'\"! eqn | mmdoc
'\"macro stdmacro
.ds Vn Version 1.2
.ds Dt 6 March 1997
.ds Re Release 1.1.0
.ds Dp Mar 17 18:05
.ds Dm  Mar  6 20:0
.ds Xs 58475 9 fog.gl
.TH GLFOG
.SH NAME
.B "glFogf, glFogi, glFogfv, glFogiv
\- specify fog parameters

.SH C SPECIFICATION
void \f3glFogf\fP(
GLenum \fIpname\fP,
.nf
.ta \w'\f3void \fPglFogf( 'u
	GLfloat \fIparam\fP )
.fi
void \f3glFogi\fP(
GLenum \fIpname\fP,
.nf
.ta \w'\f3void \fPglFogi( 'u
	GLint \fIparam\fP )
.fi

.EQ
delim $$
.EN
.SH PARAMETERS
.TP \w'\fIparams\fP\ \ 'u 
\f2pname\fP
Specifies a single-valued fog parameter.
\f3GL_FOG_MODE\fP,
\f3GL_FOG_DENSITY\fP,
\f3GL_FOG_START\fP,
\f3GL_FOG_END\fP,
and
\f3GL_FOG_INDEX\fP
are accepted.
.TP
\f2param\fP
Specifies the value that \f2pname\fP will be set to.
.SH C SPECIFICATION
void \f3glFogfv\fP(
GLenum \fIpname\fP,
.nf
.ta \w'\f3void \fPglFogfv( 'u
	const GLfloat \fI*params\fP )
.fi
void \f3glFogiv\fP(
GLenum \fIpname\fP,
.nf
.ta \w'\f3void \fPglFogiv( 'u
	const GLint \fI*params\fP )
.fi

.SH PARAMETERS
.TP
\f2pname\fP
Specifies a fog parameter.
\f3GL_FOG_MODE\fP,
\f3GL_FOG_DENSITY\fP,
\f3GL_FOG_START\fP,
\f3GL_FOG_END\fP,
\f3GL_FOG_INDEX\fP,
and
\f3GL_FOG_COLOR\fP
are accepted.
.TP
\f2params\fP
Specifies the value or values to be assigned to \f2pname\fP.
\f3GL_FOG_COLOR\fP requires an array of four values.
All other parameters accept an array containing only a single value.
.SH 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 \f3glEnable\fP and \f3glDisable\fP with argument
\f3GL_FOG\fP. 
.P
\f3glFog\fP assigns the value or values in \f2params\fP to the fog parameter
specified by \f2pname\fP.
The following values are accepted for \f2pname\fP:
.TP 20
\f3GL_FOG_MODE\fP
\f2params\fP 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:
\f3GL_LINEAR\fP,
\f3GL_EXP\fP,
and \f3GL_EXP2\fP.
The equations corresponding to these symbolic constants are defined below.
The initial fog mode is \f3GL_EXP\fP.
.TP
\f3GL_FOG_DENSITY\fP
\f2params\fP 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.
.TP
\f3GL_FOG_START\fP
\f2params\fP 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.
.TP
\f3GL_FOG_END\fP
\f2params\fP 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.
.TP
\f3GL_FOG_INDEX\fP
\f2params\fP is a single integer or floating-point value that specifies
$i sub f$,
the fog color index.
The initial fog index is 0.
.TP
\f3GL_FOG_COLOR\fP
\f2params\fP contains four integer or floating-point values that specify
$C sub f$, 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 initial fog color is (0, 0, 0, 0).
.P
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 equation for \f3GL_LINEAR\fP fog is
.ce

.EQ
f ~=~ {end ~-~ z} over {end ~-~ start}
.EN

.RE
.P
The equation for \f3GL_EXP\fP fog is
.ce

.EQ
f ~=~ e ** (-(density ~cdot~ z))
.EN

.P
The equation for \f3GL_EXP2\fP fog is
.ce

.EQ
f ~=~ e ** (-(density ~cdot~ z) ** 2)
.EN

.P
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 $C sub r$ is replaced by
.sp
.ce
.EQ
{C sub r} prime ~=~ f C sub r + (1 - f) C sub f 
.EN
.P
In color index mode, the fragment's color index $i sub r$ is replaced by
.sp
.ce
.EQ
{i sub r} prime ~=~ i sub r + (1 - f) i sub f 
.EN
.P
.SH ERRORS
\f3GL_INVALID_ENUM\fP is generated if \f2pname\fP is not an accepted value,
or if \f2pname\fP is \f3GL_FOG_MODE\fP and \f2params\fP is not an accepted value.
.P
\f3GL_INVALID_VALUE\fP is generated if \f2pname\fP is \f3GL_FOG_DENSITY\fP, 
and \f2params\fP is negative.
.P
\f3GL_INVALID_OPERATION\fP is generated if \f3glFog\fP
is executed between the execution of \f3glBegin\fP
and the corresponding execution of \f3glEnd\fP.
.SH ASSOCIATED GETS
\f3glIsEnabled\fP with argument \f3GL_FOG\fP
.br
\f3glGet\fP with argument \f3GL_FOG_COLOR\fP
.br
\f3glGet\fP with argument \f3GL_FOG_INDEX\fP
.br
\f3glGet\fP with argument \f3GL_FOG_DENSITY\fP
.br
\f3glGet\fP with argument \f3GL_FOG_START\fP
.br
\f3glGet\fP with argument \f3GL_FOG_END\fP
.br
\f3glGet\fP with argument \f3GL_FOG_MODE\fP
.SH SEE ALSO
\f3glEnable\fP