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  <div class="section" id="pixel-iterator">
<h1>Pixel Iterator</h1>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><a class="reference internal" href="#overview" id="id4">Overview</a></li>
<li><a class="reference internal" href="#fundamental-iterator" id="id5">Fundamental Iterator</a><ul>
<li><a class="reference internal" href="#models" id="id6">Models</a></li>
</ul>
</li>
<li><a class="reference internal" href="#iterator-adaptor" id="id7">Iterator Adaptor</a><ul>
<li><a class="reference internal" href="#id1" id="id8">Models</a></li>
</ul>
</li>
<li><a class="reference internal" href="#pixel-dereference-adaptor" id="id9">Pixel Dereference Adaptor</a><ul>
<li><a class="reference internal" href="#id2" id="id10">Models</a></li>
</ul>
</li>
<li><a class="reference internal" href="#step-iterator" id="id11">Step Iterator</a><ul>
<li><a class="reference internal" href="#id3" id="id12">Models</a></li>
</ul>
</li>
</ul>
</div>
<div class="section" id="overview">
<h2><a class="toc-backref" href="#id4">Overview</a></h2>
<p>Pixel iterators are random traversal iterators whose <code class="docutils literal"><span class="pre">value_type</span>
<span class="pre">models</span></code> <code class="docutils literal"><span class="pre">PixelValueConcept</span></code>.</p>
</div>
<div class="section" id="fundamental-iterator">
<h2><a class="toc-backref" href="#id5">Fundamental Iterator</a></h2>
<p>Pixel iterators provide metafunctions to determine whether they are mutable
(i.e. whether they allow for modifying the pixel they refer to), to get the
immutable (read-only) type of the iterator, and to determine whether they are
plain iterators or adaptors over another pixel iterator:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">PixelIteratorConcept</span><span class="o">&lt;</span><span class="n">RandomAccessTraversalIteratorConcept</span> <span class="n">Iterator</span><span class="o">&gt;</span>
    <span class="o">:</span> <span class="n">PixelBasedConcept</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;</span>
<span class="p">{</span>
  <span class="n">where</span> <span class="n">PixelValueConcept</span><span class="o">&lt;</span><span class="n">value_type</span><span class="o">&gt;</span><span class="p">;</span>
  <span class="k">typename</span> <span class="n">const_iterator_type</span><span class="o">&lt;</span><span class="n">It</span><span class="o">&gt;::</span><span class="n">type</span><span class="p">;</span>
      <span class="n">where</span> <span class="n">PixelIteratorConcept</span><span class="o">&lt;</span><span class="n">const_iterator_type</span><span class="o">&lt;</span><span class="n">It</span><span class="o">&gt;::</span><span class="n">type</span><span class="o">&gt;</span><span class="p">;</span>
  <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">&lt;</span><span class="n">It</span><span class="o">&gt;::</span><span class="n">value</span><span class="p">;</span>
  <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">&lt;</span><span class="n">It</span><span class="o">&gt;::</span><span class="n">value</span><span class="p">;</span>   <span class="c1">// is it an iterator adaptor</span>
<span class="p">};</span>

<span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Iterator</span><span class="o">&gt;</span>
<span class="n">concept</span> <span class="nl">MutablePixelIteratorConcept</span> <span class="p">:</span> <span class="n">PixelIteratorConcept</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;</span><span class="p">,</span> <span class="n">MutableRandomAccessIteratorConcept</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;</span> <span class="p">{};</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<ul class="last simple">
<li><a class="reference external" href="reference/group___pixel_iterator_concept_pixel_iterator.html">PixelIteratorConcept&lt;Iterator&gt;</a></li>
<li><a class="reference external" href="reference/structboost_1_1gil_1_1_mutable_pixel_iterator_concept.html">MutablePixelIteratorConcept&lt;Iterator&gt;</a></li>
</ul>
</div>
<div class="section" id="models">
<h3><a class="toc-backref" href="#id6">Models</a></h3>
<p>A built-in pointer to pixel, <code class="docutils literal"><span class="pre">pixel&lt;ChannelValue,Layout&gt;*</span></code>, is GIL model for
pixel iterator over interleaved homogeneous pixels. Similarly,
<code class="docutils literal"><span class="pre">packed_pixel&lt;PixelData,ChannelRefVec,Layout&gt;*</span></code> is GIL model for an iterator
over interleaved packed pixels.</p>
<p>For planar homogeneous pixels, GIL provides the class
<code class="docutils literal"><span class="pre">planar_pixel_iterator</span></code>, templated over a channel iterator and color space.
Here is how the standard mutable and read-only planar RGB iterators over
unsigned char are defined:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">ChannelPtr</span><span class="p">,</span> <span class="k">typename</span> <span class="n">ColorSpace</span><span class="o">&gt;</span>
<span class="k">struct</span> <span class="n">planar_pixel_iterator</span><span class="p">;</span>

<span class="c1">// GIL provided typedefs</span>
<span class="k">typedef</span> <span class="n">planar_pixel_iterator</span><span class="o">&lt;</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">&gt;</span> <span class="n">rgb8c_planar_ptr_t</span><span class="p">;</span>
<span class="k">typedef</span> <span class="n">planar_pixel_iterator</span><span class="o">&lt;</span>      <span class="n">bits8</span><span class="o">*</span><span class="p">,</span> <span class="n">rgb_t</span><span class="o">&gt;</span> <span class="n">rgb8_planar_ptr_t</span><span class="p">;</span>
</pre></div>
</div>
<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
subclasses from <code class="docutils literal"><span class="pre">homogeneous_color_base</span></code>) and, as a result, all color base
algorithms apply to it. The element type of its color base is a channel
iterator. For example, GIL implements <code class="docutils literal"><span class="pre">operator++</span></code> of planar iterators
approximately like this:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</span>
<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">&lt;</span><span class="n">T</span><span class="p">,</span><span class="n">T</span><span class="o">&gt;</span>
<span class="p">{</span>
  <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>
<span class="p">};</span>

<span class="k">template</span> <span class="o">&lt;</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">&gt;</span>
<span class="n">planar_pixel_iterator</span><span class="o">&lt;</span><span class="n">ChannelPtr</span><span class="p">,</span><span class="n">ColorSpace</span><span class="o">&gt;&amp;</span>
<span class="n">planar_pixel_iterator</span><span class="o">&lt;</span><span class="n">ChannelPtr</span><span class="p">,</span><span class="n">ColorSpace</span><span class="o">&gt;::</span><span class="k">operator</span><span class="o">++</span><span class="p">()</span>
<span class="p">{</span>
  <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">&lt;</span><span class="n">ChannelPtr</span><span class="o">&gt;</span><span class="p">());</span>
  <span class="k">return</span> <span class="o">*</span><span class="k">this</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Since <code class="docutils literal"><span class="pre">static_transform</span></code> uses compile-time recursion, incrementing an
instance of <code class="docutils literal"><span class="pre">rgb8_planar_ptr_t</span></code> amounts to three pointer increments.
GIL also uses the class <code class="docutils literal"><span class="pre">bit_aligned_pixel_iterator</span></code> as a model for a pixel
iterator over bit-aligned pixels. Internally it keeps track of the current
byte and the bit offset.</p>
</div>
</div>
<div class="section" id="iterator-adaptor">
<h2><a class="toc-backref" href="#id7">Iterator Adaptor</a></h2>
<p>Iterator adaptor is an iterator that wraps around another iterator. Its
<code class="docutils literal"><span class="pre">is_iterator_adaptor</span></code> metafunction must evaluate to true, and it needs to
provide a member method to return the base iterator, a metafunction to get its
type, and a metafunction to rebind to another base iterator:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">IteratorAdaptorConcept</span><span class="o">&lt;</span><span class="n">RandomAccessTraversalIteratorConcept</span> <span class="n">Iterator</span><span class="o">&gt;</span>
<span class="p">{</span>
  <span class="n">where</span> <span class="n">SameType</span><span class="o">&lt;</span><span class="n">is_iterator_adaptor</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;::</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">&gt;</span><span class="p">;</span>

  <span class="k">typename</span> <span class="n">iterator_adaptor_get_base</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;</span><span class="p">;</span>
      <span class="n">where</span> <span class="n">Metafunction</span><span class="o">&lt;</span><span class="n">iterator_adaptor_get_base</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;</span> <span class="o">&gt;</span><span class="p">;</span>
      <span class="n">where</span> <span class="n">boost_concepts</span><span class="o">::</span><span class="n">ForwardTraversalConcept</span><span class="o">&lt;</span><span class="n">iterator_adaptor_get_base</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;::</span><span class="n">type</span><span class="o">&gt;</span><span class="p">;</span>

  <span class="k">typename</span> <span class="n">another_iterator</span><span class="p">;</span>
  <span class="k">typename</span> <span class="n">iterator_adaptor_rebind</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="p">,</span><span class="n">another_iterator</span><span class="o">&gt;::</span><span class="n">type</span><span class="p">;</span>
      <span class="n">where</span> <span class="n">boost_concepts</span><span class="o">::</span><span class="n">ForwardTraversalConcept</span><span class="o">&lt;</span><span class="n">another_iterator</span><span class="o">&gt;</span><span class="p">;</span>
      <span class="n">where</span> <span class="n">IteratorAdaptorConcept</span><span class="o">&lt;</span><span class="n">iterator_adaptor_rebind</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="p">,</span><span class="n">another_iterator</span><span class="o">&gt;::</span><span class="n">type</span><span class="o">&gt;</span><span class="p">;</span>

  <span class="k">const</span> <span class="n">iterator_adaptor_get_base</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;::</span><span class="n">type</span><span class="o">&amp;</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>
<span class="p">};</span>

<span class="k">template</span> <span class="o">&lt;</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">&gt;</span>
<span class="n">concept</span> <span class="nl">MutableIteratorAdaptorConcept</span> <span class="p">:</span> <span class="n">IteratorAdaptorConcept</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;</span> <span class="p">{};</span>
</pre></div>
</div>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<ul class="last simple">
<li><a class="reference external" href="reference/structboost_1_1gil_1_1_iterator_adaptor_concept.html">IteratorAdaptorConcept&lt;Iterator&gt;</a></li>
<li><a class="reference external" href="reference/structboost_1_1gil_1_1_mutable_iterator_adaptor_concept.html">MutableIteratorAdaptorConcept&lt;Iterator&gt;</a></li>
</ul>
</div>
<div class="section" id="id1">
<h3><a class="toc-backref" href="#id8">Models</a></h3>
<p>GIL provides several models of <code class="docutils literal"><span class="pre">IteratorAdaptorConcept</span></code>:</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">memory_based_step_iterator&lt;Iterator&gt;</span></code>: An iterator adaptor that changes
the fundamental step of the base iterator
(see <span class="xref std std-ref">design/pixel_iterator:Step Iterator</span>)</li>
<li><code class="docutils literal"><span class="pre">dereference_iterator_adaptor&lt;Iterator,Fn&gt;</span></code>: An iterator that applies a
unary function <code class="docutils literal"><span class="pre">Fn</span></code> upon dereferencing. It is used, for example, for
on-the-fly color conversion. It can be used to construct a shallow image
&#8220;view&#8221; that pretends to have a different color space or channel depth.
See <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
model <code class="docutils literal"><span class="pre">PixelDereferenceAdaptorConcept</span></code> (see below).</li>
</ul>
</div>
</div>
<div class="section" id="pixel-dereference-adaptor">
<h2><a class="toc-backref" href="#id9">Pixel Dereference Adaptor</a></h2>
<p>Pixel dereference adaptor is a unary function that can be applied upon
dereferencing a pixel iterator. Its argument type could be anything (usually a
<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>
<div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span> <span class="o">&lt;</span><span class="n">boost</span><span class="o">::</span><span class="n">UnaryFunctionConcept</span> <span class="n">D</span><span class="o">&gt;</span>
<span class="n">concept</span> <span class="nl">PixelDereferenceAdaptorConcept</span><span class="p">:</span>
    <span class="n">DefaultConstructibleConcept</span><span class="o">&lt;</span><span class="n">D</span><span class="o">&gt;</span><span class="p">,</span>
    <span class="n">CopyConstructibleConcept</span><span class="o">&lt;</span><span class="n">D</span><span class="o">&gt;</span><span class="p">,</span>
    <span class="n">AssignableConcept</span><span class="o">&lt;</span><span class="n">D</span><span class="o">&gt;</span>
<span class="p">{</span>
  <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">&lt;</span><span class="n">const_t</span><span class="o">&gt;</span><span class="p">;</span>
  <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">&lt;</span><span class="n">value_type</span><span class="o">&gt;</span><span class="p">;</span>
  <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">&lt;</span><span class="n">remove_reference</span><span class="o">&lt;</span><span class="n">reference</span><span class="o">&gt;::</span><span class="n">type</span><span class="o">&gt;</span><span class="p">;</span>  <span class="c1">// may be mutable</span>
  <span class="k">typename</span> <span class="n">const_reference</span><span class="p">;</span>   <span class="c1">// must not be mutable</span>
  <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>

  <span class="n">where</span> <span class="n">Convertible</span><span class="o">&lt;</span><span class="n">value_type</span><span class="p">,</span> <span class="n">result_type</span><span class="o">&gt;</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
</div>
<div class="section" id="id2">
<h3><a class="toc-backref" href="#id10">Models</a></h3>
<p>GIL provides several models of <code class="docutils literal"><span class="pre">PixelDereferenceAdaptorConcept</span></code>:</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">color_convert_deref_fn</span></code>: a function object that performs color conversion</li>
<li><code class="docutils literal"><span class="pre">detail::nth_channel_deref_fn</span></code>: a function object that returns a grayscale
pixel corresponding to the n-th channel of a given pixel</li>
<li><code class="docutils literal"><span class="pre">deref_compose</span></code>: a function object that composes two models of
<code class="docutils literal"><span class="pre">PixelDereferenceAdaptorConcept</span></code>. Similar to <code class="docutils literal"><span class="pre">std::unary_compose</span></code>,
except it needs to pull the additional typedefs required by
<code class="docutils literal"><span class="pre">PixelDereferenceAdaptorConcept</span></code></li>
</ul>
<p>GIL uses pixel dereference adaptors to implement image views that perform
color conversion upon dereferencing, or that return the N-th channel of the
underlying pixel. They can be used to model virtual image views that perform
an arbitrary function upon dereferencing, for example a view of the Mandelbrot
set. <code class="docutils literal"><span class="pre">dereference_iterator_adaptor&lt;Iterator,Fn&gt;</span></code> is an iterator wrapper over
a pixel iterator <code class="docutils literal"><span class="pre">Iterator</span></code> that invokes the given dereference iterator
adaptor <code class="docutils literal"><span class="pre">Fn</span></code> upon dereferencing.</p>
</div>
</div>
<div class="section" id="step-iterator">
<h2><a class="toc-backref" href="#id11">Step Iterator</a></h2>
<p>Sometimes we want to traverse pixels with a unit step other than the one
provided by the fundamental pixel iterators. Examples where this would be
useful:</p>
<ul class="simple">
<li>a single-channel view of the red channel of an RGB interleaved image</li>
<li>left-to-right flipped image (step = -fundamental_step)</li>
<li>subsampled view, taking every N-th pixel (step = N*fundamental_step)</li>
<li>traversal in vertical direction (step = number of bytes per row)</li>
<li>any combination of the above (steps are multiplied)</li>
</ul>
<p>Step iterators are forward traversal iterators that allow changing the step
between adjacent values:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">StepIteratorConcept</span><span class="o">&lt;</span><span class="n">boost_concepts</span><span class="o">::</span><span class="n">ForwardTraversalConcept</span> <span class="n">Iterator</span><span class="o">&gt;</span>
<span class="p">{</span>
  <span class="k">template</span> <span class="o">&lt;</span><span class="n">Integral</span> <span class="n">D</span><span class="o">&gt;</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>
<span class="p">};</span>

<span class="n">concept</span> <span class="n">MutableStepIteratorConcept</span><span class="o">&lt;</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">&gt;</span>
    <span class="o">:</span> <span class="n">StepIteratorConcept</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;</span>
<span class="p">{};</span>
</pre></div>
</div>
<p>GIL currently provides a step iterator whose <code class="docutils literal"><span class="pre">value_type</span> <span class="pre">models</span></code>
<code class="docutils literal"><span class="pre">PixelValueConcept</span></code>. In addition, the step is specified in memory units
(which are bytes or bits). This is necessary, for example, when implementing
an iterator navigating along a column of pixels - the size of a row of pixels
may sometimes not be divisible by the size of a pixel; for example rows may be
word-aligned.</p>
<p>To advance in bytes/bits, the base iterator must model
<code class="docutils literal"><span class="pre">MemoryBasedIteratorConcept</span></code>. A memory-based iterator has an inherent memory
unit, which is either a bit or a byte. It must supply functions returning the
number of bits per memory unit (1 or 8), the current step in memory units, the
memory-unit distance between two iterators, and a reference a given distance
in memunits away. It must also supply a function that advances an iterator a
given distance in memory units. <code class="docutils literal"><span class="pre">memunit_advanced</span></code> and
<code class="docutils literal"><span class="pre">memunit_advanced_ref</span></code> have a default implementation but some iterators may
supply a more efficient version:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">MemoryBasedIteratorConcept</span>
<span class="o">&lt;</span>
    <span class="n">boost_concepts</span><span class="o">::</span><span class="n">RandomAccessTraversalConcept</span> <span class="n">Iterator</span>
<span class="o">&gt;</span>
<span class="p">{</span>
  <span class="k">typename</span> <span class="n">byte_to_memunit</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;</span><span class="p">;</span> <span class="n">where</span> <span class="n">metafunction</span><span class="o">&lt;</span><span class="n">byte_to_memunit</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;</span> <span class="o">&gt;</span><span class="p">;</span>
  <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">&amp;</span><span class="p">);</span>
  <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">&amp;</span> <span class="p">,</span> <span class="k">const</span> <span class="n">Iterator</span><span class="o">&amp;</span><span class="p">);</span>
  <span class="kt">void</span>                <span class="nf">memunit_advance</span><span class="p">(</span><span class="n">Iterator</span><span class="o">&amp;</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="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">&amp;</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>
  <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">&amp;</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>
<span class="p">};</span>
</pre></div>
</div>
<p>It is useful to be able to construct a step iterator over another iterator.
More generally, given a type, we want to be able to construct an equivalent
type that allows for dynamically specified horizontal step:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">HasDynamicXStepTypeConcept</span><span class="o">&lt;</span><span class="k">typename</span> <span class="n">T</span><span class="o">&gt;</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">T</span><span class="o">&gt;</span><span class="p">;</span>
      <span class="n">where</span> <span class="n">Metafunction</span><span class="o">&lt;</span><span class="n">dynamic_x_step_type</span><span class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span> <span class="o">&gt;</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
</div>
<p>All models of pixel iterators, locators and image views that GIL provides
support <code class="docutils literal"><span class="pre">HasDynamicXStepTypeConcept</span></code>.</p>
<div class="admonition seealso">
<p class="first admonition-title">See also</p>
<ul class="last simple">
<li><a class="reference external" href="reference/structboost_1_1gil_1_1_step_iterator_concept.html">StepIteratorConcept&lt;Iterator&gt;</a></li>
<li><a class="reference external" href="reference/structboost_1_1gil_1_1_mutable_step_iterator_concept.html">MutableStepIteratorConcept&lt;Iterator&gt;</a></li>
<li><a class="reference external" href="reference/structboost_1_1gil_1_1_memory_based_iterator_concept.html">MemoryBasedIteratorConcept&lt;Iterator&gt;</a></li>
<li><a class="reference external" href="reference/structboost_1_1gil_1_1_has_dynamic_x_step_type_concept.html">HasDynamicXStepTypeConcept&lt;T&gt;</a></li>
</ul>
</div>
<div class="section" id="id3">
<h3><a class="toc-backref" href="#id12">Models</a></h3>
<p>All standard memory-based iterators GIL currently provides model
<code class="docutils literal"><span class="pre">MemoryBasedIteratorConcept</span></code>. GIL provides the class
<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>,
<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
iterator as a template parameter (which must model <code class="docutils literal"><span class="pre">PixelIteratorConcept</span></code>
and <code class="docutils literal"><span class="pre">MemoryBasedIteratorConcept</span></code>) and allows changing the step dynamically.
GIL implementation contains the base iterator and a <code class="docutils literal"><span class="pre">ptrdiff_t</span></code> denoting the
number of memory units (bytes or bits) to skip for a unit step. It may also be
used with a negative number. GIL provides a function to create a step iterator
from a base iterator and a step:</p>
<div class="highlight-cpp"><div class="highlight"><pre><span class="c1">// Iterator models MemoryBasedIteratorConcept, HasDynamicXStepTypeConcept</span>
<span class="k">template</span> <span class="o">&lt;</span><span class="k">typename</span> <span class="n">Iterator</span><span class="o">&gt;</span>
<span class="k">typename</span> <span class="n">dynamic_x_step_type</span><span class="o">&lt;</span><span class="n">Iterator</span><span class="o">&gt;::</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">&amp;</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>
</pre></div>
</div>
<p>GIL also provides a model of an iterator over a virtual array of pixels,
<code class="docutils literal"><span class="pre">position_iterator</span></code>. It is a step iterator that keeps track of the pixel
position and invokes a function object to get the value of the pixel upon
dereferencing. It models <code class="docutils literal"><span class="pre">PixelIteratorConcept</span></code> and <code class="docutils literal"><span class="pre">StepIteratorConcept</span></code>
but not <code class="docutils literal"><span class="pre">MemoryBasedIteratorConcept</span></code>.</p>
</div>
</div>
</div>


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