpoint, as stated above.

Note The system stores the fog factor in the alpha component of the specular color for a vertex. If your application performs its own transformation and lighting, you can insert fog factor values manually, to be applied by the system during rendering.

The following illustration graphs these formulas, using common values as in the formula parameters.

D3DFOG_LINEAR is 1.0 at start and 0.0 at end. It is not measured relative to the near or far planes.

When Direct3D calculates fog effects, it uses the fog factor from one of the preceding equations in a blending formula, shown here.

This formula effectively scales the color of the current polygon C_i by the fog factor f, and adds the product to the fog color C_f, scaled by the bitwise inverse of the fog factor. The resulting color value is a blend of the fog color and the original color, as a factor of distance. The formula applies to all devices supported in Microsoft DirectX® 7.0 and later. For the legacy ramp device, the fog factor scales the diffuse and specular color components, clamped to the range of 0.0 and 1.0, inclusive. The fog factor typically starts at 1.0 for the near plane and decreases to 0.0 at the far plane.