fglFog(3G) OpenGL Reference fglFog(3G)NAME
fglFogf, fglFogi, fglFogfv, fglFogiv - specify fog parameters
FORTRAN SPECIFICATION
SUBROUTINE fglFogf( INTEGER*4 pname,
REAL*4 param )
SUBROUTINE fglFogi( INTEGER*4 pname,
INTEGER*4 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.
FORTRAN SPECIFICATION
SUBROUTINE fglFogfv( INTEGER*4 pname,
CHARACTER*8 params )
SUBROUTINE fglFogiv( INTEGER*4 pname,
CHARACTER*8 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 initially disabled. While enabled, fog affects rasterized
geometry, bitmaps, and pixel blocks, but not buffer clear operations. To
enable and disable fog, call fglEnable and fglDisable with argument
GL_FOG.
fglFog 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. Four symbolic constants are
accepted: GL_LINEAR, GL_EXP, GL_EXP2, and
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fglFog(3G) OpenGL Reference fglFog(3G)
GL_FOG_FUNC_SGIS. 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 i , the fog color index. The initial
f
fog index is 0.
GL_FOG_COLOR 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).
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 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. 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
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fglFog(3G) OpenGL Reference fglFog(3G)
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
.
-(density z)
f = e
The equation for GL_EXP2 fog is
. 2
-(density z)
f = e
The blending factor for GL_FOG_FUNC_SGIS fog is determined by
interpolating a set of application-defined control points.
fglFogFuncSGIS 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 red,
green, and blue colors, represented by C , are replaced by
r
'
C = fC + (1-f)C
r r f
Fog does not affect a fragment's alpha component.
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
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fglFog(3G) OpenGL Reference fglFog(3G)
Z offset relative to some reference point. This is the purpose of the
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 fglGetString when called with argument GL_EXTENSIONS,
extension SGIS_fog_func is supported by the connection. See fglIntro 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, and params is
negative.
GL_INVALID_OPERATION is generated if fglFog is executed between the
execution of fglBegin and the corresponding execution of fglEnd.
ASSOCIATED GETS
fglIsEnabled with argument GL_FOG
fglIsEnabled with argument GL_FOG_OFFSET_SGIX
fglGet with argument GL_FOG_COLOR
fglGet with argument GL_FOG_FUNC_SGIS
fglGet with argument GL_MAX_FOG_FUNC_POINTS_SGIS
fglGet with argument GL_FOG_INDEX
fglGet with argument GL_FOG_DENSITY
fglGet with argument GL_FOG_START
fglGet with argument GL_FOG_END
fglGet with argument GL_FOG_MODE
fglGet 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.
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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fglFog(3G) OpenGL Reference fglFog(3G)SEE ALSO
fglEnable, fglFogFuncSGIS
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