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 fac-
tor, 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 equa-
tion. 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 equa-
tion. 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 lin-
early 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 equa-
tion 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
ir' = ir + (1 - f) if
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
GLFOG(3G)