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<title>Generic Image Library: Adding a new io backend</title>
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<h3><a href="../index.html"><img alt="Boost GIL" src="../_static/gil.png" border="0"></a></h3>
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</td>
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<td ><h1 align="center"><a href="../index.html"></a></h1></td>
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<div><div class="header">
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<div class="headertitle"><div class="title">Adding a new io backend</div></div>
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<div class="textblock"><h1><a class="anchor" id="Overview"></a>
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of backend requirements</h1>
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<p>To add support for a new IO backend the following is required:</p><ul>
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<li>a format tag, to identify the image format, derived from boost::gil::format_tag</li>
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<li>boolean meta function is_supported<PixelType,FormatTag> must be implemented for the new format tag</li>
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<li>explicit specialisation of image_read_info<FormatTag> must be provided, containing runtime information available before/at reading the image</li>
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<li>explicit specialisation of image_write_info<FormatTag> must be provided, containing runtime encoding parameters for writing an image</li>
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<li>An image reader must be specialized: <div class="fragment"><div class="line"><span class="keyword">template</span><<span class="keyword">typename</span> IODevice, <span class="keyword">typename</span> ConversionPolicy></div>
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<div class="line"><span class="keyword">struct </span>boost::gil::reader<IODevice,FormatTag,ConversionPolicy></div>
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<div class="line">{</div>
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<div class="line"> reader( IODevice & device )</div>
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<div class="line"> reader( IODevice & device, <span class="keyword">typename</span> ConversionPolicy::color_converter_type <span class="keyword">const</span>& cc )</div>
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<div class="line"> image_read_info<FormatTag> get_info();</div>
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<div class="line"> <span class="keyword">template</span><<span class="keyword">typename</span> Image></div>
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<div class="line"> <span class="keywordtype">void</span> <a class="code hl_function" href="group___i_o.html#ga1aa312515256348fcddcebc511971c37">read_image</a>( Image &, point_t <span class="keyword">const</span>& top_left );</div>
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<div class="line"> <span class="keyword">template</span><<span class="keyword">typename</span> View></div>
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<div class="line"> <span class="keywordtype">void</span> <a class="code hl_function" href="group___i_o.html#gada0413c88413608f4f982ff50edfe0ab">read_view</a>( View &, point_t <span class="keyword">const</span>& top_left );</div>
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<div class="line">};</div>
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<div class="ttc" id="agroup___i_o_html_ga1aa312515256348fcddcebc511971c37"><div class="ttname"><a href="group___i_o.html#ga1aa312515256348fcddcebc511971c37">boost::gil::read_image</a></div><div class="ttdeci">void read_image(Reader reader, Image &img, typename std::enable_if< mp11::mp_and< detail::is_reader< Reader >, is_format_tag< typename Reader::format_tag_t >, is_read_supported< typename get_pixel_type< typename Image::view_t >::type, typename Reader::format_tag_t > >::value >::type *=nullptr)</div><div class="ttdoc">Reads an image without conversion. Image memory is allocated.</div><div class="ttdef"><b>Definition</b> read_image.hpp:32</div></div>
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<div class="ttc" id="agroup___i_o_html_gada0413c88413608f4f982ff50edfe0ab"><div class="ttname"><a href="group___i_o.html#gada0413c88413608f4f982ff50edfe0ab">boost::gil::read_view</a></div><div class="ttdeci">void read_view(Reader reader, View const &view, typename std::enable_if< mp11::mp_and< detail::is_reader< Reader >, typename is_format_tag< typename Reader::format_tag_t >::type, typename is_read_supported< typename get_pixel_type< View >::type, typename Reader::format_tag_t >::type >::value >::type *=nullptr)</div><div class="ttdoc">Reads an image view without conversion. No memory is allocated.</div><div class="ttdef"><b>Definition</b> read_view.hpp:31</div></div>
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</div><!-- fragment --></li>
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<li>An image writer must be specialized: <div class="fragment"><div class="line">\<span class="keyword">template</span> <<span class="keyword">typename</span> IODevice></div>
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<div class="line"><span class="keyword">struct </span>boost::gil::writer<IODevice,FormatTag></div>
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<div class="line">{</div>
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<div class="line"> writer( IODevice & device )</div>
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<div class="line"> <span class="keyword">template</span><<span class="keyword">typename</span> View></div>
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<div class="line"> <span class="keywordtype">void</span> apply( View <span class="keyword">const</span>&, point_t <span class="keyword">const</span>& top_left );</div>
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<div class="line"> <span class="keyword">template</span><<span class="keyword">typename</span> View></div>
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<div class="line"> <span class="keywordtype">void</span> apply( View <span class="keyword">const</span>&, point_t <span class="keyword">const</span>& top_left, image_write_info<FormatTag> <span class="keyword">const</span>& );</div>
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<div class="line">};</div>
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</div><!-- fragment --></li>
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</ul>
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<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>
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<h1><a class="anchor" id="ConversionPolicy"></a>
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Interface of the ConversionPolicy</h1>
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<p>There are two different conversion policies in use, when reading images: read_and_convert<ColorConverter> and read_and_no_convert. ColorConverter can be a user defined color converter.</p>
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<div class="fragment"><div class="line"><span class="keyword">struct </span>ConversionPolicy</div>
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<div class="line">{</div>
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<div class="line"> <span class="keyword">template</span><<span class="keyword">typename</span> InputIterator,<span class="keyword">typename</span> OutputIterator></div>
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<div class="line"> <span class="keywordtype">void</span> read( InputIterator in_begin, InputIterator in_end,</div>
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<div class="line"> OutputIterator out_end );</div>
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<div class="line">};</div>
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</div><!-- fragment --><p>Methods like read_view and read_image are supposed to bail out with an exception instead of converting the image</p>
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<h1><a class="anchor" id="IODevice"></a>
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Concept of IO Device</h1>
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<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 parameter 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>
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<div class="fragment"><div class="line"><span class="keyword">concept </span>IODevice</div>
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<div class="line">{</div>
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<div class="line"> void IODevice::read( unsigned char* data, int count );</div>
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<div class="line"> void IODevice::write( unsigned char* data, int count );</div>
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<div class="line"> void IODevice::seek(long count, int whence);</div>
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<div class="line"> void IODevice::flush();</div>
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<div class="line">};</div>
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</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><a class="code hl_namespace" href="namespaceboost.html">boost</a>{<span class="keyword">namespace </span>gil{<span class="keyword">namespace </span>detail{</div>
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<div class="line"> <span class="keyword">template</span><<span class="keyword">typename</span> Device></div>
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<div class="line"> <span class="keyword">struct </span>detail::is_input_device;</div>
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<div class="line">}}}</div>
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<div class="ttc" id="anamespaceboost_html"><div class="ttname"><a href="namespaceboost.html">boost</a></div><div class="ttdoc">defined(BOOST_NO_CXX17_HDR_MEMORY_RESOURCE)</div><div class="ttdef"><b>Definition</b> algorithm.hpp:36</div></div>
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