<div class="refentry" title="glFog"><a id="glFog"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>glFog — specify fog parameters</p></div><div class="refsynopsisdiv" title="C Specification"><h2>C Specification</h2><div class="funcsynopsis"><table class="funcprototype-table"><tr><td><code class="funcdef">void <b class="fsfunc">glFogf</b>(</code></td><td>GLenum <var class="pdparam">pname</var>, </td></tr><tr><td> </td><td>GLfloat <var class="pdparam">param</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table class="funcprototype-table"><tr><td><code class="funcdef">void <b class="fsfunc">glFogx</b>(</code></td><td>GLenum <var class="pdparam">pname</var>, </td></tr><tr><td> </td><td>GLfixed <var class="pdparam">param</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1" title="Parameters"><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>, and

<code class="constant">GL_FOG_END</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" title="C Specification"><h2>C Specification</h2><div class="funcsynopsis"><table class="funcprototype-table"><tr><td><code class="funcdef">void <b class="fsfunc">glFogfv</b>(</code></td><td>GLenum <var class="pdparam">pname</var>, </td></tr><tr><td> </td><td>const GLfloat * <var class="pdparam">params</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div><table class="funcprototype-table"><tr><td><code class="funcdef">void <b class="fsfunc">glFogxv</b>(</code></td><td>GLenum <var class="pdparam">pname</var>, </td></tr><tr><td> </td><td>const GLfixed * <var class="pdparam">params</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1" title="Parameters"><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>, and

<code class="constant">GL_FOG_COLOR</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" title="Description"><a id="description"></a><h2>Description</h2><p>If fog is 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>.

Fog is initially disabled.

</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 fixed-point or floating-point value that

specifies the equation to be used to compute the fog

blend factor <em class="replaceable"><code>f</code></em>.

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 fixed-point or floating-point value that

specifies <em class="replaceable"><code>density</code></em>,

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 fixed-point or floating-point value that

specifies <em class="replaceable"><code>start</code></em>,

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 fixed-point or floating-point value that

specifies <em class="replaceable"><code>end</code></em>,

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_COLOR</code>

</span></dt><dd><p>

<em class="parameter"><code>params</code></em>

contains four fixed-point or floating-point values that specify

<math overflow="scroll">

<msub><mi>C</mi><mi>f</mi></msub>

</math>,

the fog color. Both fixed-point and 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></dd></dl></div><p>Fog blends a fog color with each rasterized pixel

fragment's posttexturing color using a blending factor

<em class="replaceable"><code>f</code></em>. Factor <em class="replaceable"><code>f</code></em>

is computed in one of three ways, depending on the fog mode.

Let <em class="replaceable"><code>z</code></em>

be the distance in eye coordinates from the origin to the

fragment being fogged. The equation for <code class="constant">GL_LINEAR</code>

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

<mi>f</mi><mo>=</mo>

<mfrac>

<mrow><mi>end</mi><mo>-</mo><mi>z</mi></mrow>

<mrow><mi>end</mi><mo>-</mo><mi>start</mi></mrow>

</mfrac>

</math></div><p>The equation for <code class="constant">GL_EXP</code> fog is</p><div class="informalequation"><math overflow="scroll">

<mi>f</mi><mo>=</mo>

<msup>

<mi>e</mi>

<mrow><mo>-</mo><mo>(</mo><mi>density</mi><mo>-</mo><mi>z</mi><mo>)</mo></mrow>

</msup>

</math></div><p>The equation for <code class="constant">GL_EXP2</code> fog is</p><div class="informalequation"><math overflow="scroll">

<mi>f</mi><mo>=</mo>

<msup>

<mi>e</mi>

<msup>

<mrow><mo>-</mo><mo>(</mo><mi>density</mi><mo>-</mo><mi>z</mi><mo>)</mo></mrow>

<mn>2</mn>

</msup>

</msup>

</math></div><p>Regardless of the fog mode, <em class="replaceable"><code>f</code></em>

is clamped to the range [0, 1] after it is computed. Then,

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

<math overflow="scroll">

<msub><mi>C</mi><mi>r</mi></msub>

</math>,

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

<msub><mi>C'</mi><mi>r</mi></msub><mo>=</mo>

<mi>f</mi>

<msub>

<mi>C</mi>

<mi>r</mi>

</msub>

<mo>+</mo>

<mfenced><mrow><mn>1</mn><mo>-</mo><mi>f</mi></mrow></mfenced>

<msub>

<mi>C</mi>

<mi>f</mi>

</msub>

</math></div><p>Fog does not affect a fragment's alpha component.</p></div><div class="refsect1" title="Errors"><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></div><div class="refsect1" title="See Also"><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" title="Copyright"><a id="copyright"></a><h2>Copyright</h2><p>

Copyright © 2003-2004

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>