- specify fog parameters
C SPECIFICATION
PARAMETERS
_param1 Specifies a single-valued fog parameter.
GL_FOG_MODE, GL_FOG_DENSITY, GL_FOG_START,
GL_FOG_END, and GL_FOG_INDEX are accepted.
_param2 Specifies the value that _param1 will be set to.
C SPECIFICATION
PARAMETERS
_param1
Specifies a fog parameter. GL_FOG_MODE,
GL_FOG_DENSITY, GL_FOG_START, GL_FOG_END,
GL_FOG_INDEX, and GL_FOG_COLOR are accepted.
_param2
Specifies the value or values to be assigned to
_param1. GL_FOG_COLOR requires an array of four
values. All other parameters accept an array con-
taining only a single value.
DESCRIPTION
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 _param2 to the fog
parameter specified by _param1. The following values are
accepted for _param1:
GL_FOG_MODE _param2 is a single integer or float-
ing-point value that specifies the
equation to be used to compute the fog
blend factor, f. Three symbolic con-
stants are accepted: GL_LINEAR,
GL_EXP, and GL_EXP2. The equations
corresponding to these symbolic con-
stants are defined below. The initial
fog mode is GL_EXP.
GL_FOG_DENSITY _param2 is a single integer or float-
ing-point value that specifies den-
sity, the fog density used in both
exponential fog equations. Only non-
negative densities are accepted. The
initial fog density is 1.
GL_FOG_START _param2 is a single integer or float-
index is 0.
GL_FOG_COLOR _param2 contains four integer or
floating-point values that specify Cf,
the fog color. Integer values are
mapped linearly such that the most
positive representable value maps to
1.0, and the most negative repre-
sentable value maps to -1.0. Float-
ing-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 frag-
ment'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 equa-
tion for GL_LINEAR fog is
f=e_n_e<I>d_n_d<I>s_<I>t_z<I>a_r_t_
The equation for GL_EXP fog is
f=e-(density.z)
The equation for GL_EXP2 fog is
f=e-(density.z)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 Cr, are
replaced by
Cr'=fCr+(1-f)Cf
Fog does not affect a fragment's alpha component.
In color index mode, the fragment's color index ir is replaced by
ir'=ir+(1-f)if
ERRORS
GL_INVALID_ENUM is generated if _param1 is not an accepted
value, or if _param1 is GL_FOG_MODE and _param2 is not an
glGet with argument GL_FOG_START
glGet with argument GL_FOG_END
glGet with argument GL_FOG_MODE
SEE ALSO
glEnable
1