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OpenGL ES 1.0 and EGL 1.0 reference manual and man pages (written by
Claude Knaus, based on OpenGL SI man pages).

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook V4.2//EN" "DTD/docbook/docbookx.dtd">

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<refentry id="glFog">
  <refmeta>
    <refentrytitle>glFog</refentrytitle>
    <manvolnum>3G</manvolnum>
  </refmeta>

  <refnamediv>
    <refdescriptor>glFog</refdescriptor>
    <refname>glFogf</refname>
    <refname>glFogx</refname>
    <refname>glFogfv</refname>
    <refname>glFogxv</refname>
    <refpurpose>specify fog parameters</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <title>C Specification</title>

    <funcsynopsis>
      <funcprototype>
        <funcdef>void <function>glFogf</function></funcdef>
        <paramdef>GLenum <parameter>pname</parameter></paramdef>
        <paramdef>GLfloat <parameter>param</parameter></paramdef>
      </funcprototype>
      <funcprototype>
        <funcdef>void <function>glFogx</function></funcdef>
        <paramdef>GLenum <parameter>pname</parameter></paramdef>
        <paramdef>GLfixed <parameter>param</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Parameters</title>

    <variablelist>
      <varlistentry>
        <term>
          <parameter>pname</parameter>
        </term>

        <listitem>
          <para>Specifies a single-valued fog parameter. 
          <constant>GL_FOG_MODE</constant>, 
          <constant>GL_FOG_DENSITY</constant>, 
          <constant>GL_FOG_START</constant>, and 
          <constant>GL_FOG_END</constant> are accepted.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <parameter>param</parameter>
        </term>

        <listitem>
          <para>Specifies the value that 
          <parameter>pname</parameter>

          will be set to.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsynopsisdiv>
    <title>C Specification</title>

    <funcsynopsis>
      <funcprototype>
        <funcdef>void <function>glFogfv</function></funcdef>
        <paramdef>GLenum <parameter>pname</parameter></paramdef>
        <paramdef>const GLfloat * <parameter>params</parameter></paramdef>
      </funcprototype>
      <funcprototype>
        <funcdef>void <function>glFogxv</function></funcdef>
        <paramdef>GLenum <parameter>pname</parameter></paramdef>
        <paramdef>const GLfixed * <parameter>params</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Parameters</title>

    <variablelist>
      <varlistentry>
        <term>
          <parameter>pname</parameter>
        </term>

        <listitem>
          <para>Specifies a fog parameter. 
          <constant>GL_FOG_MODE</constant>, 
          <constant>GL_FOG_DENSITY</constant>, 
          <constant>GL_FOG_START</constant>, 
          <constant>GL_FOG_END</constant>, and 
          <constant>GL_FOG_COLOR</constant> are accepted.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <parameter>params</parameter>
        </term>

        <listitem>
          <para>Specifies the value or values to be assigned to 
          <parameter>pname</parameter>. 
          <constant>GL_FOG_COLOR</constant> 
          requires an array of four values. All other parameters
          accept an array containing only a single value.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Description</title>

    <para>If fog is enabled, fog affects
    rasterized geometry, bitmaps, and pixel blocks, but not buffer
    clear operations. To enable and disable fog, call 
    <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry> and 
    <citerefentry><refentrytitle>glDisable</refentrytitle></citerefentry>
    with argument <constant>GL_FOG</constant>.
    Fog is initially disabled.
    </para>

    <para>
    <function>glFog</function> assigns the value or values in 
    <parameter>params</parameter> to the fog parameter specified by 
    <parameter>pname</parameter>. The following values are accepted for 
    <parameter>pname</parameter>:</para>

    <variablelist>
      <varlistentry>
        <term>
          <constant>GL_FOG_MODE</constant>
        </term>

        <listitem>
          <para>
          <parameter>params</parameter>
          is a single fixed-point or floating-point value that
          specifies the equation to be used to compute the fog
          blend factor <replaceable>f</replaceable>.
          Three symbolic constants are accepted: 
          <constant>GL_LINEAR</constant>, 
          <constant>GL_EXP</constant>, and 
          <constant>GL_EXP2</constant>.
          The equations corresponding to these symbolic constants
          are defined below. The initial fog mode is 
          <constant>GL_EXP</constant>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <constant>GL_FOG_DENSITY</constant>
        </term>

        <listitem>
          <para>
          <parameter>params</parameter>
          is a single fixed-point or floating-point value that
          specifies <replaceable>density</replaceable>,
          the fog density used in both exponential fog equations.
          Only nonnegative densities are accepted. The initial fog
          density is 1.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <constant>GL_FOG_START</constant>
        </term>

        <listitem>
          <para>
          <parameter>params</parameter>
          is a single fixed-point or floating-point value that
          specifies <replaceable>start</replaceable>,
          the near distance used in the linear fog equation. The
          initial near distance is 0.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <constant>GL_FOG_END</constant>
        </term>

        <listitem>
          <para>
          <parameter>params</parameter>
          is a single fixed-point or floating-point value that
          specifies  <replaceable>end</replaceable>,
          the far distance used in the linear fog equation. The
          initial far distance is 1.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <constant>GL_FOG_COLOR</constant>
        </term>

        <listitem>
          <para>
          <parameter>params</parameter>
          contains four fixed-point or floating-point values that specify 
          <inlineequation><math>
            <msub><mi>C</mi><mi>f</mi></msub>
          </math></inlineequation>,
          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).</para>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>Fog blends a fog color with each rasterized pixel
    fragment's posttexturing color using a blending factor
    <replaceable>f</replaceable>. Factor <replaceable>f</replaceable>
    is computed in one of three ways, depending on the fog mode.
    Let <replaceable>z</replaceable>
    be the distance in eye coordinates from the origin to the
    fragment being fogged. The equation for <constant>GL_LINEAR</constant>
    fog is</para>

    <informalequation><math>
      <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></informalequation>

    <para>The equation for <constant>GL_EXP</constant> fog is</para>

    <informalequation><math>
      <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></informalequation>

    <para>The equation for <constant>GL_EXP2</constant> fog is</para>

    <informalequation><math>
      <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></informalequation>

    <para>Regardless of the fog mode, <replaceable>f</replaceable>
    is clamped to the range [0, 1] after it is computed. Then,
    the fragment's red, green, and blue colors, represented by 
    <inlineequation><math>
      <msub><mi>C</mi><mi>r</mi></msub>
    </math></inlineequation>,
    are replaced by</para>

    <informalequation><math>
      <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></informalequation>

    <para>Fog does not affect a fragment's alpha component.</para>
  </refsect1>

  <refsect1>
    <title>Errors</title>

    <para><constant>GL_INVALID_ENUM</constant> is generated if 
    <parameter>pname</parameter> is not an accepted value, or if 
    <parameter>pname</parameter> is 
    <constant>GL_FOG_MODE</constant> and 
    <parameter>params</parameter> is not an accepted value.</para>

    <para><constant>GL_INVALID_VALUE</constant> is generated if 
    <parameter>pname</parameter> is 
    <constant>GL_FOG_DENSITY</constant>, and 
    <parameter>params</parameter> is negative.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>

    <para>
      <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>
    </para>
  </refsect1>
</refentry>