GLFOG() MachTen Programmer’s Manual GLFOG()

NAME
glFogf, glFogi, glFogfv, glFogiv - specify fog parameters

C SPECIFICATION
void glFogf( GLenum pname,
GLfloat param )
void glFogi( GLenum pname,
GLint param )

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PARAMETERS
pname Specifies a single-valued fog parameter.
GL_FOG_MODE, GL_FOG_DENSITY, GL_FOG_START,
GL_FOG_END, and GL_FOG_INDEX are accepted.

param Specifies the value that pname will be set to.

C SPECIFICATION
void glFogfv( GLenum pname,
const GLfloat *params )
void glFogiv( GLenum pname,
const GLint *params )

PARAMETERS
pname 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.

params Specifies the value or values to be assigned to
pname. GL_FOG_COLOR requires an array of four val-
ues. All other parameters accept an array contain-
ing 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 params to the fog
parameter specified by pname. The following values are
accepted for pname:

GL_FOG_MODE params 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
constants 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 params is a single integer or float-
ing-point value that specifies $den-
sity$, 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 float-
ing-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 float-
ing-point value that specifies $end$,
the far distance used in the linear
fog equation. The initial far dis-
tance is 1.

GL_FOG_INDEX params is a single integer or float-
ing-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 float-
ing-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 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 ~=~ {end ~-~ z} over {end ~-~ start}

The equation for GL_EXP fog is

f ~=~ e ** (-(density ~cdot~ z))

The equation for GL_EXP2 fog is

f ~=~ e ** (-(density ~cdot~ 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 color $C sub r$ is replaced by

{C sub r} prime ~=~ f C sub r + (1 - f) C sub f

In color index mode, the fragment’s color index $i sub r$ is replaced by

{i sub r} prime ~=~ i sub r + (1 - f) i sub f

ERRORS
GL_INVALID_ENUM is generated if pname is not an accepted
value, or if pname is GL_FOG_MODE and params is not an
accepted value.

GL_INVALID_VALUE is generated if pname is GL_FOG_DENSITY,
and params is negative.

GL_INVALID_OPERATION is generated if glFog is executed
between the execution of glBegin and the corresponding
execution of glEnd.

ASSOCIATED GETS
glIsEnabled with argument GL_FOG
glGet with argument GL_FOG_COLOR
glGet with argument GL_FOG_INDEX
glGet with argument GL_FOG_DENSITY
glGet with argument GL_FOG_START
glGet with argument GL_FOG_END
glGet with argument GL_FOG_MODE

SEE ALSO
glEnable

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