GLFOG(3G) GLFOG(3G)
NAME
glFogf, glFogi, glFogfv, glFogiv - specify fog parameters
C SPECIFICATION
void glFogf( GLenum pname,
GLfloat param )
void glFogi( GLenum pname,
GLint param )
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
values. All other parameters accept an array containing 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 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 if, the fog
color index. The initial fog index is 0.
GL_FOG_COLOR params contains four integer or floating-point values that specify Cf, the fog
color. Integer values are mapped linearly such that the most positive repre-sentable representable
sentable 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 dis-tance distance
tance in eye coordinates from the origin to the fragment being fogged. The equation for GL_LINEAR
fog is
f = (end - z) / (end - start)
The equation for GL_EXP fog is
f = e^-(density dot z)
The equation for GL_EXP2 fog is
f = e^-( (density dot 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' = f * Cr + (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
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 cor-responding corresponding
responding 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(3G)
GLFOG(3G)
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