glfog - Man Page
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. 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, GL_FOG_COLOR, and
GL_FOG_OFFSET_VALUE_SGIX are accepted.
params
Specifies the value or values to be assigned to pname. GL_FOG_COLOR
requires an array of four values to specify the RGBA components of
the fog color. GL_FOG_OFFSET_VALUE_SGIX 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 enabled and disabled with glEnable and glDisable using the
argument GL_FOG. While enabled, fog affects rasterized geometry,
bitmaps, and pixel blocks, but not buffer clear operations.
glFog assigns the value or values in params to the fog parameter
specified by pname. The accepted values for pname are as follows:
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. Four symbolic constants are
accepted: GL_LINEAR, GL_EXP, GL_EXP2, and
GL_FOG_FUNC_SGIS. The equations corresponding to
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glFog(3G) OpenGL Reference glFog(3G)
these symbolic constants are defined below. The
default 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 default fog density is
1.0.
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 default near distance is
0.0.
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 default far distance is
1.0.
GL_FOG_INDEX params is a single integer or floating-point value
that specifies i , the fog color index. The default
fog index is 0.0.
GL_FOG_COLOR params contains four integer or floating-point values
that specify C , the fog color. 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. After
conversion, all color components are clamped to the
range [0,1]. The default fog color is (0,0,0,0).
GL_FOG_OFFSET_VALUE_SGIX
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 default 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 GL_FOG_OFFSET_SGIX is enabled, subtract the Z offset
specified by GL_FOG_OFFSET_VALUE_SGIX from z, and clamp the result so
that it is greater than or equal to zero.
The equation for GL_LINEAR fog is
f = ___________
end - start
The equation for GL_EXP fog is
f = e**(-(density . z))
The equation for GL_EXP2 fog is
f = e**(-(density . z)**2)
The blending factor for GL_FOG_FUNC_SGIS 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
is replaced by r
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 GL_FOG_OFFSET_VALUE_SGIX. 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.
GL_FOG_FUNC_SGIS is part of the SGIS_fog_func extension, not part of the
core GL command set. If GL_SGIS_fog_func is included in the string
returned by glGetString when called with argument GL_EXTENSIONS,
extension SGIS_fog_func is supported by the connection. See glIntro for
more information about using extensions.
GL_FOG_OFFSET_VALUE_SGIX is part of the SGIX_fog_offset extension, and
similar considerations apply.
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, GL_FOG_START,
or GL_FOG_END 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
glIsEnabled with argument GL_FOG_OFFSET_SGIX
glGet with argument GL_FOG_COLOR
glGet with argument GL_FOG_FUNC_SGIS
glGet with argument GL_MAX_FOG_FUNC_POINTS_SGIS
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
glGet with argument GL_FOG_OFFSET_VALUE_SGIX
MACHINE DEPENDENCIES
Fog mode GL_FOG_FUNC_SGIS is supported only on InfiniteReality systems.
The fog offset capability is supported only on InfiniteReality systems.
Fog accuracy is poor in 2- and 4-bit overlays on InfiniteReality systems.
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glFog(3G) OpenGL Reference glFog(3G)
SEE ALSO
glEnable, glFogFuncSGIS
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