210 lines
5.2 KiB
Plaintext
210 lines
5.2 KiB
Plaintext
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'\" e
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'\"! eqn | mmdoc
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'\"macro stdmacro
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.ds Vn Version 1.2
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.ds Dt 24 September 1999
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.ds Re Release 1.2.1
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.ds Dp Jan 14 18:30
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.ds Dm 01 fog.gl ->
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.ds Xs 7467 7 fog.gl
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.TH GLFOG 3G
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.SH NAME
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.B "glFogf, glFogi, glFogfv, glFogiv
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\- specify fog parameters
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.SH C SPECIFICATION
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void \f3glFogf\fP(
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GLenum \fIpname\fP,
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.nf
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.ta \w'\f3void \fPglFogf( 'u
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GLfloat \fIparam\fP )
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.fi
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void \f3glFogi\fP(
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GLenum \fIpname\fP,
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.nf
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.ta \w'\f3void \fPglFogi( 'u
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GLint \fIparam\fP )
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.fi
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.EQ
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delim $$
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.EN
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.SH PARAMETERS
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.TP \w'\fIparams\fP\ \ 'u
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\f2pname\fP
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Specifies a single-valued fog parameter.
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\%\f3GL_FOG_MODE\fP,
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\%\f3GL_FOG_DENSITY\fP,
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\%\f3GL_FOG_START\fP,
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\%\f3GL_FOG_END\fP,
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and
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\%\f3GL_FOG_INDEX\fP
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are accepted.
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.TP
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\f2param\fP
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Specifies the value that \f2pname\fP will be set to.
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.SH C SPECIFICATION
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void \f3glFogfv\fP(
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GLenum \fIpname\fP,
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.nf
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.ta \w'\f3void \fPglFogfv( 'u
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const GLfloat \fI*params\fP )
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.fi
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void \f3glFogiv\fP(
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GLenum \fIpname\fP,
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.nf
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.ta \w'\f3void \fPglFogiv( 'u
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const GLint \fI*params\fP )
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.fi
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.SH PARAMETERS
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.TP
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\f2pname\fP
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Specifies a fog parameter.
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\%\f3GL_FOG_MODE\fP,
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\%\f3GL_FOG_DENSITY\fP,
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\%\f3GL_FOG_START\fP,
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\%\f3GL_FOG_END\fP,
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\%\f3GL_FOG_INDEX\fP,
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and
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\%\f3GL_FOG_COLOR\fP
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are accepted.
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.TP
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\f2params\fP
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Specifies the value or values to be assigned to \f2pname\fP.
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\%\f3GL_FOG_COLOR\fP requires an array of four values.
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All other parameters accept an array containing only a single value.
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.SH DESCRIPTION
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Fog is initially disabled.
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While enabled, fog affects rasterized geometry,
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bitmaps, and pixel blocks, but not buffer clear operations. To enable
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and disable fog, call \%\f3glEnable\fP and \%\f3glDisable\fP with argument
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\%\f3GL_FOG\fP.
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.P
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\%\f3glFog\fP assigns the value or values in \f2params\fP to the fog parameter
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specified by \f2pname\fP.
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The following values are accepted for \f2pname\fP:
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.TP 20
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\%\f3GL_FOG_MODE\fP
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\f2params\fP is a single integer or floating-point value that specifies
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the equation to be used to compute the fog blend factor, $f$.
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Three symbolic constants are accepted:
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\%\f3GL_LINEAR\fP,
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\%\f3GL_EXP\fP,
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and \%\f3GL_EXP2\fP.
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The equations corresponding to these symbolic constants are defined below.
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The initial fog mode is \%\f3GL_EXP\fP.
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.TP
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\%\f3GL_FOG_DENSITY\fP
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\f2params\fP is a single integer or floating-point value that specifies $density$,
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the fog density used in both exponential fog equations.
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Only nonnegative densities are accepted.
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The initial fog density is 1.
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.TP
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\%\f3GL_FOG_START\fP
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\f2params\fP is a single integer or floating-point value that specifies $start$,
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the near distance used in the linear fog equation.
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The initial near distance is 0.
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.TP
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\%\f3GL_FOG_END\fP
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\f2params\fP is a single integer or floating-point value that specifies $end$,
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the far distance used in the linear fog equation.
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The initial far distance is 1.
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.TP
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\%\f3GL_FOG_INDEX\fP
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\f2params\fP is a single integer or floating-point value that specifies
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$i sub f$,
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the fog color index.
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The initial fog index is 0.
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.TP
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\%\f3GL_FOG_COLOR\fP
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\f2params\fP contains four integer or floating-point values that specify
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$C sub f$, the fog color.
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Integer values are mapped linearly such that the most positive representable
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value maps to 1.0,
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and the most negative representable value maps to \-1.0.
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Floating-point values are mapped directly.
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After conversion,
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all color components are clamped to the range [0,1].
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The initial fog color is (0, 0, 0, 0).
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.P
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Fog blends a fog color with each rasterized pixel fragment's posttexturing
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color using a blending factor $f$.
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Factor $f$ is computed in one of three ways,
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depending on the fog mode.
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Let $z$ be the distance in eye coordinates from the origin to the fragment
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being fogged.
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The equation for \%\f3GL_LINEAR\fP fog is
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.ce
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.EQ
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f ~=~ {end ~-~ z} over {end ~-~ start}
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.EN
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.RE
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.P
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The equation for \%\f3GL_EXP\fP fog is
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.ce
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.EQ
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f ~=~ e sup {-^(density ~cdot~ z)}
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.EN
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.P
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The equation for \%\f3GL_EXP2\fP fog is
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.ce
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.EQ
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f ~=~ e sup {-^(density ~cdot~ z)} sup 2
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.EN
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.P
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Regardless of the fog mode,
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$f$ is clamped to the range [0,\ 1] after it is computed.
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Then,
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if the GL is in RGBA color mode,
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the fragment's red, green, and blue colors, represented by $C sub r$,
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are replaced by
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.sp
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.ce
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.EQ
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{C sub r} sup prime ~=~ f^C sub r ~+~ (1 - f)^C sub f
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.EN
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.sp
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Fog does not affect a fragment's alpha component.
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.P
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In color index mode, the fragment's color index $i sub r$ is replaced by
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.sp
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.ce
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.EQ
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{i sub r} sup prime ~=~ i sub r ~+~ (1 - f)^i sub f
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.EN
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.P
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.SH ERRORS
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\%\f3GL_INVALID_ENUM\fP is generated if \f2pname\fP is not an accepted value,
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or if \f2pname\fP is \%\f3GL_FOG_MODE\fP and \f2params\fP is not an accepted value.
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.P
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\%\f3GL_INVALID_VALUE\fP is generated if \f2pname\fP is \%\f3GL_FOG_DENSITY\fP,
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and \f2params\fP is negative.
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.P
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\%\f3GL_INVALID_OPERATION\fP is generated if \%\f3glFog\fP
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is executed between the execution of \%\f3glBegin\fP
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and the corresponding execution of \%\f3glEnd\fP.
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.SH ASSOCIATED GETS
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\%\f3glIsEnabled\fP with argument \%\f3GL_FOG\fP
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.br
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\%\f3glGet\fP with argument \%\f3GL_FOG_COLOR\fP
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.br
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\%\f3glGet\fP with argument \%\f3GL_FOG_INDEX\fP
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.br
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\%\f3glGet\fP with argument \%\f3GL_FOG_DENSITY\fP
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.br
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\%\f3glGet\fP with argument \%\f3GL_FOG_START\fP
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.br
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\%\f3glGet\fP with argument \%\f3GL_FOG_END\fP
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.br
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\%\f3glGet\fP with argument \%\f3GL_FOG_MODE\fP
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.SH SEE ALSO
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\%\f3glEnable(3G)\fP
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