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

  <refnamediv>
    <refdescriptor>glOrtho</refdescriptor>
    <refname>glOrthof</refname>
    <refname>glOrthox</refname>
    <refpurpose>multiply the current matrix with an orthographic
    matrix</refpurpose>
  </refnamediv>

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

    <funcsynopsis>
      <funcprototype>
        <funcdef>void <function>glOrthof</function></funcdef>
        <paramdef>GLfloat <parameter>left</parameter></paramdef>
        <paramdef>GLfloat <parameter>right</parameter></paramdef>
        <paramdef>GLfloat <parameter>bottom</parameter></paramdef>
        <paramdef>GLfloat <parameter>top</parameter></paramdef>
        <paramdef>GLfloat <parameter>near</parameter></paramdef>
        <paramdef>GLfloat <parameter>far</parameter></paramdef>
      </funcprototype>
      <funcprototype>
        <funcdef>void <function>glOrthox</function></funcdef>
        <paramdef>GLfixed <parameter>left</parameter></paramdef>
        <paramdef>GLfixed <parameter>right</parameter></paramdef>
        <paramdef>GLfixed <parameter>bottom</parameter></paramdef>
        <paramdef>GLfixed <parameter>top</parameter></paramdef>
        <paramdef>GLfixed <parameter>near</parameter></paramdef>
        <paramdef>GLfixed <parameter>far</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Parameters</title>

    <variablelist>
      <varlistentry>
        <term>
        <parameter>left</parameter>, 
        <parameter>right</parameter>
        </term>

        <listitem>
          <para>Specify the coordinates for the left and right
          vertical clipping planes.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <parameter>bottom</parameter>, 
        <parameter>top</parameter>
        </term>

        <listitem>
          <para>Specify the coordinates for the bottom and top
          horizontal clipping planes.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
        <parameter>near</parameter>, 
        <parameter>far</parameter>
        </term>

        <listitem>
          <para>Specify the distances to the nearer and farther
          depth clipping planes. These values are negative if the
          plane is to be behind the viewer.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Description</title>

    <para><function>glOrtho</function>
    describes a transformation that produces a parallel projection.
    The current matrix (see 
    <citerefentry><refentrytitle>glMatrixMode</refentrytitle></citerefentry>)
    is multiplied by this matrix and the result replaces the
    current matrix, as if 
    <citerefentry><refentrytitle>glMultMatrix</refentrytitle></citerefentry>
    were called with the following matrix as its argument:</para>

    <informalequation><math>
      <mrow>
        <mo>(</mo>
        <mtable class="matrix">
          <mtr>
            <mtd>
              <mfrac>
                <mn>2</mn>
                <mrow><mi>right</mi><mo>-</mo><mi>left</mi></mrow>
              </mfrac>
            </mtd>
            <mtd><mn>0</mn></mtd>
            <mtd><mn>0</mn></mtd>
            <mtd><msub><mi>t</mi><mi>x</mi></msub></mtd>
          </mtr>
          <mtr>
            <mtd><mn>0</mn></mtd>
            <mtd>
              <mfrac>
                <mn>2</mn>
                <mrow><mi>top</mi><mo>-</mo><mi>bottom</mi></mrow>
              </mfrac>
            </mtd>
            <mtd><mn>0</mn></mtd>
            <mtd><msub><mi>t</mi><mi>y</mi></msub></mtd>
          </mtr>
          <mtr>
            <mtd><mn>0</mn></mtd>
            <mtd><mn>0</mn></mtd>
            <mtd>
              <mfrac>
                <mrow><mo>-</mo><mn>2</mn></mrow>
                <mrow><mi>far</mi><mo>-</mo><mi>near</mi></mrow>
              </mfrac>
            </mtd>
            <mtd><msub><mi>t</mi><mi>z</mi></msub></mtd>
          </mtr>
          <mtr>
            <mtd><mn>0</mn></mtd>
            <mtd><mn>0</mn></mtd>
            <mtd><mn>0</mn></mtd>
            <mtd><mn>1</mn></mtd>
          </mtr>
        </mtable>
        <mo>)</mo>
      </mrow>
    </math></informalequation>

    <para>where</para>

    <informalequation><math><mtable>
      <mtr><mtd>
        <msub><mi>t</mi><mi>x</mi></msub>
        <mgroupalign/>
        <mo>=</mo>
        <mgroupalign/>
        <mo>-</mo>
        <mfrac>
          <mrow><mi>right</mi><mo>+</mo><mi>left</mi></mrow>
          <mrow><mi>right</mi><mo>-</mo><mi>left</mi></mrow>
        </mfrac>
      </mtd></mtr>
      <mtr><mtd>
        <msub><mi>t</mi><mi>y</mi></msub>
        <mgroupalign/>
        <mo>=</mo>
        <mgroupalign/>
        <mo>-</mo>
        <mfrac>
          <mrow><mi>top</mi><mo>+</mo><mi>bottom</mi></mrow>
          <mrow><mi>top</mi><mo>-</mo><mi>bottom</mi></mrow>
        </mfrac>
      </mtd></mtr>
      <mtr><mtd>
        <msub><mi>t</mi><mi>z</mi></msub>
        <mgroupalign/>
        <mo>=</mo>
        <mgroupalign/>
        <mo>-</mo>
        <mfrac>
          <mrow><mi>far</mi><mo>+</mo><mi>near</mi></mrow>
          <mrow><mi>far</mi><mo>-</mo><mi>near</mi></mrow>
        </mfrac>
      </mtd></mtr>
    </mtable></math></informalequation>

    <para>Typically, the matrix mode is 
    <constant>GL_PROJECTION</constant>, and
    (<parameter>left</parameter>, <parameter>bottom</parameter>,
    -<parameter>near</parameter>) and
    (<parameter>right</parameter>, <parameter>top</parameter>,
    -<parameter>near</parameter>) 
    specify the points on the near clipping plane that are mapped
    to the lower left and upper right corners of the window,
    respectively, assuming that the eye is located at (0, 0, 0).
    -<parameter>far</parameter>
    specifies the location of the far clipping plane. Both 
    <parameter>near</parameter> and <parameter>far</parameter> 
    can be either positive or negative.</para>

    <para>Use 
    <citerefentry><refentrytitle>glPushMatrix</refentrytitle></citerefentry>
    and 
    <citerefentry><refentrytitle>glPopMatrix</refentrytitle></citerefentry>
    to save and restore the current matrix stack.</para>
  </refsect1>

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

    <para>
    <citerefentry><refentrytitle>glFrustum</refentrytitle></citerefentry>, 
    <citerefentry><refentrytitle>glMatrixMode</refentrytitle></citerefentry>,
    <citerefentry><refentrytitle>glMultMatrix</refentrytitle></citerefentry>,
    <citerefentry><refentrytitle>glPushMatrix</refentrytitle></citerefentry>,
    <citerefentry><refentrytitle>glViewport</refentrytitle></citerefentry>
    </para>
  </refsect1>
</refentry>