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rel="top" title="Boost.GIL documentation" href="../index.html" /> 26 <link rel="up" title="Design Guide" href="index.html" /> 27 <link rel="next" title="Pixel Locator" href="pixel_locator.html" /> 28 <link rel="prev" title="Pixel" href="pixel.html" /> 29 </head> 30 <body> 31 <div class="header"> 32 <table border="0" cellpadding="7" cellspacing="0" width="100%" summary= 33 "header"> 34 <tr> 35 <td valign="top" width="300"> 36 <h3><a href="../index.html"><img 37 alt="C++ Boost" src="../_static/gil.png" border="0"></a></h3> 38 </td> 39 40 <td > 41 <h1 align="center"><a href="../index.html"></a></h1> 42 </td> 43 <td> 44 <div id="searchbox" style="display: none"> 45 <form class="search" action="../search.html" method="get"> 46 <input type="text" name="q" size="18" /> 47 <input type="submit" value="Search" /> 48 <input type="hidden" name="check_keywords" value="yes" /> 49 <input type="hidden" name="area" value="default" /> 50 </form> 51 </div> 52 <script type="text/javascript">$('#searchbox').show(0);</script> 53 </td> 54 </tr> 55 </table> 56 </div> 57 <hr/> 58 <div class="content"> 59 <div class="navbar" style="text-align:right;"> 60 61 62 <a class="prev" title="Pixel" href="pixel.html"><img src="../_static/prev.png" alt="prev"/></a> 63 <a class="up" title="Design Guide" href="index.html"><img src="../_static/up.png" alt="up"/></a> 64 <a class="next" title="Pixel Locator" href="pixel_locator.html"><img src="../_static/next.png" alt="next"/></a> 65 66 </div> 67 68 <div class="section" id="pixel-iterator"> 69<h1>Pixel Iterator</h1> 70<div class="contents local topic" id="contents"> 71<ul class="simple"> 72<li><a class="reference internal" href="#overview" id="id4">Overview</a></li> 73<li><a class="reference internal" href="#fundamental-iterator" id="id5">Fundamental Iterator</a><ul> 74<li><a class="reference internal" href="#models" id="id6">Models</a></li> 75</ul> 76</li> 77<li><a class="reference internal" href="#iterator-adaptor" id="id7">Iterator Adaptor</a><ul> 78<li><a class="reference internal" href="#id1" id="id8">Models</a></li> 79</ul> 80</li> 81<li><a class="reference internal" href="#pixel-dereference-adaptor" id="id9">Pixel Dereference Adaptor</a><ul> 82<li><a class="reference internal" href="#id2" id="id10">Models</a></li> 83</ul> 84</li> 85<li><a class="reference internal" href="#step-iterator" id="id11">Step Iterator</a><ul> 86<li><a class="reference internal" href="#id3" id="id12">Models</a></li> 87</ul> 88</li> 89</ul> 90</div> 91<div class="section" id="overview"> 92<h2><a class="toc-backref" href="#id4">Overview</a></h2> 93<p>Pixel iterators are random traversal iterators whose <code class="docutils literal"><span class="pre">value_type</span> 94<span class="pre">models</span></code> <code class="docutils literal"><span class="pre">PixelValueConcept</span></code>.</p> 95</div> 96<div class="section" id="fundamental-iterator"> 97<h2><a class="toc-backref" href="#id5">Fundamental Iterator</a></h2> 98<p>Pixel iterators provide metafunctions to determine whether they are mutable 99(i.e. whether they allow for modifying the pixel they refer to), to get the 100immutable (read-only) type of the iterator, and to determine whether they are 101plain iterators or adaptors over another pixel iterator:</p> 102<div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">PixelIteratorConcept</span><span class="o"><</span><span class="n">RandomAccessTraversalIteratorConcept</span> <span class="n">Iterator</span><span class="o">></span> 103 <span class="o">:</span> <span class="n">PixelBasedConcept</span><span class="o"><</span><span class="n">Iterator</span><span class="o">></span> 104<span class="p">{</span> 105 <span class="n">where</span> <span class="n">PixelValueConcept</span><span class="o"><</span><span class="n">value_type</span><span class="o">></span><span class="p">;</span> 106 <span class="k">typename</span> <span class="n">const_iterator_type</span><span class="o"><</span><span class="n">It</span><span class="o">>::</span><span class="n">type</span><span class="p">;</span> 107 <span class="n">where</span> <span class="n">PixelIteratorConcept</span><span class="o"><</span><span class="n">const_iterator_type</span><span class="o"><</span><span class="n">It</span><span class="o">>::</span><span class="n">type</span><span class="o">></span><span class="p">;</span> 108 <span class="k">static</span> <span class="k">const</span> <span class="kt">bool</span> <span class="n">iterator_is_mutable</span><span class="o"><</span><span class="n">It</span><span class="o">>::</span><span class="n">value</span><span class="p">;</span> 109 <span class="k">static</span> <span class="k">const</span> <span class="kt">bool</span> <span class="n">is_iterator_adaptor</span><span class="o"><</span><span class="n">It</span><span class="o">>::</span><span class="n">value</span><span class="p">;</span> <span class="c1">// is it an iterator adaptor</span> 110<span class="p">};</span> 111 112<span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">Iterator</span><span class="o">></span> 113<span class="n">concept</span> <span class="nl">MutablePixelIteratorConcept</span> <span class="p">:</span> <span class="n">PixelIteratorConcept</span><span class="o"><</span><span class="n">Iterator</span><span class="o">></span><span class="p">,</span> <span class="n">MutableRandomAccessIteratorConcept</span><span class="o"><</span><span class="n">Iterator</span><span class="o">></span> <span class="p">{};</span> 114</pre></div> 115</div> 116<div class="admonition seealso"> 117<p class="first admonition-title">See also</p> 118<ul class="last simple"> 119<li><a class="reference external" href="reference/group___pixel_iterator_concept_pixel_iterator.html">PixelIteratorConcept<Iterator></a></li> 120<li><a class="reference external" href="reference/structboost_1_1gil_1_1_mutable_pixel_iterator_concept.html">MutablePixelIteratorConcept<Iterator></a></li> 121</ul> 122</div> 123<div class="section" id="models"> 124<h3><a class="toc-backref" href="#id6">Models</a></h3> 125<p>A built-in pointer to pixel, <code class="docutils literal"><span class="pre">pixel<ChannelValue,Layout>*</span></code>, is GIL model for 126pixel iterator over interleaved homogeneous pixels. Similarly, 127<code class="docutils literal"><span class="pre">packed_pixel<PixelData,ChannelRefVec,Layout>*</span></code> is GIL model for an iterator 128over interleaved packed pixels.</p> 129<p>For planar homogeneous pixels, GIL provides the class 130<code class="docutils literal"><span class="pre">planar_pixel_iterator</span></code>, templated over a channel iterator and color space. 131Here is how the standard mutable and read-only planar RGB iterators over 132unsigned char are defined:</p> 133<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">ChannelPtr</span><span class="p">,</span> <span class="k">typename</span> <span class="n">ColorSpace</span><span class="o">></span> 134<span class="k">struct</span> <span class="n">planar_pixel_iterator</span><span class="p">;</span> 135 136<span class="c1">// GIL provided typedefs</span> 137<span class="k">typedef</span> <span class="n">planar_pixel_iterator</span><span class="o"><</span><span class="k">const</span> <span class="n">bits8</span><span class="o">*</span><span class="p">,</span> <span class="n">rgb_t</span><span class="o">></span> <span class="n">rgb8c_planar_ptr_t</span><span class="p">;</span> 138<span class="k">typedef</span> <span class="n">planar_pixel_iterator</span><span class="o"><</span> <span class="n">bits8</span><span class="o">*</span><span class="p">,</span> <span class="n">rgb_t</span><span class="o">></span> <span class="n">rgb8_planar_ptr_t</span><span class="p">;</span> 139</pre></div> 140</div> 141<p><code class="docutils literal"><span class="pre">planar_pixel_iterator</span></code> also models <code class="docutils literal"><span class="pre">HomogeneousColorBaseConcept</span></code> (it 142subclasses from <code class="docutils literal"><span class="pre">homogeneous_color_base</span></code>) and, as a result, all color base 143algorithms apply to it. The element type of its color base is a channel 144iterator. For example, GIL implements <code class="docutils literal"><span class="pre">operator++</span></code> of planar iterators 145approximately like this:</p> 146<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> 147<span class="k">struct</span> <span class="nl">inc</span> <span class="p">:</span> <span class="k">public</span> <span class="n">std</span><span class="o">::</span><span class="n">unary_function</span><span class="o"><</span><span class="n">T</span><span class="p">,</span><span class="n">T</span><span class="o">></span> 148<span class="p">{</span> 149 <span class="n">T</span> <span class="k">operator</span><span class="p">()(</span><span class="n">T</span> <span class="n">x</span><span class="p">)</span> <span class="k">const</span> <span class="p">{</span> <span class="k">return</span> <span class="o">++</span><span class="n">x</span><span class="p">;</span> <span class="p">}</span> 150<span class="p">};</span> 151 152<span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">ChannelPtr</span><span class="p">,</span> <span class="k">typename</span> <span class="n">ColorSpace</span><span class="o">></span> 153<span class="n">planar_pixel_iterator</span><span class="o"><</span><span class="n">ChannelPtr</span><span class="p">,</span><span class="n">ColorSpace</span><span class="o">>&</span> 154<span class="n">planar_pixel_iterator</span><span class="o"><</span><span class="n">ChannelPtr</span><span class="p">,</span><span class="n">ColorSpace</span><span class="o">>::</span><span class="k">operator</span><span class="o">++</span><span class="p">()</span> 155<span class="p">{</span> 156 <span class="n">static_transform</span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">,</span><span class="o">*</span><span class="k">this</span><span class="p">,</span><span class="n">inc</span><span class="o"><</span><span class="n">ChannelPtr</span><span class="o">></span><span class="p">());</span> 157 <span class="k">return</span> <span class="o">*</span><span class="k">this</span><span class="p">;</span> 158<span class="p">}</span> 159</pre></div> 160</div> 161<p>Since <code class="docutils literal"><span class="pre">static_transform</span></code> uses compile-time recursion, incrementing an 162instance of <code class="docutils literal"><span class="pre">rgb8_planar_ptr_t</span></code> amounts to three pointer increments. 163GIL also uses the class <code class="docutils literal"><span class="pre">bit_aligned_pixel_iterator</span></code> as a model for a pixel 164iterator over bit-aligned pixels. Internally it keeps track of the current 165byte and the bit offset.</p> 166</div> 167</div> 168<div class="section" id="iterator-adaptor"> 169<h2><a class="toc-backref" href="#id7">Iterator Adaptor</a></h2> 170<p>Iterator adaptor is an iterator that wraps around another iterator. Its 171<code class="docutils literal"><span class="pre">is_iterator_adaptor</span></code> metafunction must evaluate to true, and it needs to 172provide a member method to return the base iterator, a metafunction to get its 173type, and a metafunction to rebind to another base iterator:</p> 174<div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">IteratorAdaptorConcept</span><span class="o"><</span><span class="n">RandomAccessTraversalIteratorConcept</span> <span class="n">Iterator</span><span class="o">></span> 175<span class="p">{</span> 176 <span class="n">where</span> <span class="n">SameType</span><span class="o"><</span><span class="n">is_iterator_adaptor</span><span class="o"><</span><span class="n">Iterator</span><span class="o">>::</span><span class="n">type</span><span class="p">,</span> <span class="n">mpl</span><span class="o">::</span><span class="n">true_</span><span class="o">></span><span class="p">;</span> 177 178 <span class="k">typename</span> <span class="n">iterator_adaptor_get_base</span><span class="o"><</span><span class="n">Iterator</span><span class="o">></span><span class="p">;</span> 179 <span class="n">where</span> <span class="n">Metafunction</span><span class="o"><</span><span class="n">iterator_adaptor_get_base</span><span class="o"><</span><span class="n">Iterator</span><span class="o">></span> <span class="o">></span><span class="p">;</span> 180 <span class="n">where</span> <span class="n">boost_concepts</span><span class="o">::</span><span class="n">ForwardTraversalConcept</span><span class="o"><</span><span class="n">iterator_adaptor_get_base</span><span class="o"><</span><span class="n">Iterator</span><span class="o">>::</span><span class="n">type</span><span class="o">></span><span class="p">;</span> 181 182 <span class="k">typename</span> <span class="n">another_iterator</span><span class="p">;</span> 183 <span class="k">typename</span> <span class="n">iterator_adaptor_rebind</span><span class="o"><</span><span class="n">Iterator</span><span class="p">,</span><span class="n">another_iterator</span><span class="o">>::</span><span class="n">type</span><span class="p">;</span> 184 <span class="n">where</span> <span class="n">boost_concepts</span><span class="o">::</span><span class="n">ForwardTraversalConcept</span><span class="o"><</span><span class="n">another_iterator</span><span class="o">></span><span class="p">;</span> 185 <span class="n">where</span> <span class="n">IteratorAdaptorConcept</span><span class="o"><</span><span class="n">iterator_adaptor_rebind</span><span class="o"><</span><span class="n">Iterator</span><span class="p">,</span><span class="n">another_iterator</span><span class="o">>::</span><span class="n">type</span><span class="o">></span><span class="p">;</span> 186 187 <span class="k">const</span> <span class="n">iterator_adaptor_get_base</span><span class="o"><</span><span class="n">Iterator</span><span class="o">>::</span><span class="n">type</span><span class="o">&</span> <span class="n">Iterator</span><span class="o">::</span><span class="n">base</span><span class="p">()</span> <span class="k">const</span><span class="p">;</span> 188<span class="p">};</span> 189 190<span class="k">template</span> <span class="o"><</span><span class="n">boost_concepts</span><span class="o">::</span><span class="n">Mutable_ForwardIteratorConcept</span> <span class="n">Iterator</span><span class="o">></span> 191<span class="n">concept</span> <span class="nl">MutableIteratorAdaptorConcept</span> <span class="p">:</span> <span class="n">IteratorAdaptorConcept</span><span class="o"><</span><span class="n">Iterator</span><span class="o">></span> <span class="p">{};</span> 192</pre></div> 193</div> 194<div class="admonition seealso"> 195<p class="first admonition-title">See also</p> 196<ul class="last simple"> 197<li><a class="reference external" href="reference/structboost_1_1gil_1_1_iterator_adaptor_concept.html">IteratorAdaptorConcept<Iterator></a></li> 198<li><a class="reference external" href="reference/structboost_1_1gil_1_1_mutable_iterator_adaptor_concept.html">MutableIteratorAdaptorConcept<Iterator></a></li> 199</ul> 200</div> 201<div class="section" id="id1"> 202<h3><a class="toc-backref" href="#id8">Models</a></h3> 203<p>GIL provides several models of <code class="docutils literal"><span class="pre">IteratorAdaptorConcept</span></code>:</p> 204<ul class="simple"> 205<li><code class="docutils literal"><span class="pre">memory_based_step_iterator<Iterator></span></code>: An iterator adaptor that changes 206the fundamental step of the base iterator 207(see <span class="xref std std-ref">design/pixel_iterator:Step Iterator</span>)</li> 208<li><code class="docutils literal"><span class="pre">dereference_iterator_adaptor<Iterator,Fn></span></code>: An iterator that applies a 209unary function <code class="docutils literal"><span class="pre">Fn</span></code> upon dereferencing. It is used, for example, for 210on-the-fly color conversion. It can be used to construct a shallow image 211“view” that pretends to have a different color space or channel depth. 212See <a class="reference internal" href="image_view.html"><span class="doc">Image View</span></a> for more. The unary function <code class="docutils literal"><span class="pre">Fn</span></code> must 213model <code class="docutils literal"><span class="pre">PixelDereferenceAdaptorConcept</span></code> (see below).</li> 214</ul> 215</div> 216</div> 217<div class="section" id="pixel-dereference-adaptor"> 218<h2><a class="toc-backref" href="#id9">Pixel Dereference Adaptor</a></h2> 219<p>Pixel dereference adaptor is a unary function that can be applied upon 220dereferencing a pixel iterator. Its argument type could be anything (usually a 221<code class="docutils literal"><span class="pre">PixelConcept</span></code>) and the result type must be convertible to <code class="docutils literal"><span class="pre">PixelConcept</span></code>:</p> 222<div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o"><</span><span class="n">boost</span><span class="o">::</span><span class="n">UnaryFunctionConcept</span> <span class="n">D</span><span class="o">></span> 223<span class="n">concept</span> <span class="nl">PixelDereferenceAdaptorConcept</span><span class="p">:</span> 224 <span class="n">DefaultConstructibleConcept</span><span class="o"><</span><span class="n">D</span><span class="o">></span><span class="p">,</span> 225 <span class="n">CopyConstructibleConcept</span><span class="o"><</span><span class="n">D</span><span class="o">></span><span class="p">,</span> 226 <span class="n">AssignableConcept</span><span class="o"><</span><span class="n">D</span><span class="o">></span> 227<span class="p">{</span> 228 <span class="k">typename</span> <span class="n">const_t</span><span class="p">;</span> <span class="n">where</span> <span class="n">PixelDereferenceAdaptorConcept</span><span class="o"><</span><span class="n">const_t</span><span class="o">></span><span class="p">;</span> 229 <span class="k">typename</span> <span class="n">value_type</span><span class="p">;</span> <span class="n">where</span> <span class="n">PixelValueConcept</span><span class="o"><</span><span class="n">value_type</span><span class="o">></span><span class="p">;</span> 230 <span class="k">typename</span> <span class="n">reference</span><span class="p">;</span> <span class="n">where</span> <span class="n">PixelConcept</span><span class="o"><</span><span class="n">remove_reference</span><span class="o"><</span><span class="n">reference</span><span class="o">>::</span><span class="n">type</span><span class="o">></span><span class="p">;</span> <span class="c1">// may be mutable</span> 231 <span class="k">typename</span> <span class="n">const_reference</span><span class="p">;</span> <span class="c1">// must not be mutable</span> 232 <span class="k">static</span> <span class="k">const</span> <span class="kt">bool</span> <span class="n">D</span><span class="o">::</span><span class="n">is_mutable</span><span class="p">;</span> 233 234 <span class="n">where</span> <span class="n">Convertible</span><span class="o"><</span><span class="n">value_type</span><span class="p">,</span> <span class="n">result_type</span><span class="o">></span><span class="p">;</span> 235<span class="p">};</span> 236</pre></div> 237</div> 238<div class="section" id="id2"> 239<h3><a class="toc-backref" href="#id10">Models</a></h3> 240<p>GIL provides several models of <code class="docutils literal"><span class="pre">PixelDereferenceAdaptorConcept</span></code>:</p> 241<ul class="simple"> 242<li><code class="docutils literal"><span class="pre">color_convert_deref_fn</span></code>: a function object that performs color conversion</li> 243<li><code class="docutils literal"><span class="pre">detail::nth_channel_deref_fn</span></code>: a function object that returns a grayscale 244pixel corresponding to the n-th channel of a given pixel</li> 245<li><code class="docutils literal"><span class="pre">deref_compose</span></code>: a function object that composes two models of 246<code class="docutils literal"><span class="pre">PixelDereferenceAdaptorConcept</span></code>. Similar to <code class="docutils literal"><span class="pre">std::unary_compose</span></code>, 247except it needs to pull the additional typedefs required by 248<code class="docutils literal"><span class="pre">PixelDereferenceAdaptorConcept</span></code></li> 249</ul> 250<p>GIL uses pixel dereference adaptors to implement image views that perform 251color conversion upon dereferencing, or that return the N-th channel of the 252underlying pixel. They can be used to model virtual image views that perform 253an arbitrary function upon dereferencing, for example a view of the Mandelbrot 254set. <code class="docutils literal"><span class="pre">dereference_iterator_adaptor<Iterator,Fn></span></code> is an iterator wrapper over 255a pixel iterator <code class="docutils literal"><span class="pre">Iterator</span></code> that invokes the given dereference iterator 256adaptor <code class="docutils literal"><span class="pre">Fn</span></code> upon dereferencing.</p> 257</div> 258</div> 259<div class="section" id="step-iterator"> 260<h2><a class="toc-backref" href="#id11">Step Iterator</a></h2> 261<p>Sometimes we want to traverse pixels with a unit step other than the one 262provided by the fundamental pixel iterators. Examples where this would be 263useful:</p> 264<ul class="simple"> 265<li>a single-channel view of the red channel of an RGB interleaved image</li> 266<li>left-to-right flipped image (step = -fundamental_step)</li> 267<li>subsampled view, taking every N-th pixel (step = N*fundamental_step)</li> 268<li>traversal in vertical direction (step = number of bytes per row)</li> 269<li>any combination of the above (steps are multiplied)</li> 270</ul> 271<p>Step iterators are forward traversal iterators that allow changing the step 272between adjacent values:</p> 273<div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">StepIteratorConcept</span><span class="o"><</span><span class="n">boost_concepts</span><span class="o">::</span><span class="n">ForwardTraversalConcept</span> <span class="n">Iterator</span><span class="o">></span> 274<span class="p">{</span> 275 <span class="k">template</span> <span class="o"><</span><span class="n">Integral</span> <span class="n">D</span><span class="o">></span> <span class="kt">void</span> <span class="n">Iterator</span><span class="o">::</span><span class="n">set_step</span><span class="p">(</span><span class="n">D</span> <span class="n">step</span><span class="p">);</span> 276<span class="p">};</span> 277 278<span class="n">concept</span> <span class="n">MutableStepIteratorConcept</span><span class="o"><</span><span class="n">boost_concepts</span><span class="o">::</span><span class="n">Mutable_ForwardIteratorConcept</span> <span class="n">Iterator</span><span class="o">></span> 279 <span class="o">:</span> <span class="n">StepIteratorConcept</span><span class="o"><</span><span class="n">Iterator</span><span class="o">></span> 280<span class="p">{};</span> 281</pre></div> 282</div> 283<p>GIL currently provides a step iterator whose <code class="docutils literal"><span class="pre">value_type</span> <span class="pre">models</span></code> 284<code class="docutils literal"><span class="pre">PixelValueConcept</span></code>. In addition, the step is specified in memory units 285(which are bytes or bits). This is necessary, for example, when implementing 286an iterator navigating along a column of pixels - the size of a row of pixels 287may sometimes not be divisible by the size of a pixel; for example rows may be 288word-aligned.</p> 289<p>To advance in bytes/bits, the base iterator must model 290<code class="docutils literal"><span class="pre">MemoryBasedIteratorConcept</span></code>. A memory-based iterator has an inherent memory 291unit, which is either a bit or a byte. It must supply functions returning the 292number of bits per memory unit (1 or 8), the current step in memory units, the 293memory-unit distance between two iterators, and a reference a given distance 294in memunits away. It must also supply a function that advances an iterator a 295given distance in memory units. <code class="docutils literal"><span class="pre">memunit_advanced</span></code> and 296<code class="docutils literal"><span class="pre">memunit_advanced_ref</span></code> have a default implementation but some iterators may 297supply a more efficient version:</p> 298<div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">MemoryBasedIteratorConcept</span> 299<span class="o"><</span> 300 <span class="n">boost_concepts</span><span class="o">::</span><span class="n">RandomAccessTraversalConcept</span> <span class="n">Iterator</span> 301<span class="o">></span> 302<span class="p">{</span> 303 <span class="k">typename</span> <span class="n">byte_to_memunit</span><span class="o"><</span><span class="n">Iterator</span><span class="o">></span><span class="p">;</span> <span class="n">where</span> <span class="n">metafunction</span><span class="o"><</span><span class="n">byte_to_memunit</span><span class="o"><</span><span class="n">Iterator</span><span class="o">></span> <span class="o">></span><span class="p">;</span> 304 <span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span> <span class="n">memunit_step</span><span class="p">(</span><span class="k">const</span> <span class="n">Iterator</span><span class="o">&</span><span class="p">);</span> 305 <span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span> <span class="n">memunit_distance</span><span class="p">(</span><span class="k">const</span> <span class="n">Iterator</span><span class="o">&</span> <span class="p">,</span> <span class="k">const</span> <span class="n">Iterator</span><span class="o">&</span><span class="p">);</span> 306 <span class="kt">void</span> <span class="nf">memunit_advance</span><span class="p">(</span><span class="n">Iterator</span><span class="o">&</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span> <span class="n">diff</span><span class="p">);</span> 307 <span class="n">Iterator</span> <span class="nf">memunit_advanced</span><span class="p">(</span><span class="k">const</span> <span class="n">Iterator</span><span class="o">&</span> <span class="n">p</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span> <span class="n">diff</span><span class="p">)</span> <span class="p">{</span> <span class="n">Iterator</span> <span class="n">tmp</span><span class="p">;</span> <span class="n">memunit_advance</span><span class="p">(</span><span class="n">tmp</span><span class="p">,</span><span class="n">diff</span><span class="p">);</span> <span class="k">return</span> <span class="n">tmp</span><span class="p">;</span> <span class="p">}</span> 308 <span class="n">Iterator</span><span class="o">::</span><span class="n">reference</span> <span class="n">memunit_advanced_ref</span><span class="p">(</span><span class="k">const</span> <span class="n">Iterator</span><span class="o">&</span> <span class="n">p</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span> <span class="n">diff</span><span class="p">)</span> <span class="p">{</span> <span class="k">return</span> <span class="o">*</span><span class="n">memunit_advanced</span><span class="p">(</span><span class="n">p</span><span class="p">,</span><span class="n">diff</span><span class="p">);</span> <span class="p">}</span> 309<span class="p">};</span> 310</pre></div> 311</div> 312<p>It is useful to be able to construct a step iterator over another iterator. 313More generally, given a type, we want to be able to construct an equivalent 314type that allows for dynamically specified horizontal step:</p> 315<div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">HasDynamicXStepTypeConcept</span><span class="o"><</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span> 316<span class="p">{</span> 317 <span class="k">typename</span> <span class="n">dynamic_x_step_type</span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="p">;</span> 318 <span class="n">where</span> <span class="n">Metafunction</span><span class="o"><</span><span class="n">dynamic_x_step_type</span><span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="o">></span><span class="p">;</span> 319<span class="p">};</span> 320</pre></div> 321</div> 322<p>All models of pixel iterators, locators and image views that GIL provides 323support <code class="docutils literal"><span class="pre">HasDynamicXStepTypeConcept</span></code>.</p> 324<div class="admonition seealso"> 325<p class="first admonition-title">See also</p> 326<ul class="last simple"> 327<li><a class="reference external" href="reference/structboost_1_1gil_1_1_step_iterator_concept.html">StepIteratorConcept<Iterator></a></li> 328<li><a class="reference external" href="reference/structboost_1_1gil_1_1_mutable_step_iterator_concept.html">MutableStepIteratorConcept<Iterator></a></li> 329<li><a class="reference external" href="reference/structboost_1_1gil_1_1_memory_based_iterator_concept.html">MemoryBasedIteratorConcept<Iterator></a></li> 330<li><a class="reference external" href="reference/structboost_1_1gil_1_1_has_dynamic_x_step_type_concept.html">HasDynamicXStepTypeConcept<T></a></li> 331</ul> 332</div> 333<div class="section" id="id3"> 334<h3><a class="toc-backref" href="#id12">Models</a></h3> 335<p>All standard memory-based iterators GIL currently provides model 336<code class="docutils literal"><span class="pre">MemoryBasedIteratorConcept</span></code>. GIL provides the class 337<code class="docutils literal"><span class="pre">memory_based_step_iterator</span></code> which models <code class="docutils literal"><span class="pre">PixelIteratorConcept</span></code>, 338<code class="docutils literal"><span class="pre">StepIteratorConcept</span></code>, and <code class="docutils literal"><span class="pre">MemoryBasedIteratorConcept</span></code>. It takes the base 339iterator as a template parameter (which must model <code class="docutils literal"><span class="pre">PixelIteratorConcept</span></code> 340and <code class="docutils literal"><span class="pre">MemoryBasedIteratorConcept</span></code>) and allows changing the step dynamically. 341GIL implementation contains the base iterator and a <code class="docutils literal"><span class="pre">ptrdiff_t</span></code> denoting the 342number of memory units (bytes or bits) to skip for a unit step. It may also be 343used with a negative number. GIL provides a function to create a step iterator 344from a base iterator and a step:</p> 345<div class="highlight-cpp"><div class="highlight"><pre><span class="c1">// Iterator models MemoryBasedIteratorConcept, HasDynamicXStepTypeConcept</span> 346<span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">Iterator</span><span class="o">></span> 347<span class="k">typename</span> <span class="n">dynamic_x_step_type</span><span class="o"><</span><span class="n">Iterator</span><span class="o">>::</span><span class="n">type</span> <span class="n">make_step_iterator</span><span class="p">(</span><span class="n">Iterator</span> <span class="k">const</span><span class="o">&</span> <span class="n">it</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span> <span class="n">step</span><span class="p">);</span> 348</pre></div> 349</div> 350<p>GIL also provides a model of an iterator over a virtual array of pixels, 351<code class="docutils literal"><span class="pre">position_iterator</span></code>. It is a step iterator that keeps track of the pixel 352position and invokes a function object to get the value of the pixel upon 353dereferencing. It models <code class="docutils literal"><span class="pre">PixelIteratorConcept</span></code> and <code class="docutils literal"><span class="pre">StepIteratorConcept</span></code> 354but not <code class="docutils literal"><span class="pre">MemoryBasedIteratorConcept</span></code>.</p> 355</div> 356</div> 357</div> 358 359 360 <div class="navbar" style="text-align:right;"> 361 362 363 <a class="prev" title="Pixel" href="pixel.html"><img src="../_static/prev.png" alt="prev"/></a> 364 <a class="up" title="Design Guide" href="index.html"><img src="../_static/up.png" alt="up"/></a> 365 <a class="next" title="Pixel Locator" href="pixel_locator.html"><img src="../_static/next.png" alt="next"/></a> 366 367 </div> 368 </div> 369 <div class="footer" role="contentinfo"> 370 Last updated on 2020-08-11 15:08:48. 371 Created using <a href="http://sphinx-doc.org/">Sphinx</a> 1.5.6. 372 </div> 373 </body> 374</html>
