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  68   <div class="section" id="color-space-and-layout">
  69 <h1>Color Space and Layout</h1>
  70 <div class="contents local topic" id="contents">
  71 <ul class="simple">
  72 <li><a class="reference internal" href="#overview" id="id1">Overview</a></li>
  73 <li><a class="reference internal" href="#models" id="id2">Models</a></li>
  74 </ul>
  75 </div>
  76 <div class="section" id="overview">
  77 <h2><a class="toc-backref" href="#id1">Overview</a></h2>
  78 <p>A color space captures the set and interpretation of channels comprising a
  79 pixel. In Boost.GIL, color space is defined as an MPL random access sequence
  80 containing the types of all elements in the color space.</p>
  81 <p>Two color spaces are considered <em>compatible</em> if they are equal (i.e. have the
  82 same set of colors in the same order).</p>
  83 <div class="admonition seealso">
  84 <p class="first admonition-title">See also</p>
  85 <ul class="last simple">
  86 <li><a class="reference external" href="reference/structboost_1_1gil_1_1_color_space_concept.html">ColorSpaceConcept&lt;ColorSpace&gt;</a></li>
  87 <li><a class="reference external" href="reference/structboost_1_1gil_1_1_color_spaces_compatible_concept.html">ColorSpacesCompatibleConcept&lt;ColorSpace1,ColorSpace2&gt;</a></li>
  88 <li><a class="reference external" href="reference/structboost_1_1gil_1_1_channel_mapping_concept.html">ChannelMappingConcept&lt;Mapping&gt;</a></li>
  89 </ul>
  90 </div>
  91 </div>
  92 <div class="section" id="models">
  93 <h2><a class="toc-backref" href="#id2">Models</a></h2>
  94 <p>GIL currently provides the following color spaces:</p>
  95 <ul class="simple">
  96 <li><code class="docutils literal"><span class="pre">gray_t</span></code></li>
  97 <li><code class="docutils literal"><span class="pre">rgb_t</span></code></li>
  98 <li><code class="docutils literal"><span class="pre">rgba_t</span></code></li>
  99 <li><code class="docutils literal"><span class="pre">cmyk_t</span></code></li>
 100 </ul>
 101 <p>It also provides unnamed N-channel color spaces of two to five channels:</p>
 102 <ul class="simple">
 103 <li><code class="docutils literal"><span class="pre">devicen_t&lt;2&gt;</span></code></li>
 104 <li><code class="docutils literal"><span class="pre">devicen_t&lt;3&gt;</span></code></li>
 105 <li><code class="docutils literal"><span class="pre">devicen_t&lt;4&gt;</span></code></li>
 106 <li><code class="docutils literal"><span class="pre">devicen_t&lt;5&gt;</span></code></li>
 107 </ul>
 108 <p>Besides the standard layouts, it also provides:</p>
 109 <ul class="simple">
 110 <li><code class="docutils literal"><span class="pre">bgr_layout_t</span></code></li>
 111 <li><code class="docutils literal"><span class="pre">bgra_layout_t</span></code></li>
 112 <li><code class="docutils literal"><span class="pre">abgr_layout_t</span></code></li>
 113 <li><code class="docutils literal"><span class="pre">argb_layout_t</span></code></li>
 114 </ul>
 115 <p>As an example, here is how GIL defines the RGBA color space:</p>
 116 <div class="highlight-c++"><div class="highlight"><pre><span class="p">..</span> <span class="n">code</span><span class="o">-</span><span class="n">block</span><span class="o">::</span> <span class="n">cpp</span>
 117 </pre></div>
 118 </div>
 119 <blockquote>
 120 <div>struct red_t {};
 121 struct green_t {};
 122 struct blue_t {};
 123 struct alpha_t {};
 124 rgba_t = using mpl::vector4&lt;red_t, green_t, blue_t, alpha_t&gt;;</div></blockquote>
 125 <p>The ordering of the channels in the color space definition specifies their
 126 semantic order. For example, <code class="docutils literal"><span class="pre">red_t</span></code> is the first semantic channel of
 127 <code class="docutils literal"><span class="pre">rgba_t</span></code>. While there is a unique semantic ordering of the channels in a
 128 color space, channels may vary in their physical ordering in memory</p>
 129 <p>The mapping of channels is specified by <code class="docutils literal"><span class="pre">ChannelMappingConcept</span></code>, which is
 130 an MPL random access sequence of integral types.
 131 A color space and its associated mapping are often used together.</p>
 132 <p>Thus they are grouped in GIL&#8217;s layout:</p>
 133 <div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span>
 134 <span class="o">&lt;</span>
 135     <span class="k">typename</span> <span class="n">ColorSpace</span><span class="p">,</span>
 136     <span class="k">typename</span> <span class="n">ChannelMapping</span> <span class="o">=</span> <span class="n">mpl</span><span class="o">::</span><span class="n">range_c</span><span class="o">&lt;</span><span class="kt">int</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="n">mpl</span><span class="o">::</span><span class="n">size</span><span class="o">&lt;</span><span class="n">ColorSpace</span><span class="o">&gt;::</span><span class="n">value</span><span class="o">&gt;</span>
 137 <span class="o">&gt;</span>
 138 <span class="k">struct</span> <span class="n">layout</span>
 139 <span class="p">{</span>
 140   <span class="k">using</span> <span class="n">color_space_t</span> <span class="o">=</span> <span class="n">ColorSpace</span><span class="p">;</span>
 141   <span class="k">using</span> <span class="n">channel_mapping_t</span> <span class="o">=</span> <span class="n">ChannelMapping</span><span class="p">;</span>
 142 <span class="p">};</span>
 143 </pre></div>
 144 </div>
 145 <p>Here is how to create layouts for the RGBA color space:</p>
 146 <div class="highlight-cpp"><div class="highlight"><pre><span class="k">using</span> <span class="n">rgba_layout_t</span> <span class="o">=</span> <span class="n">layout</span><span class="o">&lt;</span><span class="n">rgba_t</span><span class="o">&gt;</span><span class="p">;</span> <span class="c1">// default ordering is 0,1,2,3...</span>
 147 <span class="k">using</span> <span class="n">bgra_layout_t</span> <span class="o">=</span> <span class="n">layout</span><span class="o">&lt;</span><span class="n">rgba_t</span><span class="p">,</span> <span class="n">mpl</span><span class="o">::</span><span class="n">vector4_c</span><span class="o">&lt;</span><span class="kt">int</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">0</span><span class="p">,</span><span class="mi">3</span><span class="o">&gt;&gt;</span><span class="p">;</span>
 148 <span class="k">using</span> <span class="n">argb_layout_t</span> <span class="o">=</span> <span class="n">layout</span><span class="o">&lt;</span><span class="n">rgba_t</span><span class="p">,</span> <span class="n">mpl</span><span class="o">::</span><span class="n">vector4_c</span><span class="o">&lt;</span><span class="kt">int</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">0</span><span class="o">&gt;&gt;</span><span class="p">;</span>
 149 <span class="k">using</span> <span class="n">abgr_layout_t</span> <span class="o">=</span> <span class="n">layout</span><span class="o">&lt;</span><span class="n">rgba_t</span><span class="p">,</span> <span class="n">mpl</span><span class="o">::</span><span class="n">vector4_c</span><span class="o">&lt;</span><span class="kt">int</span><span class="p">,</span><span class="mi">3</span><span class="p">,</span><span class="mi">2</span><span class="p">,</span><span class="mi">1</span><span class="p">,</span><span class="mi">0</span><span class="o">&gt;&gt;</span><span class="p">;</span>
 150 </pre></div>
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