sdk/1.2/docs/man/imageFunctionsInc.xml

  <para>
    <!-- core spec p. 288, section 6.12.14 --> The built-in functions defined in this section
    can only be used with image memory objects.  An image memory object can be accessed by
    specific function calls that read from and/or write to specific locations in the image.
  </para>

<!-- Note: use <function> instead of <citerefentry> for the read and write functions in the text below. -->

  <para>
    Image memory objects that are being read by a kernel should be declared with the <citerefentry
    href="accessQualifiers"><refentrytitle>__read_only</refentrytitle></citerefentry>
    qualifier. <function>write_image</function> calls to image memory objects declared with the
    <citerefentry href="accessQualifiers"><refentrytitle>__read_only</refentrytitle></citerefentry>
    qualifier will generate a compilation error. Image memory objects that
    are being written to by a kernel should be declared with the <citerefentry
    href="accessQualifiers"><refentrytitle>__write_only</refentrytitle></citerefentry> qualifier.
    <function>read_image</function> calls to image memory objects declared with the <citerefentry
    href="accessQualifiers"><refentrytitle>__write_only</refentrytitle></citerefentry>
    qualifier will generate a compilation error. <function>read_image</function> and
    <function>write_image</function> calls to the same image memory object in a kernel are
    not supported.
  </para>

  <para>
    The <function>read_image</function> calls returns a four component floating-point, integer or
    unsigned integer color value. The color values returned by <function>read_image</function>
    are identified as <code>x, y, z, w</code> where <code>x</code> refers to the red component,
    <code>y</code> refers to the green component, <code>z</code> refers to the blue component
    and <code>w</code> refers to the alpha component.
  </para>

<!-- 16-Oct-2011 -->