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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="glViewport">
  <refmeta>
    <refentrytitle>glViewport</refentrytitle>
    <manvolnum>3G</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>glViewport</refname>
    <refpurpose>set the viewport</refpurpose>
  </refnamediv>

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

    <funcsynopsis>
      <funcprototype>
        <funcdef>void <function>glViewport</function></funcdef>
        <paramdef>GLint <parameter>x</parameter></paramdef>
        <paramdef>GLint <parameter>y</parameter></paramdef>
        <paramdef>GLsizei <parameter>width</parameter></paramdef>
        <paramdef>GLsizei <parameter>height</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Parameters</title>

    <variablelist>
      <varlistentry>
        <term>
          <parameter>x</parameter>, 
          <parameter>y</parameter>
        </term>
        <listitem>
          <para>Specify the lower left corner of the viewport
          rectangle, in pixels. The initial value is (0, 0).</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <parameter>width</parameter>, 
          <parameter>height</parameter>
        </term>
        <listitem>
          <para>Specify the width and height of the viewport. When
          a GL context is first attached to a surface (e.g. window), 
          <parameter>width</parameter>
          and 
          <parameter>height</parameter>
          are set to the dimensions of that surface.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Description</title>

    <para>
    <function>glViewport</function>

    specifies the affine transformation of 
    <replaceable>x</replaceable> and 
    <replaceable>y</replaceable>
    from normalized device coordinates to window coordinates. Let
    <inlineequation><math><mfenced>
      <msub><mi>x</mi><mi>nd</mi></msub>
      <msub><mi>y</mi><mi>nd</mi></msub>
    </mfenced></math></inlineequation>
    be normalized device coordinates. Then the window coordinates
    <inlineequation><math><mfenced>
      <msub><mi>x</mi><mi>w</mi></msub>
      <msub><mi>y</mi><mi>w</mi></msub>
    </mfenced></math></inlineequation>
    are computed as follows:</para>

    <informalequation><math>
      <mtable><mtr><mtd>
        <msub><mi>x</mi><mi>w</mi></msub>
        <maligngroup/>
        <mo>=</mo>
        <maligngroup/>
        <mo>(</mo>
          <msub><mi>x</mi><mi>nd</mi></msub>
          <mo>+</mo>
          <mn>1</mn>
        <mo>)</mo>
        <mfrac>
          <mi>width</mi>
          <mn>2</mn>
        </mfrac>
        <mo>+</mo>
        <mi>x</mi>
      </mtd></mtr>
      <mtr><mtd>
        <msub><mi>y</mi><mi>w</mi></msub>
        <maligngroup/>
        <mo>=</mo>
        <maligngroup/>
        <mo>(</mo>
          <msub><mi>y</mi><mi>nd</mi></msub>
          <mo>+</mo>
          <mn>1</mn>
        <mo>)</mo>
        <mfrac>
          <mi>height</mi>
          <mn>2</mn>
        </mfrac>
        <mo>+</mo>
        <mi>y</mi>
      </mtd></mtr></mtable>
    </math></informalequation>

    <para>Viewport width and height are silently clamped to a range
    that depends on the implementation. To query this range, call 
    <citerefentry><refentrytitle>glGetInteger</refentrytitle></citerefentry>
    with argument <constant>GL_MAX_VIEWPORT_DIMS</constant>.</para>
  </refsect1>

  <refsect1>
    <title>Errors</title>

    <para><constant>GL_INVALID_VALUE</constant> is generated if either 
      <parameter>width</parameter> or 
      <parameter>height</parameter> is negative.</para>
  </refsect1>

  <refsect1>
    <title>Associated Gets</title>

    <para>
      <citerefentry><refentrytitle>glGetInteger</refentrytitle></citerefentry>
      with argument <constant>GL_MAX_VIEWPORT_DIMS</constant>
    </para>
  </refsect1>

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

    <para>
      <citerefentry><refentrytitle>glDepthRange</refentrytitle></citerefentry>, 
      <citerefentry><refentrytitle>glGetInteger</refentrytitle></citerefentry>
    </para>
  </refsect1>
</refentry>