Fog is initially disabled. While enabled, fog affects rasterized geometry, bitmaps, and pixel blocks, but not buffer clear operations. To enable and disable fog, call
glEnable and glDisable with argument GL_FOG.glFog assigns the value or values in params to the fog parameter specified by pname. The following values are accepted for pname:
GL_FOG_MODE
params is a single integer or floating-point value that specifies the equation to be used to compute the fog blend factor, $f$. Three symbolic constants are accepted: GL_LINEAR, GL_EXP, and GL_EXP2. The equations corresponding to these symbolic constants are defined below. The initial fog mode is GL_EXP.
GL_FOG_DENSITY
params is a single integer or floating-point value that specifies $density$, the fog density used in both exponential fog equations. Only nonnegative densities are accepted. The initial fog density is 1.
GL_FOG_START
params is a single integer or floating-point value that specifies $start$, the near distance used in the linear fog equation. The initial near distance is 0.
GL_FOG_END
params is a single integer or floating-point value that specifies $end$, the far distance used in the linear fog equation. The initial far distance is 1.
GL_FOG_INDEX
params is a single integer or floating-point value that specifies $i sub f$, the fog color index. The initial fog index is 0.
GL_FOG_COLOR
params contains four integer or floating-point values that specify $C sub f$, the fog color. Integer values are mapped linearly such that the most positive representable value maps to 1.0, and the most negative representable value maps to -1.0. Floating-point values are mapped directly. After conversion, all color components are clamped to the range [0,1]. The initial fog color is (0, 0, 0, 0).
Fog blends a fog color with each rasterized pixel fragment's posttexturing color using a blending factor $f$. Factor $f$ is computed in one of three ways, depending on the fog mode. Let $z$ be the distance in eye coordinates from the origin to the fragment being fogged. The equation for GL_LINEAR fog is f ~=~ {end ~-~ z} over {end ~-~ start}
The equation for GL_EXP fog is f ~=~ e sup {-^(density ~cdot~ z)}
The equation for GL_EXP2 fog is f ~=~ e sup {-^(density ~cdot~ z)} sup 2
Regardless of the fog mode, $f$ is clamped to the range [0, 1] after it is computed. Then, if the GL is in RGBA color mode, the fragment's red, green, and blue colors, represented by $C sub r$, are replaced by
{C sub r} sup prime ~=~ f^C sub r ~+~ (1 - f)^C sub f
Fog does not affect a fragment's alpha component.
In color index mode, the fragment's color index $i sub r$ is replaced by
{i sub r} sup prime ~=~ i sub r ~+~ (1 - f)^i sub f