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301 lines
14 KiB
XML
301 lines
14 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="glAccum">
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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>glAccum</refentrytitle>
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<manvolnum>3G</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>glAccum</refname>
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<refpurpose>operate on the accumulation buffer</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>glAccum</function></funcdef>
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<paramdef>GLenum <parameter>op</parameter></paramdef>
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<paramdef>GLfloat <parameter>value</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>op</parameter></term>
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<listitem>
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<para>
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Specifies the accumulation buffer operation.
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Symbolic constants
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<constant>GL_ACCUM</constant>,
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<constant>GL_LOAD</constant>,
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<constant>GL_ADD</constant>,
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<constant>GL_MULT</constant>,
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and
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<constant>GL_RETURN</constant> 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>value</parameter></term>
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<listitem>
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<para>
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Specifies a floating-point value used in the accumulation buffer operation.
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<parameter>op</parameter> determines how <parameter>value</parameter> is used.
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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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The accumulation buffer is an extended-range color buffer.
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Images are not rendered into it.
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Rather,
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images rendered into one of the color buffers
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are added to the contents of the accumulation buffer after rendering.
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Effects such as antialiasing (of points, lines, and polygons),
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motion blur,
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and depth of field can be created
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by accumulating images generated with different transformation matrices.
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</para>
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<para>
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Each pixel in the accumulation buffer consists of
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red, green, blue, and alpha values.
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The number of bits per component in the accumulation buffer
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depends on the implementation. You can examine this number
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by calling <citerefentry><refentrytitle>glGetIntegerv</refentrytitle></citerefentry> four times,
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with arguments <constant>GL_ACCUM_RED_BITS</constant>,
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<constant>GL_ACCUM_GREEN_BITS</constant>,
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<constant>GL_ACCUM_BLUE_BITS</constant>,
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and <constant>GL_ACCUM_ALPHA_BITS</constant>.
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Regardless of the number of bits per component,
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the range of values stored by each component is
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<inlineequation><mml:math>
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<!-- eqn: [-1,1]:-->
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<mml:mfenced open="[" close="]">
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<mml:mn>-1</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 accumulation buffer pixels are mapped one-to-one with frame buffer pixels.
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</para>
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<para>
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<function>glAccum</function> operates on the accumulation buffer.
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The first argument, <parameter>op</parameter>,
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is a symbolic constant that selects an accumulation buffer operation.
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The second argument, <parameter>value</parameter>,
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is a floating-point value to be used in that operation.
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Five operations are specified:
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<constant>GL_ACCUM</constant>, <constant>GL_LOAD</constant>, <constant>GL_ADD</constant>,
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<constant>GL_MULT</constant>, and <constant>GL_RETURN</constant>.
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</para>
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<para>
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All accumulation buffer operations are limited
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to the area of the current scissor box and applied identically to
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the red, green, blue, and alpha components of each pixel.
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If a <function>glAccum</function> operation results in a value outside the range
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<inlineequation><mml:math>
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<!-- eqn: [-1,1]:-->
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<mml:mfenced open="[" close="]">
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<mml:mn>-1</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 contents of an accumulation buffer pixel component are undefined.
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</para>
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<para>
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The operations are as follows:
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</para>
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<variablelist>
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<varlistentry>
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<term><constant>GL_ACCUM</constant></term>
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<listitem>
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<para>
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Obtains R, G, B, and A values
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from the buffer currently selected for reading (see <citerefentry><refentrytitle>glReadBuffer</refentrytitle></citerefentry>).
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Each component value is divided by
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<inlineequation><mml:math>
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<!-- eqn: 2 sup n - 1:-->
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<mml:mrow>
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<mml:msup><mml:mn>2</mml:mn>
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<mml:mi mathvariant="italic">n</mml:mi>
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</mml:msup>
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<mml:mo>-</mml:mo>
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<mml:mn>1</mml:mn>
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</mml:mrow>
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</mml:math></inlineequation>,
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where
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<inlineequation><mml:math><mml:mi mathvariant="italic">n</mml:mi></mml:math></inlineequation>
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is the number of bits allocated to each color component
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in the currently selected buffer.
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The result is a floating-point value in 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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which is multiplied by <parameter>value</parameter> and added to the corresponding pixel component
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in the accumulation buffer,
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thereby updating the accumulation buffer.
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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_LOAD</constant></term>
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<listitem>
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<para>
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Similar to <constant>GL_ACCUM</constant>,
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except that the current value in the accumulation buffer is not used
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in the calculation of the new value.
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That is, the R, G, B, and A values from the currently selected buffer
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are divided by
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<inlineequation><mml:math>
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<!-- eqn: 2 sup n - 1:-->
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<mml:mrow>
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<mml:msup><mml:mn>2</mml:mn>
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<mml:mi mathvariant="italic">n</mml:mi>
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</mml:msup>
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<mml:mo>-</mml:mo>
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<mml:mn>1</mml:mn>
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</mml:mrow>
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</mml:math></inlineequation>,
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multiplied by <parameter>value</parameter>,
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and then stored in the corresponding accumulation buffer cell,
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overwriting the current value.
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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_ADD</constant></term>
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<listitem>
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<para>
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Adds <parameter>value</parameter> to each R, G, B, and A
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in the accumulation buffer.
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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_MULT</constant></term>
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<listitem>
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<para>
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Multiplies each R, G, B, and A
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in the accumulation buffer by <parameter>value</parameter> and returns the scaled component
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to its corresponding accumulation buffer location.
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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_RETURN</constant></term>
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<listitem>
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<para>
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Transfers accumulation buffer values
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to the color buffer or buffers currently selected for writing.
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Each R, G, B, and A component is multiplied by <parameter>value</parameter>,
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then multiplied by
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<inlineequation><mml:math>
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<!-- eqn: 2 sup n - 1:-->
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<mml:mrow>
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<mml:msup><mml:mn>2</mml:mn>
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<mml:mi mathvariant="italic">n</mml:mi>
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</mml:msup>
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<mml:mo>-</mml:mo>
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<mml:mn>1</mml:mn>
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</mml:mrow>
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</mml:math></inlineequation>,
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clamped to the range
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<inlineequation><mml:math>
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<!-- eqn: [0, 2 sup n - 1]:-->
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<mml:mfenced open="[" close="]">
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<mml:mn>0</mml:mn>
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<mml:mrow>
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<mml:msup><mml:mn>2</mml:mn>
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<mml:mi mathvariant="italic">n</mml:mi>
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</mml:msup>
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<mml:mo>-</mml:mo>
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<mml:mn>1</mml:mn>
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</mml:mrow>
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</mml:mfenced>
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</mml:math></inlineequation>,
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and stored
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in the corresponding display buffer cell.
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The only fragment operations that are applied to this transfer are
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pixel ownership,
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scissor,
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dithering,
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and color writemasks.
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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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To clear the accumulation buffer, call <citerefentry><refentrytitle>glClearAccum</refentrytitle></citerefentry> with R, G, B,
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and A values to set it to, then call <citerefentry><refentrytitle>glClear</refentrytitle></citerefentry> with the
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accumulation buffer enabled.
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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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Only pixels within the current scissor box are updated by a
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<function>glAccum</function> operation.
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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>op</parameter> is not an accepted value.
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</para>
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<para>
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<constant>GL_INVALID_OPERATION</constant> is generated if there is no accumulation buffer.
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</para>
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<para>
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<constant>GL_INVALID_OPERATION</constant> is generated if <function>glAccum</function>
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is executed between the execution of
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<citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry> and the corresponding execution of <citerefentry><refentrytitle>glEnd</refentrytitle></citerefentry>.
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</para>
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</refsect1>
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<refsect1 id="associatedgets"><title>Associated Gets</title>
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<para>
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<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_ACCUM_RED_BITS</constant>
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</para>
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<para>
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<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_ACCUM_GREEN_BITS</constant>
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</para>
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<para>
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<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_ACCUM_BLUE_BITS</constant>
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</para>
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<para>
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<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_ACCUM_ALPHA_BITS</constant>
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</para>
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</refsect1>
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<refsect1 id="seealso"><title>See Also</title>
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<para>
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<citerefentry><refentrytitle>glClear</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glClearAccum</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glCopyPixels</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glDrawBuffer</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glGet</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glReadBuffer</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glReadPixels</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glScissor</refentrytitle></citerefentry>,
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<citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry>
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</para>
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</refsect1>
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<refsect1 id="Copyright"><title>Copyright</title>
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<para>
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Copyright <trademark class="copyright"></trademark> 1991-2006
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Silicon Graphics, Inc. This document is licensed under the SGI
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Free Software B License. For details, see
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<ulink url="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</ulink>.
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</para>
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</refsect1>
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</refentry>
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