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    <title>read_image (2D, Half)</title>
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      <div xmlns="" class="refnamediv">
        <a xmlns="http://www.w3.org/1999/xhtml" id="read_image2d"></a>
        <h1>read_imageh (2D, Half)</h1>
        <p>
            Do an element lookup in the 2D image object, returning half floating-point values.
        </p>
      </div>
      <div class="refsynopsisdiv">
        <h2></h2>
        <div class="funcsynopsis">
          <table xmlns="" border="0" summary="Function synopsis" cellspacing="0" cellpadding="0">
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              <td>
                <code xmlns="http://www.w3.org/1999/xhtml" class="funcdef">
                    <a class="link" href="vectorDataTypes.html" target="_top">half4</a>
                    <strong class="fsfunc">                   
                        read_imageh
                    </strong>
                (</code>
                <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="otherDataTypes.html" target="_top">image2d_t</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">image</var>, </td>
              </td>
            </tr>
            <tr valign="top">
              <td> </td>
              <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="sampler_t.html" target="_top">sampler_t</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">sampler</var>, </td>
            </tr>
            <tr valign="top">
              <td> </td>
              <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="vectorDataTypes.html" target="_top">int2</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">coord</var><code>)</code></td>
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          <table xmlns="" border="0" summary="Function synopsis" cellspacing="0" cellpadding="0">
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                <code xmlns="http://www.w3.org/1999/xhtml" class="funcdef">
                    <a class="link" href="vectorDataTypes.html" target="_top">half4</a>
                    <strong class="fsfunc">                   
                        read_imageh
                    </strong>
                (</code>
                <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="otherDataTypes.html" target="_top">image2d_t</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">image</var>, </td>
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              <td> </td>
              <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="sampler_t.html" target="_top">sampler_t</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">sampler</var>, </td>
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              <td> </td>
              <td><a xmlns="http://www.w3.org/1999/xhtml" class="link" href="vectorDataTypes.html" target="_top">float2</a> <var xmlns="http://www.w3.org/1999/xhtml" class="pdparam">coord</var><code>)</code></td>
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      <div class="refsect1">
        <a id="description"></a>
        <h2>Description</h2>
        <p>
        Use the coordinate <span class="emphasis"><em>(x, y)</em></span> to do an element lookup in the 2D image object specified by <code class="varname">image</code>.
        </p>
        <p>      
<code class="function">read_imageh</code> returns half floating-point values in the range [0.0... 1.0] for image objects created with <code class="varname">image_channel_data_type</code> set to one of the predefined packed formats or CL_UNORM_INT8 or CL_UNORM_INT16.
        </p>
        <p>  
<code class="function">read_imageh</code> returns half floating-point values in the range [-1.0... 1.0] for image objects created with <code class="varname">image_channel_data_type</code> set to CL_SNORM_INT8, or CL_SNORM_INT16.
        </p>
        <p>  
<code class="function">read_imageh</code> returns half floating-point values for image objects created with <code class="varname">image_channel_data_type</code> set to CL_HALF_FLOAT.
        </p>
        <p>  
The <code class="function">read_imageh</code> calls that take integer coordinates must use a sampler with filter mode set to CLK_FILTER_NEAREST, normalized coordinates set to CLK_NORMALIZED_COORDS_FALSE and addressing mode set to CLK_ADDRESS_CLAMP_TO_EDGE, CLK_ADDRESS_CLAMP or CLK_ADDRESS_NONE; otherwise the values returned are undefined.
        </p>
        <p>  
Values returned by <code class="function">read_imageh</code> for image objects with <code class="varname">image_channel_data_type</code> values not specified in the description above are undefined.
        </p>
      </div>
      <div class="refsect1">
        <a id="notes"></a>
        <h2>Notes</h2>
        <p>  
        The built-in functions defined in this section can only be used with image memory objects
created with <a class="citerefentry" href="clCreateImage2D.html"><span class="citerefentry"><span class="refentrytitle">clCreateImage2D</span></span></a>, or <a class="citerefentry" href="clCreateImage3D.html"><span class="citerefentry"><span class="refentrytitle">clCreateImage3D</span></span></a>. An image memory object can be
accessed by specific function calls that read from and/or write to specific locations in the image.
        </p>
        <p>
Image memory objects that are being read by a kernel should be declared with the
<a class="citerefentry" href="accessQualifiers.html"><span class="citerefentry"><span class="refentrytitle">__read_only</span></span></a> qualifier. <a class="citerefentry" href="write_image.html"><span class="citerefentry"><span class="refentrytitle">write_image</span></span></a> calls to image memory objects declared with the
<a class="citerefentry" href="accessQualifiers.html"><span class="citerefentry"><span class="refentrytitle">__read_only</span></span></a> qualifier will generate a compilation error. Image memory objects that are
being written to by a kernel should be declared with the <a class="citerefentry" href="accessQualifiers.html"><span class="citerefentry"><span class="refentrytitle">__write_only</span></span></a> qualifier.
<a class="citerefentry" href="read_image.html"><span class="citerefentry"><span class="refentrytitle">read_image</span></span></a> calls to image memory objects declared with the <a class="citerefentry" href="accessQualifiers.html"><span class="citerefentry"><span class="refentrytitle">__write_only</span></span></a> qualifier will
generate a compilation error. <a class="citerefentry" href="read_image.html"><span class="citerefentry"><span class="refentrytitle">read_image</span></span></a> 
and <a class="citerefentry" href="write_image.html"><span class="citerefentry"><span class="refentrytitle">write_image</span></span></a> calls to the same image memory
object in a kernel are not supported.
        </p>
        <p>
The <a class="citerefentry" href="read_image.html"><span class="citerefentry"><span class="refentrytitle">read_image</span></span></a> calls returns a four component floating-point, integer or unsigned integer color
value. The color values returned by <a class="citerefentry" href="read_image.html"><span class="citerefentry"><span class="refentrytitle">read_image</span></span></a> are identified as <em class="replaceable"><code>x, y, z, w</code></em> where <em class="replaceable"><code>x</code></em> refers to
the red component, <em class="replaceable"><code>y</code></em> refers to the green component, <em class="replaceable"><code>z</code></em> refers to the blue component and <em class="replaceable"><code>w</code></em> refers to the alpha component.
        </p>
        <p>
        The image read functions take a sampler argument. The sampler can be passed as an argument
to the kernel using <a class="citerefentry" href="clSetKernelArg.html"><span class="citerefentry"><span class="refentrytitle">clSetKernelArg</span></span></a>, or it can be a constant variable of type <a class="citerefentry" href="sampler_t.html"><span class="citerefentry"><span class="refentrytitle">sampler_t</span></span></a> declared in the program source.
        </p>
        <p>
            A kernel that uses a sampler with the CL_ADDRESS_CLAMP addressing mode with
multiple images may result in additional samplers being used internally by an implementation. If
the same sampler is used with multiple images called via <code class="function">read_image{f | i | ui}</code>, then it is
possible that an implementation may need to allocate an additional sampler to handle the
different border color values that may be needed depending on the image formats being used.
These implementation allocated samplers will count against the maximum sampler values
supported by the device and given by CL_DEVICE_MAX_SAMPLERS. Enqueuing a kernel that
requires more samplers than the implementation can support will result in a
CL_OUT_OF_RESOURCES error being returned.
        </p>
      </div>
      <div class="refsect1">
        <a id="specification"></a>
        <h2>Specification</h2>
        <p>
            <img src="pdficon_small1.gif" />
                                               
            <a href="http://www.khronos.org/registry/cl/specs/opencl-1.1.pdf#page=234" target="OpenCL Spec">OpenCL Specification</a>
        </p>
      </div>
      <div class="refsect1">
        <a id="seealso"></a>
        <h2>Also see</h2>
        <p>
            <a class="citerefentry" href="imageFunctions.html"><span class="citerefentry"><span class="refentrytitle">Image Functions</span></span></a> 
        </p>
      </div>
      <div xmlns="" class="refsect3" lang="en" xml:lang="en"><a xmlns="http://www.w3.org/1999/xhtml" id="Copyright"></a><h4 xmlns="http://www.w3.org/1999/xhtml"></h4><img xmlns="http://www.w3.org/1999/xhtml" src="KhronosLogo.jpg" /><p xmlns="http://www.w3.org/1999/xhtml"></p>Copyright © 2007-2010 The Khronos Group Inc.
Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and/or associated documentation files (the
"Materials"), to deal in the Materials without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Materials, and to
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