<div class="refentry" lang="en" xml:lang="en"><a id="glFog"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>glFog — specify fog parameters</p></div><div class="refsynopsisdiv"><h2>C Specification</h2><div class="funcsynopsis"><table><tr><td><code class="funcdef">void <b class="fsfunc">glFogf</b>(</code></td><td>GLenum  </td><td><var class="pdparam">pname</var>, </td></tr><tr><td> </td><td>GLfloat  </td><td><var class="pdparam">param</var><code>)</code>;</td></tr></table></div><div class="funcsynopsis"><table><tr><td><code class="funcdef">void <b class="fsfunc">glFogi</b>(</code></td><td>GLenum  </td><td><var class="pdparam">pname</var>, </td></tr><tr><td> </td><td>GLint  </td><td><var class="pdparam">param</var><code>)</code>;</td></tr></table></div></div><div class="refsect1" lang="en" xml:lang="en"><a id="parameters"></a><h2>Parameters</h2><div class="variablelist"><dl><dt><span class="term"><em class="parameter"><code>pname</code></em></span></dt><dd><p>

Specifies a single-valued fog parameter.

<code class="constant">GL_FOG_MODE</code>,

<code class="constant">GL_FOG_DENSITY</code>,

<code class="constant">GL_FOG_START</code>,

<code class="constant">GL_FOG_END</code>,

<code class="constant">GL_FOG_INDEX</code>, and

<code class="constant">GL_FOG_COORD_SRC</code>

are accepted.

</p></dd><dt><span class="term"><em class="parameter"><code>param</code></em></span></dt><dd><p>

Specifies the value that <em class="parameter"><code>pname</code></em> will be set to.

</p></dd></dl></div></div><div class="refsynopsisdiv"><h2>C Specification</h2><div class="funcsynopsis"><table><tr><td><code class="funcdef">void <b class="fsfunc">glFogfv</b>(</code></td><td>GLenum  </td><td><var class="pdparam">pname</var>, </td></tr><tr><td> </td><td>const GLfloat *  </td><td><var class="pdparam">params</var><code>)</code>;</td></tr></table></div><div class="funcsynopsis"><table><tr><td><code class="funcdef">void <b class="fsfunc">glFogiv</b>(</code></td><td>GLenum  </td><td><var class="pdparam">pname</var>, </td></tr><tr><td> </td><td>const GLint *  </td><td><var class="pdparam">params</var><code>)</code>;</td></tr></table></div></div><div class="refsect1" lang="en" xml:lang="en"><a id="parameters2"></a><h2>Parameters</h2><div class="variablelist"><dl><dt><span class="term"><em class="parameter"><code>pname</code></em></span></dt><dd><p>

Specifies a fog parameter.

<code class="constant">GL_FOG_MODE</code>,

<code class="constant">GL_FOG_DENSITY</code>,

<code class="constant">GL_FOG_START</code>,

<code class="constant">GL_FOG_END</code>,

<code class="constant">GL_FOG_INDEX</code>,

<code class="constant">GL_FOG_COLOR</code>, and

<code class="constant">GL_FOG_COORD_SRC</code>

are accepted.

</p></dd><dt><span class="term"><em class="parameter"><code>params</code></em></span></dt><dd><p>

Specifies the value or values to be assigned to <em class="parameter"><code>pname</code></em>.

<code class="constant">GL_FOG_COLOR</code> requires an array of four values.

All other parameters accept an array containing only a single value.

</p></dd></dl></div></div><div class="refsect1" lang="en" xml:lang="en"><a id="description"></a><h2>Description</h2><p>

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 <a class="citerefentry" href="glEnable"><span class="citerefentry"><span class="refentrytitle">glEnable</span></span></a> and <a class="citerefentry" href="glDisable"><span class="citerefentry"><span class="refentrytitle">glDisable</span></span></a> with argument

<code class="constant">GL_FOG</code>.

</p><p>

<code class="function">glFog</code> assigns the value or values in <em class="parameter"><code>params</code></em> to the fog parameter

specified by <em class="parameter"><code>pname</code></em>.

The following values are accepted for <em class="parameter"><code>pname</code></em>:

</p><div class="variablelist"><dl><dt><span class="term"><code class="constant">GL_FOG_MODE</code></span></dt><dd><p>

<em class="parameter"><code>params</code></em> is a single integer or floating-point value that specifies

the equation to be used to compute the fog blend factor,

<math overflow="scroll"><mi mathvariant="italic">f</mi></math>.

Three symbolic constants are accepted:

<code class="constant">GL_LINEAR</code>,

<code class="constant">GL_EXP</code>,

and <code class="constant">GL_EXP2</code>.

The equations corresponding to these symbolic constants are defined below.

The initial fog mode is <code class="constant">GL_EXP</code>.

</p></dd><dt><span class="term"><code class="constant">GL_FOG_DENSITY</code></span></dt><dd><p>

<em class="parameter"><code>params</code></em> is a single integer or floating-point value that specifies

<math overflow="scroll"><mi mathvariant="italic">density</mi></math>,

the fog density used in both exponential fog equations.

Only nonnegative densities are accepted.

The initial fog density is 1.

</p></dd><dt><span class="term"><code class="constant">GL_FOG_START</code></span></dt><dd><p>

<em class="parameter"><code>params</code></em> is a single integer or floating-point value that specifies

<math overflow="scroll"><mi mathvariant="italic">start</mi></math>,

the near distance used in the linear fog equation.

The initial near distance is 0.

</p></dd><dt><span class="term"><code class="constant">GL_FOG_END</code></span></dt><dd><p>

<em class="parameter"><code>params</code></em> is a single integer or floating-point value that specifies

<math overflow="scroll"><mi mathvariant="italic">end</mi></math>,

the far distance used in the linear fog equation.

The initial far distance is 1.

</p></dd><dt><span class="term"><code class="constant">GL_FOG_INDEX</code></span></dt><dd><p>

<em class="parameter"><code>params</code></em> is a single integer or floating-point value that specifies

<math overflow="scroll">

<msub><mi mathvariant="italic">i</mi>

<mi mathvariant="italic">f</mi>

</msub>

</math>,

the fog color index.

The initial fog index is 0.

</p></dd><dt><span class="term"><code class="constant">GL_FOG_COLOR</code></span></dt><dd><p>

<em class="parameter"><code>params</code></em> contains four integer or floating-point values that specify

<math overflow="scroll">

<msub><mi mathvariant="italic">C</mi>

<mi mathvariant="italic">f</mi>

</msub>

</math>,

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

<math overflow="scroll">

<mn>-1.0</mn>

</math>.

Floating-point values are mapped directly.

After conversion,

all color components are clamped to the range

<math overflow="scroll">

<mfenced open="[" close="]">

<mn>0</mn>

<mn>1</mn>

</mfenced>

</math>.

The initial fog color is (0, 0, 0, 0).

</p></dd><dt><span class="term"><code class="constant">GL_FOG_COORD_SRC</code></span></dt><dd><p>

<em class="parameter"><code>params</code></em> contains either of the following symbolic constants:

<code class="constant">GL_FOG_COORD</code> or <code class="constant">GL_FRAGMENT_DEPTH</code>. <code class="constant">GL_FOG_COORD</code>

specifies that the current fog coordinate should be used as distance value

in the fog color computation. <code class="constant">GL_FRAGMENT_DEPTH</code> specifies that the

current fragment depth should be used as distance value in the fog

computation.

</p></dd></dl></div><p>

Fog blends a fog color with each rasterized pixel fragment's post-texturing

color using a blending factor

<math overflow="scroll"><mi mathvariant="italic">f</mi></math>.

Factor

<math overflow="scroll"><mi mathvariant="italic">f</mi></math>

is computed in one of three ways,

depending on the fog mode.

Let

<math overflow="scroll"><mi mathvariant="italic">c</mi></math>

be either the distance in eye coordinate from the origin (in the

case that the <code class="constant">GL_FOG_COORD_SRC</code> is <code class="constant">GL_FRAGMENT_DEPTH</code>) or

the current fog coordinate (in the case that <code class="constant">GL_FOG_COORD_SRC</code>

is <code class="constant">GL_FOG_COORD</code>).

The equation for <code class="constant">GL_LINEAR</code> fog is

</p><div class="informalequation"><math overflow="scroll">

<mrow>

<mi mathvariant="italic">f</mi>

<mo>=</mo>

<mfrac>

<mfenced open="" close="">

<mrow>

<mi mathvariant="italic">end</mi>

<mo>-</mo>

<mi mathvariant="italic">c</mi>

</mrow>

</mfenced>

<mfenced open="" close="">

<mrow>

<mi mathvariant="italic">end</mi>

<mo>-</mo>

<mi mathvariant="italic">start</mi>

</mrow>

</mfenced>

</mfrac>

</mrow>

</math></div><p>

</p><p>

The equation for <code class="constant">GL_EXP</code> fog is

</p><div class="informalequation"><math overflow="scroll">

<mrow>

<mi mathvariant="italic">f</mi>

<mo>=</mo>

<msup><mi mathvariant="italic">e</mi>

<mfenced open="" close="">

<mrow>

<mo>-</mo>

<mfenced open="(" close=")">

<mrow>

<mi mathvariant="italic">density</mi>

<mo>·</mo>

<mi mathvariant="italic">c</mi>

</mrow>

</mfenced>

</mrow>

</mfenced>

</msup>

</mrow>

</math></div><p>

</p><p>

The equation for <code class="constant">GL_EXP2</code> fog is

</p><div class="informalequation"><math overflow="scroll">

<mrow>

<mi mathvariant="italic">f</mi>

<mo>=</mo>

<msup><mi mathvariant="italic">e</mi>

<msup><mfenced open="" close="">

<mrow>

<mo>-</mo>

<mfenced open="(" close=")">

<mrow>

<mi mathvariant="italic">density</mi>

<mo>·</mo>

<mi mathvariant="italic">c</mi>

</mrow>

</mfenced>

</mrow>

</mfenced>

<mn>2</mn>

</msup></msup>

</mrow>

</math></div><p>

</p><p>

Regardless of the fog mode,

<math overflow="scroll"><mi mathvariant="italic">f</mi></math>

is clamped to the range

<math overflow="scroll">

<mfenced open="[" close="]">

<mn>0</mn>

<mn>1</mn>

</mfenced>

</math>

after it is computed.

Then,

if the GL is in RGBA color mode,

the fragment's red, green, and blue colors, represented by

<math overflow="scroll">

<msub><mi mathvariant="italic">C</mi>

<mi mathvariant="italic">r</mi>

</msub>

</math>,

are replaced by

</p><p>

</p><div class="informalequation"><math overflow="scroll">

<mrow>

<msup><mfenced open="" close="">

<msub><mi mathvariant="italic">C</mi>

<mi mathvariant="italic">r</mi>

</msub>

</mfenced>

<mo>″</mo>

</msup>

<mo>=</mo>

<mrow>

<mrow>

<mi mathvariant="italic">f</mi>

<mo>×</mo>

<msub><mi mathvariant="italic">C</mi>

<mi mathvariant="italic">r</mi>

</msub>

</mrow>

<mo>+</mo>

<mrow>

<mfenced open="(" close=")">

<mrow>

<mn>1</mn>

<mo>-</mo>

<mi mathvariant="italic">f</mi>

</mrow>

</mfenced>

<mo>×</mo>

<msub><mi mathvariant="italic">C</mi>

<mi mathvariant="italic">f</mi>

</msub>

</mrow>

</mrow>

</mrow>

</math></div><p>

</p><p>

Fog does not affect a fragment's alpha component.

</p><p>

In color index mode, the fragment's color index

<math overflow="scroll">

<msub><mi mathvariant="italic">i</mi>

<mi mathvariant="italic">r</mi>

</msub>

</math>

is replaced by

</p><p>

</p><div class="informalequation"><math overflow="scroll">

<mrow>

<msup><mfenced open="" close="">

<msub><mi mathvariant="italic">i</mi>

<mi mathvariant="italic">r</mi>

</msub>

</mfenced>

<mo>″</mo>

</msup>

<mo>=</mo>

<mrow>

<msub><mi mathvariant="italic">i</mi>

<mi mathvariant="italic">r</mi>

</msub>

<mo>+</mo>

<mrow>

<mfenced open="(" close=")">

<mrow>

<mn>1</mn>

<mo>-</mo>

<mi mathvariant="italic">f</mi>

</mrow>

</mfenced>

<mo>×</mo>

<msub><mi mathvariant="italic">i</mi>

<mi mathvariant="italic">f</mi>

</msub>

</mrow>

</mrow>

</mrow>

</math></div><p>

</p><p>

</p></div><div class="refsect1" lang="en" xml:lang="en"><a id="notes"></a><h2>Notes</h2><p>

<code class="constant">GL_FOG_COORD_SRC</code> is available only if the GL version is 1.4 or

greater.

</p></div><div class="refsect1" lang="en" xml:lang="en"><a id="errors"></a><h2>Errors</h2><p>

<code class="constant">GL_INVALID_ENUM</code> is generated if <em class="parameter"><code>pname</code></em> is not an accepted value,

or if <em class="parameter"><code>pname</code></em> is <code class="constant">GL_FOG_MODE</code> and <em class="parameter"><code>params</code></em> is not an accepted value.

</p><p>

<code class="constant">GL_INVALID_VALUE</code> is generated if <em class="parameter"><code>pname</code></em> is <code class="constant">GL_FOG_DENSITY</code>

and <em class="parameter"><code>params</code></em> is negative.

</p><p>

<code class="constant">GL_INVALID_OPERATION</code> is generated if <code class="function">glFog</code>

is executed between the execution of <a class="citerefentry" href="glBegin"><span class="citerefentry"><span class="refentrytitle">glBegin</span></span></a>

and the corresponding execution of <a class="citerefentry" href="glEnd"><span class="citerefentry"><span class="refentrytitle">glEnd</span></span></a>.

</p></div><div class="refsect1" lang="en" xml:lang="en"><a id="associatedgets"></a><h2>Associated Gets</h2><p>

<a class="citerefentry" href="glIsEnabled"><span class="citerefentry"><span class="refentrytitle">glIsEnabled</span></span></a> with argument <code class="constant">GL_FOG</code>

</p><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_FOG_COLOR</code>

</p><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_FOG_INDEX</code>

</p><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_FOG_DENSITY</code>

</p><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_FOG_START</code>

</p><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_FOG_END</code>

</p><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_FOG_MODE</code>

</p></div>

{$pipelinestall}{$examples}

<div class="refsect1" lang="en" xml:lang="en"><a id="seealso"></a><h2>See Also</h2><p>

<a class="citerefentry" href="glEnable"><span class="citerefentry"><span class="refentrytitle">glEnable</span></span></a>

</p></div><div class="refsect1" lang="en" xml:lang="en"><div id="Copyright"><h2>Copyright</h2><p>

Copyright © 1991-2006

Silicon Graphics, Inc. This document is licensed under the SGI

Free Software B License. For details, see

<a class="ulink" href="https://web.archive.org/web/20171022161616/http://oss.sgi.com/projects/FreeB/" target="_top">https://web.archive.org/web/20171022161616/http://oss.sgi.com/projects/FreeB/</a>.

</p></div></div></div>