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<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 -->
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