glFog(3G) OpenGL Reference 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. GLFOGMODE,
GLFOGDENSITY, GLFOGSTART, GLFOGEND, and GLFOGINDEX 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. GLFOGMODE, GLFOGDENSITY,
GLFOGSTART, GLFOGEND, GLFOGINDEX, GLFOGCOLOR, and
GLFOGOFFSETVALUESGIX are accepted.
params
Specifies the value or values to be assigned to pname. GLFOGCOLOR
requires an array of four values to specify the RGBA components of
the fog color. GLFOGOFFSETVALUESGIX requires an array of four
values to specify a reference point in eye-space coordinates, and a
Z offset in eye-space coordinates. 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 GLFOG.
glFog assigns the value or values in params to the fog parameter
specified by pname. The following values are accepted for pname:
GLFOGMODE params is a single integer or floating-point value
that specifies the equation to be used to compute the
fog blend factor, f. Four symbolic constants are
accepted: GLLINEAR, GLEXP, GLEXP2, and
GLFOGFUNCSGIS. The equations corresponding to
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glFog(3G) OpenGL Reference glFog(3G)
these symbolic constants are defined below. The
initial fog mode is GLEXP.
GLFOGDENSITY 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.
GLFOGSTART 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.
GLFOGEND 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.
GLFOGINDEX params is a single integer or floating-point value
that specifies i , the fog color index. The initial
f
fog index is 0.
GLFOGCOLOR params contains four integer or floating-point values
that specify C , the fog color. Integer values are
f
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).
GLFOGOFFSETVALUESGIX
params contains four integer or floating-point
values. The first three specify a reference point as
X, Y, and Z eye coordinates. The fourth specifies a
Z offset in eye coordinates. The offset is used to
modify z in the fog equations described below, so
that bright objects (such as emissive objects
representing light sources) will have a more
realistic appearance in heavily-fogged environments.
The significance of the reference point is discussed
below. 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.
The initial value 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 four ways, depending on the fog mode.
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glFog(3G) OpenGL Reference glFog(3G)
Let z be the distance in eye coordinates from the origin to the fragment
being fogged. If GLFOGOFFSETSGIX is enabled, subtract the Z offset
specified by GLFOGOFFSETVALUESGIX from z, and clamp the result so
that it is greater than or equal to zero.
The equation for GLLINEAR fog is
__end_-_z__
f =
end - start
The equation for GLEXP fog is
-density * z
f = e
The equation for GLEXP2 fog is
2
-(density * z)
f = e
The blending factor for GLFOGFUNCSGIS fog is determined by
interpolating a set of application-defined control points. glFogFuncSGIS
is used to specify the control points. Each control point consists of a
pair of floating-point numbers. The first number of the pair specifies a
value of z, and the second number of the pair specifies a value of f.
The GL fits a curve through all the control points. This curve may be
piecewise linear or it may be smoothed, but it will pass through the
control points exactly (limited only by the resolution of the
implementation). If a given z is less than or equal to the z of the
first control point, then the resulting value of f is the f of the first
control point. If z is greater than or equal to the z of the last
control point, then the resulting value of f is the f of the last control
point. Otherwise, f is determined by finding the point on the curve that
corresponds to the given z.
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
r
is replaced by
C ' = fC +(1-f)C
r r f
In color index mode, the fragment's color index i is replaced by
r
i ' = i +(1-f)i
r r f
NOTES
In perspective projections, a Z offset in eye coordinates will map to
different Z offsets in window coordinates at various points in the range
of Z. Machines that evaluate the fog blending factor by using a window Z
coordinate to index a table must therefore compute the window-coordinate
Z offset relative to some reference point. This is the purpose of the
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glFog(3G) OpenGL Reference glFog(3G)
reference point specified by the first three values in params when pname
is GLFOGOFFSETVALUESGIX. For best fog accuracy in perspective
projections, this reference point must be close to the objects being
fogged.
In parallel projections the window-coordinate Z offset does not vary with
position in the Z range, so the reference point is ignored.
GLFOGFUNCSGIS is part of the SGISfogfunc extension, not part of the
core GL command set. If GLSGISfogfunc is included in the string
returned by glGetString when called with argument GLEXTENSIONS,
extension SGISfogfunc is supported by the connection. See glIntro for
more information about using extensions.
GLFOGOFFSETVALUESGIX is part of the SGIXfogoffset extension, and
similar considerations apply.
ERRORS
GLINVALIDENUM is generated if pname is not an accepted value, or if
pname is GLFOGMODE and params is not an accepted value.
GLINVALIDVALUE is generated if pname is GLFOGDENSITY and params is
negative.
GLINVALIDOPERATION is generated if glFog is executed between the
execution of glBegin and the corresponding execution of glEnd.
ASSOCIATED GETS
glIsEnabled with argument GLFOG
glIsEnabled with argument GLFOGOFFSETSGIX
glGet with argument GLFOGCOLOR
glGet with argument GLFOGFUNCSGIS
glGet with argument GLMAXFOGFUNCPOINTSSGIS
glGet with argument GLFOGINDEX
glGet with argument GLFOGDENSITY
glGet with argument GLFOGSTART
glGet with argument GLFOGEND
glGet with argument GLFOGMODE
glGet with argument GLFOGOFFSETVALUESGIX
MACHINE DEPENDENCIES
Fog mode GLFOGFUNCSGIS is supported only on InfiniteReality systems.
The fog offset capability is supported only on InfiniteReality systems.
On InfiniteReality systems, fog cannot be used with color-index visuals
that are 2 or 4 bits deep. Accordingly, these visuals are marked as
non-conforming.
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glFog(3G) OpenGL Reference glFog(3G)
SEE ALSO
glEnable, glFogFuncSGIS
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