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<div class="title">Adding a new io backend </div>  </div>
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<div class="textblock"><h1><a class="anchor" id="Overview"></a>
of backend requirements</h1>
<p>To add support for a new IO backend the following is required:</p><ul>
<li>a format tag, to identify the image format, derived from boost::gil::format_tag</li>
<li>boolean meta function is_supported&lt;PixelType,FormatTag&gt; must be implemented for the new format tag</li>
<li>explicit specialisation of image_read_info&lt;FormatTag&gt; must be provided, containing runtime information available before/at reading the image</li>
<li>explicit specialisation of image_write_info&lt;FormatTag&gt; must be provided, containing runtime encoding parameters for writing an image</li>
<li>An image reader must be specialized: <div class="fragment"><div class="line"><span class="keyword">template</span>&lt;<span class="keyword">typename</span> IODevice, <span class="keyword">typename</span> ConversionPolicy&gt;</div><div class="line"><span class="keyword">struct </span>boost::gil::reader&lt;IODevice,FormatTag,ConversionPolicy&gt;</div><div class="line">{</div><div class="line">  reader( IODevice &amp; device )</div><div class="line">  reader( IODevice &amp; device, <span class="keyword">typename</span> ConversionPolicy::color_converter_type <span class="keyword">const</span>&amp; cc )</div><div class="line">  image_read_info&lt;FormatTag&gt; get_info();</div><div class="line">  <span class="keyword">template</span>&lt;<span class="keyword">typename</span> Image&gt;</div><div class="line">  <span class="keywordtype">void</span> <a class="code" href="group___i_o.html#gac1e65b2ad8cd28fe8d694f1663df0d78">read_image</a>( Image &amp;, point_t <span class="keyword">const</span>&amp; top_left );</div><div class="line">  <span class="keyword">template</span>&lt;<span class="keyword">typename</span> View&gt;</div><div class="line">  <span class="keywordtype">void</span> <a class="code" href="group___i_o.html#ga9b5ae3d12c73aa1d81245fd5f8772847">read_view</a>( View &amp;, point_t <span class="keyword">const</span>&amp; top_left );</div><div class="line">};</div></div><!-- fragment --></li>
<li>An image writer must be specialized: <div class="fragment"><div class="line">\template &lt;typename IODevice&gt;</div><div class="line"><span class="keyword">struct </span>boost::gil::writer&lt;IODevice,FormatTag&gt;</div><div class="line">{</div><div class="line">  writer( IODevice &amp; device )</div><div class="line">  <span class="keyword">template</span>&lt;<span class="keyword">typename</span> View&gt;</div><div class="line">  <span class="keywordtype">void</span> apply( View <span class="keyword">const</span>&amp;, point_t <span class="keyword">const</span>&amp; top_left );</div><div class="line">  <span class="keyword">template</span>&lt;<span class="keyword">typename</span> View&gt;</div><div class="line">  <span class="keywordtype">void</span> apply( View <span class="keyword">const</span>&amp;, point_t <span class="keyword">const</span>&amp; top_left, image_write_info&lt;FormatTag&gt; <span class="keyword">const</span>&amp; );</div><div class="line">};</div></div><!-- fragment --></li>
</ul>
<p>Or instead of the items above implement overloads of read_view, read_and_convert_view, read_image, read_and_convert_image, write_view and read_image_info.</p>
<h1><a class="anchor" id="ConversionPolicy"></a>
Interface of the ConversionPolicy</h1>
<p>There are two different conversion policies in use, when reading images: read_and_convert&lt;ColorConverter&gt; and read_and_no_convert. ColorConverter can be a user defined color converter.</p>
<div class="fragment"><div class="line"><span class="keyword">struct </span>ConversionPolicy</div><div class="line">{</div><div class="line">   <span class="keyword">template</span>&lt;<span class="keyword">typename</span> InputIterator,<span class="keyword">typename</span> OutputIterator&gt;</div><div class="line">   <span class="keywordtype">void</span> read( InputIterator in_begin, InputIterator in_end,</div><div class="line">         OutputIterator out_end );</div><div class="line">};</div></div><!-- fragment --><p>Methods like read_view and read_image are supposed to bail out with an exception instead of converting the image</p>
<h1><a class="anchor" id="IODevice"></a>
Concept of IO Device</h1>
<p>A Device is simply an object used to read and write data to and from a stream. The IODevice was added as a template paramter to be able to replace the file_name access functionality. This is only an interim solution, as soon as boost provides a good IO library, interfaces/constraints provided by that library could be used.</p>
<div class="fragment"><div class="line">concept IODevice</div><div class="line">{</div><div class="line">    <span class="keywordtype">void</span> IODevice::read( <span class="keywordtype">unsigned</span> <span class="keywordtype">char</span>* data, <span class="keywordtype">int</span> count );</div><div class="line">    <span class="keywordtype">void</span> IODevice::write( <span class="keywordtype">unsigned</span> <span class="keywordtype">char</span>* data, <span class="keywordtype">int</span> count );</div><div class="line">    <span class="keywordtype">void</span> IODevice::seek(<span class="keywordtype">long</span> count, <span class="keywordtype">int</span> whence);</div><div class="line">    <span class="keywordtype">void</span> IODevice::flush();</div><div class="line">};</div></div><!-- fragment --><p>For the time being a boolean meta function must be specialized: </p><div class="fragment"><div class="line"><span class="keyword">namespace </span>boost{<span class="keyword">namespace </span>gil{<span class="keyword">namespace </span>detail{</div><div class="line"> <span class="keyword">template</span>&lt;<span class="keyword">typename</span> Device&gt;</div><div class="line"> <span class="keyword">struct </span>detail::is_input_device;</div><div class="line">}}}</div></div><!-- fragment --> </div></div><!-- PageDoc -->
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