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  <div class="section" id="color-space-and-layout">
<h1>Color Space and Layout</h1>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><a class="reference internal" href="#overview" id="id1">Overview</a></li>
<li><a class="reference internal" href="#models" id="id2">Models</a></li>
</ul>
</div>
<div class="section" id="overview">
<h2><a class="toc-backref" href="#id1">Overview</a></h2>
<p>A color space captures the set and interpretation of channels comprising a
pixel. In Boost.GIL, color space is defined as an MPL random access sequence
containing the types of all elements in the color space.</p>
<p>Two color spaces are considered <em>compatible</em> if they are equal (i.e. have the
same set of colors in the same order).</p>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<ul class="last simple">
<li><a class="reference external" href="reference/structboost_1_1gil_1_1_color_space_concept.html">ColorSpaceConcept&lt;ColorSpace&gt;</a></li>
<li><a class="reference external" href="reference/structboost_1_1gil_1_1_color_spaces_compatible_concept.html">ColorSpacesCompatibleConcept&lt;ColorSpace1,ColorSpace2&gt;</a></li>
<li><a class="reference external" href="reference/structboost_1_1gil_1_1_channel_mapping_concept.html">ChannelMappingConcept&lt;Mapping&gt;</a></li>
</ul>
</div>
</div>
<div class="section" id="models">
<h2><a class="toc-backref" href="#id2">Models</a></h2>
<p>GIL currently provides the following color spaces:</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">gray_t</span></code></li>
<li><code class="docutils literal"><span class="pre">rgb_t</span></code></li>
<li><code class="docutils literal"><span class="pre">rgba_t</span></code></li>
<li><code class="docutils literal"><span class="pre">cmyk_t</span></code></li>
</ul>
<p>It also provides unnamed N-channel color spaces of two to five channels:</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">devicen_t&lt;2&gt;</span></code></li>
<li><code class="docutils literal"><span class="pre">devicen_t&lt;3&gt;</span></code></li>
<li><code class="docutils literal"><span class="pre">devicen_t&lt;4&gt;</span></code></li>
<li><code class="docutils literal"><span class="pre">devicen_t&lt;5&gt;</span></code></li>
</ul>
<p>Besides the standard layouts, it also provides:</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">bgr_layout_t</span></code></li>
<li><code class="docutils literal"><span class="pre">bgra_layout_t</span></code></li>
<li><code class="docutils literal"><span class="pre">abgr_layout_t</span></code></li>
<li><code class="docutils literal"><span class="pre">argb_layout_t</span></code></li>
</ul>
<p>As an example, here is how GIL defines the RGBA color space:</p>
<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>
</pre></div>
</div>
<blockquote>
<div>struct red_t {};
struct green_t {};
struct blue_t {};
struct alpha_t {};
rgba_t = using mpl::vector4&lt;red_t, green_t, blue_t, alpha_t&gt;;</div></blockquote>
<p>The ordering of the channels in the color space definition specifies their
semantic order. For example, <code class="docutils literal"><span class="pre">red_t</span></code> is the first semantic channel of
<code class="docutils literal"><span class="pre">rgba_t</span></code>. While there is a unique semantic ordering of the channels in a
color space, channels may vary in their physical ordering in memory</p>
<p>The mapping of channels is specified by <code class="docutils literal"><span class="pre">ChannelMappingConcept</span></code>, which is
an MPL random access sequence of integral types.
A color space and its associated mapping are often used together.</p>
<p>Thus they are grouped in GIL&#8217;s layout:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span>
<span class="o">&lt;</span>
    <span class="k">typename</span> <span class="n">ColorSpace</span><span class="p">,</span>
    <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>
<span class="o">&gt;</span>
<span class="k">struct</span> <span class="n">layout</span>
<span class="p">{</span>
  <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>
  <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>
<span class="p">};</span>
</pre></div>
</div>
<p>Here is how to create layouts for the RGBA color space:</p>
<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>
<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>
<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>
<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>
</pre></div>
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