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  68   <div class="section" id="dynamic-images-and-image-views">
  69 <h1>Dynamic images and image views</h1>
  70 <p>The GIL extension called <code class="docutils literal"><span class="pre">dynamic_image</span></code> allows for images, image views
  71 or any GIL constructs to have their parameters defined at run time.</p>
  72 <p>The color space, channel depth, channel ordering, and interleaved/planar
  73 structure of an image are defined by the type of its template argument, which
  74 makes them compile-time bound. Often some of these parameters are available
  75 only at run time. Consider, for example, writing a module that opens the image
  76 at a given file path, rotates it and saves it back in its original color space
  77 and channel depth. How can we possibly write this using our generic image?
  78 What type is the image loading code supposed to return?</p>
  79 <p>Here is an example:</p>
  80 <div class="highlight-cpp"><div class="highlight"><pre><span class="cp">#include</span> <span class="cpf">&lt;boost/gil/extension/dynamic_image/dynamic_image_all.hpp&gt;</span><span class="cp"></span>
  81 <span class="k">using</span> <span class="k">namespace</span> <span class="n">boost</span><span class="p">;</span>
  82
  83 <span class="cp">#define ASSERT_SAME(A,B) static_assert(is_same&lt; A,B &gt;::value, &quot;&quot;)</span>
  84
  85 <span class="c1">// Define the set of allowed images</span>
  86 <span class="k">typedef</span> <span class="n">mpl</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="n">rgb8_image_t</span><span class="p">,</span> <span class="n">cmyk16_planar_image_t</span><span class="o">&gt;</span> <span class="n">my_images_t</span><span class="p">;</span>
  87
  88 <span class="c1">// Create any_image class (or any_image_view) class</span>
  89 <span class="k">typedef</span> <span class="n">any_image</span><span class="o">&lt;</span><span class="n">my_images_t</span><span class="o">&gt;</span> <span class="n">my_any_image_t</span><span class="p">;</span>
  90
  91 <span class="c1">// Associated view types are available (equivalent to the ones in image_t)</span>
  92 <span class="k">typedef</span> <span class="n">any_image_view</span><span class="o">&lt;</span><span class="n">mpl</span><span class="o">::</span><span class="n">vector2</span><span class="o">&lt;</span><span class="n">rgb8_view_t</span><span class="p">,</span>  <span class="n">cmyk16_planar_view_t</span> <span class="o">&gt;</span> <span class="o">&gt;</span> <span class="n">AV</span><span class="p">;</span>
  93 <span class="n">ASSERT_SAME</span><span class="p">(</span><span class="n">my_any_image_t</span><span class="o">::</span><span class="n">view_t</span><span class="p">,</span> <span class="n">AV</span><span class="p">);</span>
  94
  95 <span class="k">typedef</span> <span class="n">any_image_view</span><span class="o">&lt;</span><span class="n">mpl</span><span class="o">::</span><span class="n">vector2</span><span class="o">&lt;</span><span class="n">rgb8c_view_t</span><span class="p">,</span> <span class="n">cmyk16c_planar_view_t</span><span class="o">&gt;</span> <span class="o">&gt;</span> <span class="n">CAV</span><span class="p">;</span>
  96 <span class="n">ASSERT_SAME</span><span class="p">(</span><span class="n">my_any_image_t</span><span class="o">::</span><span class="n">const_view_t</span><span class="p">,</span> <span class="n">CAV</span><span class="p">);</span>
  97 <span class="n">ASSERT_SAME</span><span class="p">(</span><span class="n">my_any_image_t</span><span class="o">::</span><span class="n">const_view_t</span><span class="p">,</span> <span class="n">my_any_image_t</span><span class="o">::</span><span class="n">view_t</span><span class="o">::</span><span class="n">const_t</span><span class="p">);</span>
  98
  99 <span class="k">typedef</span> <span class="n">any_image_view</span><span class="o">&lt;</span><span class="n">mpl</span><span class="o">::</span><span class="n">vector2</span><span class="o">&lt;</span><span class="n">rgb8_step_view_t</span><span class="p">,</span> <span class="n">cmyk16_planar_step_view_t</span><span class="o">&gt;</span> <span class="o">&gt;</span> <span class="n">SAV</span><span class="p">;</span>
 100 <span class="n">ASSERT_SAME</span><span class="p">(</span><span class="k">typename</span> <span class="n">dynamic_x_step_type</span><span class="o">&lt;</span><span class="n">my_any_image_t</span><span class="o">::</span><span class="n">view_t</span><span class="o">&gt;::</span><span class="n">type</span><span class="p">,</span> <span class="n">SAV</span><span class="p">);</span>
 101
 102 <span class="c1">// Assign it a concrete image at run time:</span>
 103 <span class="n">my_any_image_t</span> <span class="n">myImg</span> <span class="o">=</span> <span class="n">my_any_image_t</span><span class="p">(</span><span class="n">rgb8_image_t</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span><span class="mi">100</span><span class="p">));</span>
 104
 105 <span class="c1">// Change it to another at run time. The previous image gets destroyed</span>
 106 <span class="n">myImg</span> <span class="o">=</span> <span class="n">cmyk16_planar_image_t</span><span class="p">(</span><span class="mi">200</span><span class="p">,</span><span class="mi">100</span><span class="p">);</span>
 107
 108 <span class="c1">// Assigning to an image not in the allowed set throws an exception</span>
 109 <span class="n">myImg</span> <span class="o">=</span> <span class="n">gray8_image_t</span><span class="p">();</span>        <span class="c1">// will throw std::bad_cast</span>
 110 </pre></div>
 111 </div>
 112 <p>The <code class="docutils literal"><span class="pre">any_image</span></code> and <code class="docutils literal"><span class="pre">any_image_view</span></code> subclass from GIL <code class="docutils literal"><span class="pre">variant</span></code> class,
 113 which breaks down the instantiated type into a non-templated underlying base
 114 type and a unique instantiation type identifier. The underlying base instance
 115 is represented as a block of bytes.
 116 The block is large enough to hold the largest of the specified types.</p>
 117 <p>GIL variant is similar to <code class="docutils literal"><span class="pre">boost::variant</span></code> in spirit (hence we borrow the
 118 name from there) but it differs in several ways from the current boost
 119 implementation. Perhaps the biggest difference is that GIL variant always
 120 takes a single argument, which is a model of MPL Random Access Sequence
 121 enumerating the allowed types. Having a single interface allows GIL variant
 122 to be used easier in generic code. Synopsis:</p>
 123 <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">Types</span><span class="o">&gt;</span>    <span class="c1">// models MPL Random Access Container</span>
 124 <span class="k">class</span> <span class="nc">variant</span>
 125 <span class="p">{</span>
 126   <span class="p">...</span>         <span class="n">_bits</span><span class="p">;</span>
 127   <span class="n">std</span><span class="o">::</span><span class="kt">size_t</span> <span class="n">_index</span><span class="p">;</span>
 128 <span class="k">public</span><span class="o">:</span>
 129   <span class="k">typedef</span> <span class="n">Types</span> <span class="n">types_t</span><span class="p">;</span>
 130
 131   <span class="n">variant</span><span class="p">();</span>
 132   <span class="n">variant</span><span class="p">(</span><span class="k">const</span> <span class="n">variant</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">);</span>
 133   <span class="k">virtual</span> <span class="o">~</span><span class="n">variant</span><span class="p">();</span>
 134
 135   <span class="n">variant</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">variant</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">);</span>
 136   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">TS</span><span class="o">&gt;</span> <span class="k">friend</span> <span class="kt">bool</span> <span class="k">operator</span><span class="o">==</span><span class="p">(</span><span class="k">const</span> <span class="n">variant</span><span class="o">&lt;</span><span class="n">TS</span><span class="o">&gt;&amp;</span> <span class="n">x</span><span class="p">,</span> <span class="k">const</span> <span class="n">variant</span><span class="o">&lt;</span><span class="n">TS</span><span class="o">&gt;&amp;</span> <span class="n">y</span><span class="p">);</span>
 137   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">TS</span><span class="o">&gt;</span> <span class="k">friend</span> <span class="kt">bool</span> <span class="k">operator</span><span class="o">!=</span><span class="p">(</span><span class="k">const</span> <span class="n">variant</span><span class="o">&lt;</span><span class="n">TS</span><span class="o">&gt;&amp;</span> <span class="n">x</span><span class="p">,</span> <span class="k">const</span> <span class="n">variant</span><span class="o">&lt;</span><span class="n">TS</span><span class="o">&gt;&amp;</span> <span class="n">y</span><span class="p">);</span>
 138
 139   <span class="c1">// Construct/assign to type T. Throws std::bad_cast if T is not in Types</span>
 140   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="k">explicit</span> <span class="n">variant</span><span class="p">(</span><span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">obj</span><span class="p">);</span>
 141   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="n">variant</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">obj</span><span class="p">);</span>
 142
 143   <span class="c1">// Construct/assign by swapping T with its current instance. Only possible if they are swappable</span>
 144   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="k">explicit</span> <span class="n">variant</span><span class="p">(</span><span class="n">T</span><span class="o">&amp;</span> <span class="n">obj</span><span class="p">,</span> <span class="kt">bool</span> <span class="n">do_swap</span><span class="p">);</span>
 145   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="kt">void</span> <span class="n">move_in</span><span class="p">(</span><span class="n">T</span><span class="o">&amp;</span> <span class="n">obj</span><span class="p">);</span>
 146
 147   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="k">static</span> <span class="kt">bool</span> <span class="n">has_type</span><span class="p">();</span>
 148
 149   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">_dynamic_cast</span><span class="p">()</span> <span class="k">const</span><span class="p">;</span>
 150   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span>       <span class="n">T</span><span class="o">&amp;</span> <span class="n">_dynamic_cast</span><span class="p">();</span>
 151
 152   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="kt">bool</span> <span class="n">current_type_is</span><span class="p">()</span> <span class="k">const</span><span class="p">;</span>
 153 <span class="p">};</span>
 154
 155 <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">UOP</span><span class="p">,</span> <span class="k">typename</span> <span class="n">Types</span><span class="o">&gt;</span>
 156  <span class="n">UOP</span><span class="o">::</span><span class="n">result_type</span> <span class="n">apply_operation</span><span class="p">(</span><span class="n">variant</span><span class="o">&lt;</span><span class="n">Types</span><span class="o">&gt;&amp;</span> <span class="n">v</span><span class="p">,</span> <span class="n">UOP</span> <span class="n">op</span><span class="p">);</span>
 157 <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">UOP</span><span class="p">,</span> <span class="k">typename</span> <span class="n">Types</span><span class="o">&gt;</span>
 158  <span class="n">UOP</span><span class="o">::</span><span class="n">result_type</span> <span class="n">apply_operation</span><span class="p">(</span><span class="k">const</span> <span class="n">variant</span><span class="o">&lt;</span><span class="n">Types</span><span class="o">&gt;&amp;</span> <span class="n">v</span><span class="p">,</span> <span class="n">UOP</span> <span class="n">op</span><span class="p">);</span>
 159
 160 <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">BOP</span><span class="p">,</span> <span class="k">typename</span> <span class="n">Types1</span><span class="p">,</span> <span class="k">typename</span> <span class="n">Types2</span><span class="o">&gt;</span>
 161  <span class="n">BOP</span><span class="o">::</span><span class="n">result_type</span> <span class="n">apply_operation</span><span class="p">(</span>      <span class="n">variant</span><span class="o">&lt;</span><span class="n">Types1</span><span class="o">&gt;&amp;</span> <span class="n">v1</span><span class="p">,</span>       <span class="n">variant</span><span class="o">&lt;</span><span class="n">Types2</span><span class="o">&gt;&amp;</span> <span class="n">v2</span><span class="p">,</span> <span class="n">UOP</span> <span class="n">op</span><span class="p">);</span>
 162
 163 <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">BOP</span><span class="p">,</span> <span class="k">typename</span> <span class="n">Types1</span><span class="p">,</span> <span class="k">typename</span> <span class="n">Types2</span><span class="o">&gt;</span>
 164  <span class="n">BOP</span><span class="o">::</span><span class="n">result_type</span> <span class="n">apply_operation</span><span class="p">(</span><span class="k">const</span> <span class="n">variant</span><span class="o">&lt;</span><span class="n">Types1</span><span class="o">&gt;&amp;</span> <span class="n">v1</span><span class="p">,</span>       <span class="n">variant</span><span class="o">&lt;</span><span class="n">Types2</span><span class="o">&gt;&amp;</span> <span class="n">v2</span><span class="p">,</span> <span class="n">UOP</span> <span class="n">op</span><span class="p">);</span>
 165
 166 <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">BOP</span><span class="p">,</span> <span class="k">typename</span> <span class="n">Types1</span><span class="p">,</span> <span class="k">typename</span> <span class="n">Types2</span><span class="o">&gt;</span>
 167  <span class="n">BOP</span><span class="o">::</span><span class="n">result_type</span> <span class="n">apply_operation</span><span class="p">(</span><span class="k">const</span> <span class="n">variant</span><span class="o">&lt;</span><span class="n">Types1</span><span class="o">&gt;&amp;</span> <span class="n">v1</span><span class="p">,</span> <span class="k">const</span> <span class="n">variant</span><span class="o">&lt;</span><span class="n">Types2</span><span class="o">&gt;&amp;</span> <span class="n">v2</span><span class="p">,</span> <span class="n">UOP</span> <span class="n">op</span><span class="p">);</span>
 168 </pre></div>
 169 </div>
 170 <p>GIL <code class="docutils literal"><span class="pre">any_image_view</span></code> and <code class="docutils literal"><span class="pre">any_image</span></code> are subclasses of <code class="docutils literal"><span class="pre">variant</span></code>:</p>
 171 <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">ImageViewTypes</span><span class="o">&gt;</span>
 172 <span class="k">class</span> <span class="nc">any_image_view</span> <span class="o">:</span> <span class="k">public</span> <span class="n">variant</span><span class="o">&lt;</span><span class="n">ImageViewTypes</span><span class="o">&gt;</span>
 173 <span class="p">{</span>
 174 <span class="k">public</span><span class="o">:</span>
 175   <span class="k">typedef</span> <span class="p">...</span> <span class="n">const_t</span><span class="p">;</span> <span class="c1">// immutable equivalent of this</span>
 176   <span class="k">typedef</span> <span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span> <span class="n">x_coord_t</span><span class="p">;</span>
 177   <span class="k">typedef</span> <span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span> <span class="n">y_coord_t</span><span class="p">;</span>
 178   <span class="k">typedef</span> <span class="n">point</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span><span class="o">&gt;</span> <span class="n">point_t</span><span class="p">;</span>
 179
 180   <span class="n">any_image_view</span><span class="p">();</span>
 181   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="k">explicit</span> <span class="n">any_image_view</span><span class="p">(</span><span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">obj</span><span class="p">);</span>
 182   <span class="n">any_image_view</span><span class="p">(</span><span class="k">const</span> <span class="n">any_image_view</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">);</span>
 183
 184   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="n">any_image_view</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">obj</span><span class="p">);</span>
 185   <span class="n">any_image_view</span><span class="o">&amp;</span>                       <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">any_image_view</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">);</span>
 186
 187   <span class="c1">// parameters of the currently instantiated view</span>
 188   <span class="n">std</span><span class="o">::</span><span class="kt">size_t</span> <span class="n">num_channels</span><span class="p">()</span>  <span class="k">const</span><span class="p">;</span>
 189   <span class="n">point_t</span>     <span class="nf">dimensions</span><span class="p">()</span>    <span class="k">const</span><span class="p">;</span>
 190   <span class="n">x_coord_t</span>   <span class="nf">width</span><span class="p">()</span>         <span class="k">const</span><span class="p">;</span>
 191   <span class="n">y_coord_t</span>   <span class="nf">height</span><span class="p">()</span>        <span class="k">const</span><span class="p">;</span>
 192 <span class="p">};</span>
 193
 194 <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">ImageTypes</span><span class="o">&gt;</span>
 195 <span class="k">class</span> <span class="nc">any_image</span> <span class="o">:</span> <span class="k">public</span> <span class="n">variant</span><span class="o">&lt;</span><span class="n">ImageTypes</span><span class="o">&gt;</span>
 196 <span class="p">{</span>
 197   <span class="k">typedef</span> <span class="n">variant</span><span class="o">&lt;</span><span class="n">ImageTypes</span><span class="o">&gt;</span> <span class="n">parent_t</span><span class="p">;</span>
 198 <span class="k">public</span><span class="o">:</span>
 199   <span class="k">typedef</span> <span class="p">...</span> <span class="n">const_view_t</span><span class="p">;</span>
 200   <span class="k">typedef</span> <span class="p">...</span> <span class="n">view_t</span><span class="p">;</span>
 201   <span class="k">typedef</span> <span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span> <span class="n">x_coord_t</span><span class="p">;</span>
 202   <span class="k">typedef</span> <span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span> <span class="n">y_coord_t</span><span class="p">;</span>
 203   <span class="k">typedef</span> <span class="n">point</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span><span class="o">&gt;</span> <span class="n">point_t</span><span class="p">;</span>
 204
 205   <span class="n">any_image</span><span class="p">();</span>
 206   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="k">explicit</span> <span class="n">any_image</span><span class="p">(</span><span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">obj</span><span class="p">);</span>
 207   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="k">explicit</span> <span class="n">any_image</span><span class="p">(</span><span class="n">T</span><span class="o">&amp;</span> <span class="n">obj</span><span class="p">,</span> <span class="kt">bool</span> <span class="n">do_swap</span><span class="p">);</span>
 208   <span class="n">any_image</span><span class="p">(</span><span class="k">const</span> <span class="n">any_image</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">);</span>
 209
 210   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span> <span class="n">any_image</span><span class="o">&amp;</span> <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">T</span><span class="o">&amp;</span> <span class="n">obj</span><span class="p">);</span>
 211   <span class="n">any_image</span><span class="o">&amp;</span>                       <span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span> <span class="n">any_image</span><span class="o">&amp;</span> <span class="n">v</span><span class="p">);</span>
 212
 213   <span class="kt">void</span> <span class="nf">recreate</span><span class="p">(</span><span class="k">const</span> <span class="n">point_t</span><span class="o">&amp;</span> <span class="n">dims</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="n">alignment</span><span class="o">=</span><span class="mi">1</span><span class="p">);</span>
 214   <span class="kt">void</span> <span class="nf">recreate</span><span class="p">(</span><span class="n">x_coord_t</span> <span class="n">width</span><span class="p">,</span> <span class="n">y_coord_t</span> <span class="n">height</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="n">alignment</span><span class="o">=</span><span class="mi">1</span><span class="p">);</span>
 215
 216   <span class="n">std</span><span class="o">::</span><span class="kt">size_t</span> <span class="n">num_channels</span><span class="p">()</span>  <span class="k">const</span><span class="p">;</span>
 217   <span class="n">point_t</span>     <span class="nf">dimensions</span><span class="p">()</span>    <span class="k">const</span><span class="p">;</span>
 218   <span class="n">x_coord_t</span>   <span class="nf">width</span><span class="p">()</span>         <span class="k">const</span><span class="p">;</span>
 219   <span class="n">y_coord_t</span>   <span class="nf">height</span><span class="p">()</span>        <span class="k">const</span><span class="p">;</span>
 220 <span class="p">};</span>
 221 </pre></div>
 222 </div>
 223 <p>Operations are invoked on variants via <code class="docutils literal"><span class="pre">apply_operation</span></code> passing a
 224 function object to perform the operation. The code for every allowed
 225 type in the variant is instantiated and the appropriate instantiation
 226 is selected via a switch statement. Since image view algorithms
 227 typically have time complexity at least linear on the number of
 228 pixels, the single switch statement of image view variant adds
 229 practically no measurable performance overhead compared to templated
 230 image views.</p>
 231 <p>Variants behave like the underlying type. Their copy constructor will
 232 invoke the copy constructor of the underlying instance. Equality
 233 operator will check if the two instances are of the same type and then
 234 invoke their <code class="docutils literal"><span class="pre">operator==</span></code>, etc. The default constructor of a variant
 235 will default-construct the first type. That means that
 236 <code class="docutils literal"><span class="pre">any_image_view</span></code> has shallow default-constructor, copy-constructor,
 237 assignment and equality comparison, whereas <code class="docutils literal"><span class="pre">any_image</span></code> has deep
 238 ones.</p>
 239 <p>It is important to note that even though <code class="docutils literal"><span class="pre">any_image_view</span></code> and
 240 <code class="docutils literal"><span class="pre">any_image</span></code> resemble the static <code class="docutils literal"><span class="pre">image_view</span></code> and <code class="docutils literal"><span class="pre">image</span></code>, they
 241 do not model the full requirements of <code class="docutils literal"><span class="pre">ImageViewConcept</span></code> and
 242 <code class="docutils literal"><span class="pre">ImageConcept</span></code>. In particular they don&#8217;t provide access to the
 243 pixels. There is no &#8220;any_pixel&#8221; or &#8220;any_pixel_iterator&#8221; in GIL. Such
 244 constructs could be provided via the <code class="docutils literal"><span class="pre">variant</span></code> mechanism, but doing
 245 so would result in inefficient algorithms, since the type resolution
 246 would have to be performed per pixel. Image-level algorithms should be
 247 implemented via <code class="docutils literal"><span class="pre">apply_operation</span></code>. That said, many common operations
 248 are shared between the static and dynamic types. In addition, all of
 249 the image view transformations and many STL-like image view algorithms
 250 have overloads operating on <code class="docutils literal"><span class="pre">any_image_view</span></code>, as illustrated with
 251 <code class="docutils literal"><span class="pre">copy_pixels</span></code>:</p>
 252 <div class="highlight-cpp"><div class="highlight"><pre><span class="n">rgb8_view_t</span> <span class="nf">v1</span><span class="p">(...);</span>  <span class="c1">// concrete image view</span>
 253 <span class="n">bgr8_view_t</span> <span class="nf">v2</span><span class="p">(...);</span>  <span class="c1">// concrete image view compatible with v1 and of the same size</span>
 254 <span class="n">any_image_view</span><span class="o">&lt;</span><span class="n">Types</span><span class="o">&gt;</span>  <span class="n">av</span><span class="p">(...);</span>  <span class="c1">// run-time specified image view</span>
 255
 256 <span class="c1">// Copies the pixels from v1 into v2.</span>
 257 <span class="c1">// If the pixels are incompatible triggers compile error</span>
 258 <span class="n">copy_pixels</span><span class="p">(</span><span class="n">v1</span><span class="p">,</span><span class="n">v2</span><span class="p">);</span>
 259
 260 <span class="c1">// The source or destination (or both) may be run-time instantiated.</span>
 261 <span class="c1">// If they happen to be incompatible, throws std::bad_cast</span>
 262 <span class="n">copy_pixels</span><span class="p">(</span><span class="n">v1</span><span class="p">,</span> <span class="n">av</span><span class="p">);</span>
 263 <span class="n">copy_pixels</span><span class="p">(</span><span class="n">av</span><span class="p">,</span> <span class="n">v2</span><span class="p">);</span>
 264 <span class="n">copy_pixels</span><span class="p">(</span><span class="n">av</span><span class="p">,</span> <span class="n">av</span><span class="p">);</span>
 265 </pre></div>
 266 </div>
 267 <p>By having algorithm overloads supporting dynamic constructs, we create
 268 a base upon which it is possible to write algorithms that can work
 269 with either compile-time or runtime images or views. The following
 270 code, for example, uses the GIL I/O extension to turn an image on disk
 271 upside down:</p>
 272 <div class="highlight-cpp"><div class="highlight"><pre><span class="cp">#include</span> <span class="cpf">&lt;boost\gil\extension\io\jpeg_dynamic_io.hpp&gt;</span><span class="cp"></span>
 273
 274 <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Image</span><span class="o">&gt;</span>    <span class="c1">// Could be rgb8_image_t or any_image&lt;...&gt;</span>
 275 <span class="kt">void</span> <span class="n">save_180rot</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&amp;</span> <span class="n">file_name</span><span class="p">)</span>
 276 <span class="p">{</span>
 277   <span class="n">Image</span> <span class="n">img</span><span class="p">;</span>
 278   <span class="n">jpeg_read_image</span><span class="p">(</span><span class="n">file_name</span><span class="p">,</span> <span class="n">img</span><span class="p">);</span>
 279   <span class="n">jpeg_write_view</span><span class="p">(</span><span class="n">file_name</span><span class="p">,</span> <span class="n">rotated180_view</span><span class="p">(</span><span class="n">view</span><span class="p">(</span><span class="n">img</span><span class="p">)));</span>
 280 <span class="p">}</span>
 281 </pre></div>
 282 </div>
 283 <p>It can be instantiated with either a compile-time or a runtime image
 284 because all functions it uses have overloads taking runtime
 285 constructs. For example, here is how <code class="docutils literal"><span class="pre">rotated180_view</span></code> is
 286 implemented:</p>
 287 <div class="highlight-cpp"><div class="highlight"><pre><span class="c1">// implementation using templated view</span>
 288 <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">View</span><span class="o">&gt;</span>
 289 <span class="k">typename</span> <span class="n">dynamic_xy_step_type</span><span class="o">&lt;</span><span class="n">View</span><span class="o">&gt;::</span><span class="n">type</span> <span class="n">rotated180_view</span><span class="p">(</span><span class="k">const</span> <span class="n">View</span><span class="o">&amp;</span> <span class="n">src</span><span class="p">)</span> <span class="p">{</span> <span class="p">...</span> <span class="p">}</span>
 290
 291 <span class="k">namespace</span> <span class="n">detail</span>
 292 <span class="p">{</span>
 293   <span class="c1">// the function, wrapped inside a function object</span>
 294   <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Result</span><span class="o">&gt;</span> <span class="k">struct</span> <span class="n">rotated180_view_fn</span>
 295   <span class="p">{</span>
 296       <span class="k">typedef</span> <span class="n">Result</span> <span class="n">result_type</span><span class="p">;</span>
 297       <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">View</span><span class="o">&gt;</span> <span class="n">result_type</span> <span class="k">operator</span><span class="p">()(</span><span class="k">const</span> <span class="n">View</span><span class="o">&amp;</span> <span class="n">src</span><span class="p">)</span> <span class="k">const</span>
 298 <span class="p">{</span>
 299           <span class="k">return</span> <span class="n">result_type</span><span class="p">(</span><span class="n">rotated180_view</span><span class="p">(</span><span class="n">src</span><span class="p">));</span>
 300       <span class="p">}</span>
 301   <span class="p">};</span>
 302 <span class="p">}</span>
 303
 304 <span class="c1">// overloading of the function using variant. Takes and returns run-time bound view.</span>
 305 <span class="c1">// The returned view has a dynamic step</span>
 306 <span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">ViewTypes</span><span class="o">&gt;</span> <span class="kr">inline</span> <span class="c1">// Models MPL Random Access Container of models of ImageViewConcept</span>
 307 <span class="k">typename</span> <span class="n">dynamic_xy_step_type</span><span class="o">&lt;</span><span class="n">any_image_view</span><span class="o">&lt;</span><span class="n">ViewTypes</span><span class="o">&gt;</span> <span class="o">&gt;::</span><span class="n">type</span> <span class="n">rotated180_view</span><span class="p">(</span><span class="k">const</span> <span class="n">any_image_view</span><span class="o">&lt;</span><span class="n">ViewTypes</span><span class="o">&gt;&amp;</span> <span class="n">src</span><span class="p">)</span>
 308 <span class="p">{</span>
 309   <span class="k">return</span> <span class="n">apply_operation</span><span class="p">(</span><span class="n">src</span><span class="p">,</span><span class="n">detail</span><span class="o">::</span><span class="n">rotated180_view_fn</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">dynamic_xy_step_type</span><span class="o">&lt;</span><span class="n">any_image_view</span><span class="o">&lt;</span><span class="n">ViewTypes</span><span class="o">&gt;</span> <span class="o">&gt;::</span><span class="n">type</span><span class="o">&gt;</span><span class="p">());</span>
 310 <span class="p">}</span>
 311 </pre></div>
 312 </div>
 313 <p>Variants should be used with caution (especially algorithms that take
 314 more than one variant) because they instantiate the algorithm for
 315 every possible model that the variant can take. This can take a toll
 316 on compile time and executable size. Despite these limitations,
 317 <code class="docutils literal"><span class="pre">variant</span></code> is a powerful technique that allows us to combine the
 318 speed of compile-time resolution with the flexibility of run-time
 319 resolution. It allows us to treat images of different parameters
 320 uniformly as a collection and store them in the same container.</p>
 321 </div>
 322
 323
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