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<div class="section" id="channel">
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<h1>Channel</h1>
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<div class="contents local topic" id="contents">
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<ul class="simple">
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<li><a class="reference internal" href="#overview" id="id1">Overview</a></li>
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<li><a class="reference internal" href="#models" id="id2">Models</a></li>
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<li><a class="reference internal" href="#algorithms" id="id3">Algorithms</a></li>
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</ul>
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</div>
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<div class="section" id="overview">
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<h2><a class="toc-backref" href="#id1">Overview</a></h2>
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<p>A channel indicates the intensity of a color component (for example, the red
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channel in an RGB pixel). Typical channel operations are getting, comparing
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and setting the channel values. Channels have associated minimum and maximum
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value. GIL channels model the following concept:</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">ChannelConcept</span><span class="o"><</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span> <span class="o">:</span> <span class="n">EqualityComparable</span><span class="o"><</span><span class="n">T</span><span class="o">></span>
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<span class="p">{</span>
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<span class="k">typename</span> <span class="n">value_type</span> <span class="o">=</span> <span class="n">T</span><span class="p">;</span> <span class="c1">// use channel_traits<T>::value_type to access it</span>
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<span class="n">where</span> <span class="n">ChannelValueConcept</span><span class="o"><</span><span class="n">value_type</span><span class="o">></span><span class="p">;</span>
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<span class="k">typename</span> <span class="n">reference</span> <span class="o">=</span> <span class="n">T</span><span class="o">&</span><span class="p">;</span> <span class="c1">// use channel_traits<T>::reference to access it</span>
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<span class="k">typename</span> <span class="n">pointer</span> <span class="o">=</span> <span class="n">T</span><span class="o">*</span><span class="p">;</span> <span class="c1">// use channel_traits<T>::pointer to access it</span>
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<span class="k">typename</span> <span class="n">const_reference</span> <span class="o">=</span> <span class="k">const</span> <span class="n">T</span><span class="o">&</span><span class="p">;</span> <span class="c1">// use channel_traits<T>::const_reference to access it</span>
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<span class="k">typename</span> <span class="n">const_pointer</span> <span class="o">=</span> <span class="k">const</span> <span class="n">T</span><span class="o">*</span><span class="p">;</span> <span class="c1">// use channel_traits<T>::const_pointer to access it</span>
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<span class="k">static</span> <span class="k">const</span> <span class="kt">bool</span> <span class="n">is_mutable</span><span class="p">;</span> <span class="c1">// use channel_traits<T>::is_mutable to access it</span>
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<span class="k">static</span> <span class="n">T</span> <span class="nf">min_value</span><span class="p">();</span> <span class="c1">// use channel_traits<T>::min_value to access it</span>
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<span class="k">static</span> <span class="n">T</span> <span class="nf">max_value</span><span class="p">();</span> <span class="c1">// use channel_traits<T>::min_value to access it</span>
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<span class="p">};</span>
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<span class="n">concept</span> <span class="n">MutableChannelConcept</span><span class="o"><</span><span class="n">ChannelConcept</span> <span class="n">T</span><span class="o">></span> <span class="o">:</span> <span class="n">Swappable</span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="p">,</span> <span class="n">Assignable</span><span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="p">{};</span>
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<span class="n">concept</span> <span class="n">ChannelValueConcept</span><span class="o"><</span><span class="n">ChannelConcept</span> <span class="n">T</span><span class="o">></span> <span class="o">:</span> <span class="n">Regular</span><span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="p">{};</span>
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</pre></div>
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</div>
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<p>GIL allows built-in integral and floating point types to be channels.
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Therefore the associated types and range information are defined in
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<code class="docutils literal"><span class="pre">channel_traits</span></code> with the following default implementation:</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span>
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<span class="k">struct</span> <span class="n">channel_traits</span>
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<span class="p">{</span>
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<span class="k">typedef</span> <span class="n">T</span> <span class="n">value_type</span><span class="p">;</span>
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<span class="k">typedef</span> <span class="n">T</span><span class="o">&</span> <span class="n">reference</span><span class="p">;</span>
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<span class="k">typedef</span> <span class="n">T</span><span class="o">*</span> <span class="n">pointer</span><span class="p">;</span>
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<span class="k">typedef</span> <span class="n">T</span><span class="o">&</span> <span class="k">const</span> <span class="n">const_reference</span><span class="p">;</span>
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<span class="k">typedef</span> <span class="n">T</span><span class="o">*</span> <span class="k">const</span> <span class="n">const_pointer</span><span class="p">;</span>
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<span class="k">static</span> <span class="n">value_type</span> <span class="nf">min_value</span><span class="p">()</span> <span class="p">{</span> <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">numeric_limits</span><span class="o"><</span><span class="n">T</span><span class="o">>::</span><span class="n">min</span><span class="p">();</span> <span class="p">}</span>
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<span class="k">static</span> <span class="n">value_type</span> <span class="nf">max_value</span><span class="p">()</span> <span class="p">{</span> <span class="k">return</span> <span class="n">std</span><span class="o">::</span><span class="n">numeric_limits</span><span class="o"><</span><span class="n">T</span><span class="o">>::</span><span class="n">max</span><span class="p">();</span> <span class="p">}</span>
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<span class="p">};</span>
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</pre></div>
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</div>
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<p>Two channel types are <em>compatible</em> if they have the same value type:</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">ChannelsCompatibleConcept</span><span class="o"><</span><span class="n">ChannelConcept</span> <span class="n">T1</span><span class="p">,</span> <span class="n">ChannelConcept</span> <span class="n">T2</span><span class="o">></span>
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<span class="p">{</span>
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<span class="n">where</span> <span class="n">SameType</span><span class="o"><</span><span class="n">T1</span><span class="o">::</span><span class="n">value_type</span><span class="p">,</span> <span class="n">T2</span><span class="o">::</span><span class="n">value_type</span><span class="o">></span><span class="p">;</span>
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<span class="p">};</span>
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</pre></div>
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</div>
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<p>A channel may be <em>convertible</em> to another channel:</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o"><</span><span class="n">ChannelConcept</span> <span class="n">Src</span><span class="p">,</span> <span class="n">ChannelValueConcept</span> <span class="n">Dst</span><span class="o">></span>
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<span class="n">concept</span> <span class="n">ChannelConvertibleConcept</span>
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<span class="p">{</span>
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<span class="n">Dst</span> <span class="n">channel_convert</span><span class="p">(</span><span class="n">Src</span><span class="p">);</span>
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<span class="p">};</span>
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</pre></div>
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</div>
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<p>Note that <code class="docutils literal"><span class="pre">ChannelConcept</span></code> and <code class="docutils literal"><span class="pre">MutableChannelConcept</span></code> do not require a
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default constructor. Channels that also support default construction (and thus
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are regular types) model <code class="docutils literal"><span class="pre">ChannelValueConcept</span></code>.
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To understand the motivation for this distinction, consider a 16-bit RGB pixel
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in a “565” bit pattern. Its channels correspond to bit ranges. To support such
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channels, we need to create a custom proxy class corresponding to a reference
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to a sub-byte channel.
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Such a proxy reference class models only <code class="docutils literal"><span class="pre">ChannelConcept</span></code>, because, similar
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to native C++ references, it may not have a default constructor.</p>
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<p>Note also that algorithms may impose additional requirements on channels,
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such as support for arithmetic operations.</p>
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<div class="admonition seealso">
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<p class="first admonition-title">See also</p>
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<ul class="last simple">
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<li><a class="reference external" href="reference/structboost_1_1gil_1_1_channel_concept.html">ChannelConcept<T></a></li>
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<li><a class="reference external" href="reference/structboost_1_1gil_1_1_channel_value_concept.html">ChannelValueConcept<T></a></li>
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<li><a class="reference external" href="reference/structboost_1_1gil_1_1_mutable_channel_concept.html">MutableChannelConcept<T></a></li>
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<li><a class="reference external" href="reference/structboost_1_1gil_1_1_channels_compatible_concept.html">ChannelsCompatibleConcept<T1,T2></a></li>
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<li><a class="reference external" href="reference/structboost_1_1gil_1_1_channel_convertible_concept.html">ChannelConvertibleConcept<SrcChannel,DstChannel></a></li>
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</ul>
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</div>
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</div>
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<div class="section" id="models">
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<h2><a class="toc-backref" href="#id2">Models</a></h2>
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<p>All C++11 fundamental integer and float point types are valid channels.</p>
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<p>The minimum and maximum values of a channel modeled by a built-in type
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correspond to the minimum and maximum physical range of the built-in type, as
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specified by its <code class="docutils literal"><span class="pre">std::numeric_limits</span></code>. Sometimes the physical range is not
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appropriate. GIL provides <code class="docutils literal"><span class="pre">scoped_channel_value</span></code>, a model for a channel
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adapter that allows for specifying a custom range.
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We use it to define a <code class="docutils literal"><span class="pre">[0..1]</span></code> floating point channel type as follows:</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="k">struct</span> <span class="n">float_zero</span> <span class="p">{</span> <span class="k">static</span> <span class="kt">float</span> <span class="n">apply</span><span class="p">()</span> <span class="p">{</span> <span class="k">return</span> <span class="mf">0.0f</span><span class="p">;</span> <span class="p">}</span> <span class="p">};</span>
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<span class="k">struct</span> <span class="n">float_one</span> <span class="p">{</span> <span class="k">static</span> <span class="kt">float</span> <span class="n">apply</span><span class="p">()</span> <span class="p">{</span> <span class="k">return</span> <span class="mf">1.0f</span><span class="p">;</span> <span class="p">}</span> <span class="p">};</span>
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<span class="k">typedef</span> <span class="n">scoped_channel_value</span><span class="o"><</span><span class="kt">float</span><span class="p">,</span><span class="n">float_zero</span><span class="p">,</span><span class="n">float_one</span><span class="o">></span> <span class="n">bits32f</span><span class="p">;</span>
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</pre></div>
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</div>
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<p>GIL also provides models for channels corresponding to ranges of bits:</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="c1">// Value of a channel defined over NumBits bits. Models ChannelValueConcept</span>
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<span class="k">template</span> <span class="o"><</span><span class="kt">int</span> <span class="n">NumBits</span><span class="o">></span> <span class="k">class</span> <span class="nc">packed_channel_value</span><span class="p">;</span>
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<span class="c1">// Reference to a channel defined over NumBits bits. Models ChannelConcept</span>
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<span class="k">template</span> <span class="o"><</span><span class="kt">int</span> <span class="n">FirstBit</span><span class="p">,</span>
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<span class="kt">int</span> <span class="n">NumBits</span><span class="p">,</span> <span class="c1">// Defines the sequence of bits in the data value that contain the channel</span>
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<span class="kt">bool</span> <span class="n">Mutable</span><span class="o">></span> <span class="c1">// true if the reference is mutable</span>
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<span class="k">class</span> <span class="nc">packed_channel_reference</span><span class="p">;</span>
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<span class="c1">// Reference to a channel defined over NumBits bits. Its FirstBit is a run-time parameter. Models ChannelConcept</span>
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<span class="k">template</span> <span class="o"><</span><span class="kt">int</span> <span class="n">NumBits</span><span class="p">,</span> <span class="c1">// Defines the sequence of bits in the data value that contain the channel</span>
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<span class="kt">bool</span> <span class="n">Mutable</span><span class="o">></span> <span class="c1">// true if the reference is mutable</span>
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<span class="k">class</span> <span class="nc">packed_dynamic_channel_reference</span><span class="p">;</span>
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</pre></div>
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</div>
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<p>Note that there are two models of a reference proxy which differ based on
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whether the offset of the channel range is specified as a template or a
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run-time parameter. The first model is faster and more compact while the
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second model is more flexible. For example, the second model allows us to
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construct an iterator over bit range channels.</p>
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</div>
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<div class="section" id="algorithms">
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<h2><a class="toc-backref" href="#id3">Algorithms</a></h2>
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<p>Here is how to construct the three channels of a 16-bit “565” pixel and set
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them to their maximum value:</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="k">using</span> <span class="n">channel16_0_5_reference_t</span> <span class="o">=</span> <span class="n">packed_channel_reference</span><span class="o"><</span><span class="mi">0</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="nb">true</span><span class="o">></span><span class="p">;</span>
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<span class="k">using</span> <span class="n">channel16_5_6_reference_t</span> <span class="o">=</span> <span class="n">packed_channel_reference</span><span class="o"><</span><span class="mi">5</span><span class="p">,</span> <span class="mi">6</span><span class="p">,</span> <span class="nb">true</span><span class="o">></span><span class="p">;</span>
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<span class="k">using</span> <span class="n">channel16_11_5_reference_t</span> <span class="o">=</span> <span class="n">packed_channel_reference</span><span class="o"><</span><span class="mi">11</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="nb">true</span><span class="o">></span><span class="p">;</span>
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<span class="n">std</span><span class="o">::</span><span class="kt">uint16_t</span> <span class="n">data</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span>
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<span class="n">channel16_0_5_reference_t</span> <span class="nf">channel1</span><span class="p">(</span><span class="o">&</span><span class="n">data</span><span class="p">);</span>
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<span class="n">channel16_5_6_reference_t</span> <span class="nf">channel2</span><span class="p">(</span><span class="o">&</span><span class="n">data</span><span class="p">);</span>
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<span class="n">channel16_11_5_reference_t</span> <span class="nf">channel3</span><span class="p">(</span><span class="o">&</span><span class="n">data</span><span class="p">);</span>
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<span class="n">channel1</span> <span class="o">=</span> <span class="n">channel_traits</span><span class="o"><</span><span class="n">channel16_0_5_reference_t</span><span class="o">>::</span><span class="n">max_value</span><span class="p">();</span>
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<span class="n">channel2</span> <span class="o">=</span> <span class="n">channel_traits</span><span class="o"><</span><span class="n">channel16_5_6_reference_t</span><span class="o">>::</span><span class="n">max_value</span><span class="p">();</span>
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<span class="n">channel3</span> <span class="o">=</span> <span class="n">channel_traits</span><span class="o"><</span><span class="n">channel16_11_5_reference_t</span><span class="o">>::</span><span class="n">max_value</span><span class="p">();</span>
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<span class="n">assert</span><span class="p">(</span><span class="n">data</span> <span class="o">==</span> <span class="mi">65535</span><span class="p">);</span>
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</pre></div>
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</div>
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<p>Assignment, equality comparison and copy construction are defined only between
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compatible channels:</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="n">packed_channel_value</span><span class="o"><</span><span class="mi">5</span><span class="o">></span> <span class="n">channel_6bit</span> <span class="o">=</span> <span class="n">channel1</span><span class="p">;</span>
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<span class="n">channel_6bit</span> <span class="o">=</span> <span class="n">channel3</span><span class="p">;</span>
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<span class="c1">// compile error: Assignment between incompatible channels</span>
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<span class="c1">//channel_6bit = channel2;</span>
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</pre></div>
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</div>
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<p>All channel models provided by GIL are pairwise convertible:</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="n">channel1</span> <span class="o">=</span> <span class="n">channel_traits</span><span class="o"><</span><span class="n">channel16_0_5_reference_t</span><span class="o">>::</span><span class="n">max_value</span><span class="p">();</span>
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<span class="n">assert</span><span class="p">(</span><span class="n">channel1</span> <span class="o">==</span> <span class="mi">31</span><span class="p">);</span>
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<span class="n">bits16</span> <span class="n">chan16</span> <span class="o">=</span> <span class="n">channel_convert</span><span class="o"><</span><span class="n">bits16</span><span class="o">></span><span class="p">(</span><span class="n">channel1</span><span class="p">);</span>
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<span class="n">assert</span><span class="p">(</span><span class="n">chan16</span> <span class="o">==</span> <span class="mi">65535</span><span class="p">);</span>
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</pre></div>
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</div>
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<p>Channel conversion is a lossy operation. GIL’s channel conversion is a linear
|
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transformation between the ranges of the source and destination channel.
|
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It maps precisely the minimum to the minimum and the maximum to the maximum.
|
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(For example, to convert from uint8_t to uint16_t GIL does not do a bit shift
|
|
because it will not properly match the maximum values. Instead GIL multiplies
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|
the source by 257).</p>
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<p>All channel models that GIL provides are convertible from/to an integral or
|
|
floating point type. Thus they support arithmetic operations. Here are the
|
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channel-level algorithms that GIL provides:</p>
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<div class="highlight-cpp"><div class="highlight"><pre><span class="c1">// Converts a source channel value into a destination channel.</span>
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<span class="c1">// Linearly maps the value of the source into the range of the destination.</span>
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<span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">DstChannel</span><span class="p">,</span> <span class="k">typename</span> <span class="n">SrcChannel</span><span class="o">></span>
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<span class="k">typename</span> <span class="n">channel_traits</span><span class="o"><</span><span class="n">DstChannel</span><span class="o">>::</span><span class="n">value_type</span> <span class="n">channel_convert</span><span class="p">(</span><span class="n">SrcChannel</span> <span class="n">src</span><span class="p">);</span>
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<span class="c1">// returns max_value - x + min_value</span>
|
|
<span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">Channel</span><span class="o">></span>
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<span class="k">typename</span> <span class="n">channel_traits</span><span class="o"><</span><span class="n">Channel</span><span class="o">>::</span><span class="n">value_type</span> <span class="n">channel_invert</span><span class="p">(</span><span class="n">Channel</span> <span class="n">x</span><span class="p">);</span>
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<span class="c1">// returns a * b / max_value</span>
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<span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">Channel</span><span class="o">></span>
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<span class="k">typename</span> <span class="n">channel_traits</span><span class="o"><</span><span class="n">Channel</span><span class="o">>::</span><span class="n">value_type</span> <span class="n">channel_multiply</span><span class="p">(</span><span class="n">Channel</span> <span class="n">a</span><span class="p">,</span> <span class="n">Channel</span> <span class="n">b</span><span class="p">);</span>
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</pre></div>
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