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493 lines
23 KiB
XML
493 lines
23 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN"
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"http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd">
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<refentry id="glFog">
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<refmeta>
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<refmetainfo>
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<copyright>
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<year>1991-2006</year>
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<holder>Silicon Graphics, Inc.</holder>
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</copyright>
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</refmetainfo>
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<refentrytitle>glFog</refentrytitle>
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<manvolnum>3G</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>glFog</refname>
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<refpurpose>specify fog parameters</refpurpose>
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</refnamediv>
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<refsynopsisdiv><title>C Specification</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>void <function>glFogf</function></funcdef>
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<paramdef>GLenum <parameter>pname</parameter></paramdef>
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<paramdef>GLfloat <parameter>param</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<funcsynopsis>
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<funcprototype>
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<funcdef>void <function>glFogi</function></funcdef>
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<paramdef>GLenum <parameter>pname</parameter></paramdef>
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<paramdef>GLint <parameter>param</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<!-- eqn: ignoring delim $$ -->
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<refsect1 id="parameters"><title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><parameter>pname</parameter></term>
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<listitem>
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<para>
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Specifies a single-valued fog parameter.
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<constant>GL_FOG_MODE</constant>,
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<constant>GL_FOG_DENSITY</constant>,
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<constant>GL_FOG_START</constant>,
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<constant>GL_FOG_END</constant>,
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<constant>GL_FOG_INDEX</constant>, and
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<constant>GL_FOG_COORD_SRC</constant>
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are accepted.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>param</parameter></term>
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<listitem>
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<para>
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Specifies the value that <parameter>pname</parameter> will be set to.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsynopsisdiv><title>C Specification</title>
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<funcsynopsis>
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<funcprototype>
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<funcdef>void <function>glFogfv</function></funcdef>
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<paramdef>GLenum <parameter>pname</parameter></paramdef>
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<paramdef>const GLfloat * <parameter>params</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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<funcsynopsis>
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<funcprototype>
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<funcdef>void <function>glFogiv</function></funcdef>
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<paramdef>GLenum <parameter>pname</parameter></paramdef>
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<paramdef>const GLint * <parameter>params</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1 id="parameters2"><title>Parameters</title>
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<variablelist>
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<varlistentry>
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<term><parameter>pname</parameter></term>
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<listitem>
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<para>
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Specifies a fog parameter.
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<constant>GL_FOG_MODE</constant>,
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<constant>GL_FOG_DENSITY</constant>,
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<constant>GL_FOG_START</constant>,
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<constant>GL_FOG_END</constant>,
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<constant>GL_FOG_INDEX</constant>,
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<constant>GL_FOG_COLOR</constant>, and
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<constant>GL_FOG_COORD_SRC</constant>
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are accepted.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>params</parameter></term>
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<listitem>
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<para>
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Specifies the value or values to be assigned to <parameter>pname</parameter>.
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<constant>GL_FOG_COLOR</constant> 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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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1 id="description"><title>Description</title>
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<para>
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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 <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry> and <citerefentry><refentrytitle>glDisable</refentrytitle></citerefentry> with argument
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<constant>GL_FOG</constant>.
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</para>
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<para>
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<function>glFog</function> assigns the value or values in <parameter>params</parameter> to the fog parameter
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specified by <parameter>pname</parameter>.
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The following values are accepted for <parameter>pname</parameter>:
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</para>
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<variablelist>
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<varlistentry>
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<term><constant>GL_FOG_MODE</constant></term>
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<listitem>
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<para>
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<parameter>params</parameter> 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,
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<inlineequation><mml:math><mml:mi mathvariant="italic">f</mml:mi></mml:math></inlineequation>.
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Three symbolic constants are accepted:
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<constant>GL_LINEAR</constant>,
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<constant>GL_EXP</constant>,
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and <constant>GL_EXP2</constant>.
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The equations corresponding to these symbolic constants are defined below.
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The initial fog mode is <constant>GL_EXP</constant>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_FOG_DENSITY</constant></term>
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<listitem>
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<para>
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<parameter>params</parameter> is a single integer or floating-point value that specifies
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<inlineequation><mml:math><mml:mi mathvariant="italic">density</mml:mi></mml:math></inlineequation>,
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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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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_FOG_START</constant></term>
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<listitem>
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<para>
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<parameter>params</parameter> is a single integer or floating-point value that specifies
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<inlineequation><mml:math><mml:mi mathvariant="italic">start</mml:mi></mml:math></inlineequation>,
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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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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_FOG_END</constant></term>
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<listitem>
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<para>
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<parameter>params</parameter> is a single integer or floating-point value that specifies
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<inlineequation><mml:math><mml:mi mathvariant="italic">end</mml:mi></mml:math></inlineequation>,
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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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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_FOG_INDEX</constant></term>
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<listitem>
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<para>
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<parameter>params</parameter> is a single integer or floating-point value that specifies
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<inlineequation><mml:math>
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<!-- eqn: i sub f:-->
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<mml:msub><mml:mi mathvariant="italic">i</mml:mi>
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<mml:mi mathvariant="italic">f</mml:mi>
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</mml:msub>
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</mml:math></inlineequation>,
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the fog color index.
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The initial fog index is 0.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_FOG_COLOR</constant></term>
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<listitem>
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<para>
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<parameter>params</parameter> contains four integer or floating-point values that specify
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<inlineequation><mml:math>
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<!-- eqn: C sub f:-->
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<mml:msub><mml:mi mathvariant="italic">C</mml:mi>
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<mml:mi mathvariant="italic">f</mml:mi>
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</mml:msub>
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</mml:math></inlineequation>,
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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
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<inlineequation><mml:math>
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<!-- eqn: -1.0:-->
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<mml:mn>-1.0</mml:mn>
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</mml:math></inlineequation>.
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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
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<inlineequation><mml:math>
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<!-- eqn: [0,1]:-->
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<mml:mfenced open="[" close="]">
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<mml:mn>0</mml:mn>
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<mml:mn>1</mml:mn>
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</mml:mfenced>
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</mml:math></inlineequation>.
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The initial fog color is (0, 0, 0, 0).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>GL_FOG_COORD_SRC</constant></term>
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<listitem>
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<para>
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<parameter>params</parameter> contains either of the following symbolic constants:
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<constant>GL_FOG_COORD</constant> or <constant>GL_FRAGMENT_DEPTH</constant>. <constant>GL_FOG_COORD</constant>
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specifies that the current fog coordinate should be used as distance value
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in the fog color computation. <constant>GL_FRAGMENT_DEPTH</constant> specifies that the
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current fragment depth should be used as distance value in the fog
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computation.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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Fog blends a fog color with each rasterized pixel fragment's post-texturing
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color using a blending factor
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<inlineequation><mml:math><mml:mi mathvariant="italic">f</mml:mi></mml:math></inlineequation>.
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Factor
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<inlineequation><mml:math><mml:mi mathvariant="italic">f</mml:mi></mml:math></inlineequation>
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is computed in one of three ways,
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depending on the fog mode.
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Let
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<inlineequation><mml:math><mml:mi mathvariant="italic">c</mml:mi></mml:math></inlineequation>
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be either the distance in eye coordinate from the origin (in the
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case that the <constant>GL_FOG_COORD_SRC</constant> is <constant>GL_FRAGMENT_DEPTH</constant>) or
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the current fog coordinate (in the case that <constant>GL_FOG_COORD_SRC</constant>
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is <constant>GL_FOG_COORD</constant>).
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The equation for <constant>GL_LINEAR</constant> fog is
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<informalequation><mml:math>
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<!-- eqn: f = {end - c} over {end - start}:-->
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<mml:mrow>
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<mml:mi mathvariant="italic">f</mml:mi>
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<mml:mo>=</mml:mo>
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<mml:mfrac>
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<mml:mfenced open="" close="">
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<mml:mrow>
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<mml:mi mathvariant="italic">end</mml:mi>
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<mml:mo>-</mml:mo>
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<mml:mi mathvariant="italic">c</mml:mi>
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</mml:mrow>
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</mml:mfenced>
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<mml:mfenced open="" close="">
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<mml:mrow>
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<mml:mi mathvariant="italic">end</mml:mi>
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<mml:mo>-</mml:mo>
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<mml:mi mathvariant="italic">start</mml:mi>
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</mml:mrow>
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</mml:mfenced>
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</mml:mfrac>
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</mml:mrow>
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</mml:math></informalequation>
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</para>
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<para>
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The equation for <constant>GL_EXP</constant> fog is
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<informalequation><mml:math>
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<!-- eqn: f = e sup {-(density cdot c)}:-->
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<mml:mrow>
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<mml:mi mathvariant="italic">f</mml:mi>
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<mml:mo>=</mml:mo>
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<mml:msup><mml:mi mathvariant="italic">e</mml:mi>
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<mml:mfenced open="" close="">
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<mml:mrow>
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<mml:mo>-</mml:mo>
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<mml:mfenced open="(" close=")">
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<mml:mrow>
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<mml:mi mathvariant="italic">density</mml:mi>
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<mml:mo>·</mml:mo>
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<mml:mi mathvariant="italic">c</mml:mi>
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</mml:mrow>
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</mml:mfenced>
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</mml:mrow>
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</mml:mfenced>
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</mml:msup>
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</mml:mrow>
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</mml:math></informalequation>
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</para>
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<para>
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The equation for <constant>GL_EXP2</constant> fog is
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<informalequation><mml:math>
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<!-- eqn: f = e sup {-(density cdot c)} sup 2:-->
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<mml:mrow>
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<mml:mi mathvariant="italic">f</mml:mi>
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<mml:mo>=</mml:mo>
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<mml:msup><mml:mi mathvariant="italic">e</mml:mi>
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<mml:msup><mml:mfenced open="" close="">
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<mml:mrow>
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<mml:mo>-</mml:mo>
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<mml:mfenced open="(" close=")">
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<mml:mrow>
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<mml:mi mathvariant="italic">density</mml:mi>
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<mml:mo>·</mml:mo>
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<mml:mi mathvariant="italic">c</mml:mi>
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</mml:mrow>
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</mml:mfenced>
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</mml:mrow>
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</mml:mfenced>
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<mml:mn>2</mml:mn>
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</mml:msup></mml:msup>
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</mml:mrow>
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</mml:math></informalequation>
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</para>
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<para>
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Regardless of the fog mode,
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<inlineequation><mml:math><mml:mi mathvariant="italic">f</mml:mi></mml:math></inlineequation>
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is clamped to the range
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<inlineequation><mml:math>
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<!-- eqn: [0,1]:-->
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<mml:mfenced open="[" close="]">
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<mml:mn>0</mml:mn>
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<mml:mn>1</mml:mn>
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</mml:mfenced>
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</mml:math></inlineequation>
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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
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<inlineequation><mml:math>
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<!-- eqn: C sub r:-->
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<mml:msub><mml:mi mathvariant="italic">C</mml:mi>
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<mml:mi mathvariant="italic">r</mml:mi>
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</mml:msub>
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</mml:math></inlineequation>,
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are replaced by
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</para>
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<para>
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<informalequation><mml:math>
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<!-- eqn: {C sub r} sup prime = f * C sub r + (1 - f) * C sub f:-->
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<mml:mrow>
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<mml:msup><mml:mfenced open="" close="">
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<mml:msub><mml:mi mathvariant="italic">C</mml:mi>
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<mml:mi mathvariant="italic">r</mml:mi>
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</mml:msub>
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</mml:mfenced>
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<mml:mo>″</mml:mo>
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</mml:msup>
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<mml:mo>=</mml:mo>
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<mml:mrow>
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<mml:mrow>
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<mml:mi mathvariant="italic">f</mml:mi>
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<mml:mo>×</mml:mo>
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<mml:msub><mml:mi mathvariant="italic">C</mml:mi>
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<mml:mi mathvariant="italic">r</mml:mi>
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</mml:msub>
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</mml:mrow>
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<mml:mo>+</mml:mo>
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<mml:mrow>
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<mml:mfenced open="(" close=")">
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<mml:mrow>
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<mml:mn>1</mml:mn>
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<mml:mo>-</mml:mo>
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<mml:mi mathvariant="italic">f</mml:mi>
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</mml:mrow>
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</mml:mfenced>
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<mml:mo>×</mml:mo>
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<mml:msub><mml:mi mathvariant="italic">C</mml:mi>
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<mml:mi mathvariant="italic">f</mml:mi>
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</mml:msub>
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</mml:mrow>
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</mml:mrow>
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</mml:mrow>
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</mml:math></informalequation>
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</para>
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<para>
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Fog does not affect a fragment's alpha component.
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</para>
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<para>
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In color index mode, the fragment's color index
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<inlineequation><mml:math>
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<!-- eqn: i sub r:-->
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<mml:msub><mml:mi mathvariant="italic">i</mml:mi>
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<mml:mi mathvariant="italic">r</mml:mi>
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</mml:msub>
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</mml:math></inlineequation>
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is replaced by
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</para>
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<para>
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<informalequation><mml:math>
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<!-- eqn: {i sub r} sup prime = i sub r + (1 - f) * i sub f:-->
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<mml:mrow>
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<mml:msup><mml:mfenced open="" close="">
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<mml:msub><mml:mi mathvariant="italic">i</mml:mi>
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<mml:mi mathvariant="italic">r</mml:mi>
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</mml:msub>
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</mml:mfenced>
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<mml:mo>″</mml:mo>
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</mml:msup>
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<mml:mo>=</mml:mo>
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<mml:mrow>
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<mml:msub><mml:mi mathvariant="italic">i</mml:mi>
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<mml:mi mathvariant="italic">r</mml:mi>
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</mml:msub>
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<mml:mo>+</mml:mo>
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<mml:mrow>
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<mml:mfenced open="(" close=")">
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<mml:mrow>
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<mml:mn>1</mml:mn>
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<mml:mo>-</mml:mo>
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<mml:mi mathvariant="italic">f</mml:mi>
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</mml:mrow>
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</mml:mfenced>
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<mml:mo>×</mml:mo>
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<mml:msub><mml:mi mathvariant="italic">i</mml:mi>
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<mml:mi mathvariant="italic">f</mml:mi>
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</mml:msub>
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</mml:mrow>
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</mml:mrow>
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</mml:mrow>
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</mml:math></informalequation>
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</para>
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<para>
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</para>
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</refsect1>
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<refsect1 id="notes"><title>Notes</title>
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<para>
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<constant>GL_FOG_COORD_SRC</constant> is available only if the GL version is 1.4 or
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greater.
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</para>
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</refsect1>
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<refsect1 id="errors"><title>Errors</title>
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<para>
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<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>
|
|
<para>
|
|
<constant>GL_INVALID_OPERATION</constant> is generated if <function>glFog</function>
|
|
is executed between the execution of <citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry>
|
|
and the corresponding execution of <citerefentry><refentrytitle>glEnd</refentrytitle></citerefentry>.
|
|
</para>
|
|
</refsect1>
|
|
<refsect1 id="associatedgets"><title>Associated Gets</title>
|
|
<para>
|
|
<citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry> with argument <constant>GL_FOG</constant>
|
|
</para>
|
|
<para>
|
|
<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_FOG_COLOR</constant>
|
|
</para>
|
|
<para>
|
|
<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_FOG_INDEX</constant>
|
|
</para>
|
|
<para>
|
|
<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_FOG_DENSITY</constant>
|
|
</para>
|
|
<para>
|
|
<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_FOG_START</constant>
|
|
</para>
|
|
<para>
|
|
<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_FOG_END</constant>
|
|
</para>
|
|
<para>
|
|
<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_FOG_MODE</constant>
|
|
</para>
|
|
</refsect1>
|
|
<refsect1 id="seealso"><title>See Also</title>
|
|
<para>
|
|
<citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>
|
|
</para>
|
|
</refsect1>
|
|
<refsect1 id="Copyright"><title>Copyright</title>
|
|
<para>
|
|
Copyright <trademark class="copyright"></trademark> 1991-2006
|
|
Silicon Graphics, Inc. This document is licensed under the SGI
|
|
Free Software B License. For details, see
|
|
<ulink url="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</ulink>.
|
|
</para>
|
|
</refsect1>
|
|
</refentry>
|