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- <div class="section" id="pixel-locator">
- <h1>Pixel Locator</h1>
- <div class="contents local topic" id="contents">
- <ul class="simple">
- <li><a class="reference internal" href="#overview" id="id1">Overview</a></li>
- <li><a class="reference internal" href="#models" id="id2">Models</a></li>
- <li><a class="reference internal" href="#iterator-over-2d-image" id="id3">Iterator over 2D image</a></li>
- </ul>
- </div>
- <div class="section" id="overview">
- <h2><a class="toc-backref" href="#id1">Overview</a></h2>
- <p>A Locator allows for navigation in two or more dimensions. Locators are
- N-dimensional iterators in spirit, but we use a different name because they
- don’t satisfy all the requirements of iterators. For example, they don’t
- supply increment and decrement operators because it is unclear which dimension
- the operators should advance along.
- N-dimensional locators model the following concept:</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">RandomAccessNDLocatorConcept</span><span class="o"><</span><span class="n">Regular</span> <span class="n">Loc</span><span class="o">></span>
- <span class="p">{</span>
- <span class="k">typename</span> <span class="n">value_type</span><span class="p">;</span> <span class="c1">// value over which the locator navigates</span>
- <span class="k">typename</span> <span class="n">reference</span><span class="p">;</span> <span class="c1">// result of dereferencing</span>
- <span class="k">typename</span> <span class="n">difference_type</span><span class="p">;</span> <span class="n">where</span> <span class="n">PointNDConcept</span><span class="o"><</span><span class="n">difference_type</span><span class="o">></span><span class="p">;</span> <span class="c1">// return value of operator-.</span>
- <span class="k">typename</span> <span class="n">const_t</span><span class="p">;</span> <span class="c1">// same as Loc, but operating over immutable values</span>
- <span class="k">typename</span> <span class="n">cached_location_t</span><span class="p">;</span> <span class="c1">// type to store relative location (for efficient repeated access)</span>
- <span class="k">typename</span> <span class="n">point_t</span> <span class="o">=</span> <span class="n">difference_type</span><span class="p">;</span>
- <span class="k">static</span> <span class="k">const</span> <span class="kt">size_t</span> <span class="n">num_dimensions</span><span class="p">;</span> <span class="c1">// dimensionality of the locator</span>
- <span class="n">where</span> <span class="n">num_dimensions</span> <span class="o">=</span> <span class="n">point_t</span><span class="o">::</span><span class="n">num_dimensions</span><span class="p">;</span>
- <span class="c1">// The difference_type and iterator type along each dimension. The iterators may only differ in</span>
- <span class="c1">// difference_type. Their value_type must be the same as Loc::value_type</span>
- <span class="k">template</span> <span class="o"><</span><span class="kt">size_t</span> <span class="n">D</span><span class="o">></span> <span class="k">struct</span> <span class="n">axis</span> <span class="p">{</span>
- <span class="k">typename</span> <span class="n">coord_t</span> <span class="o">=</span> <span class="n">point_t</span><span class="o">::</span><span class="n">axis</span><span class="o"><</span><span class="n">D</span><span class="o">>::</span><span class="n">coord_t</span><span class="p">;</span>
- <span class="k">typename</span> <span class="n">iterator</span><span class="p">;</span> <span class="n">where</span> <span class="n">RandomAccessTraversalConcept</span><span class="o"><</span><span class="n">iterator</span><span class="o">></span><span class="p">;</span> <span class="c1">// iterator along D-th axis.</span>
- <span class="n">where</span> <span class="n">iterator</span><span class="o">::</span><span class="n">value_type</span> <span class="o">==</span> <span class="n">value_type</span><span class="p">;</span>
- <span class="p">};</span>
- <span class="c1">// Defines the type of a locator similar to this type, except it invokes Deref upon dereferencing</span>
- <span class="k">template</span> <span class="o"><</span><span class="n">PixelDereferenceAdaptorConcept</span> <span class="n">Deref</span><span class="o">></span> <span class="k">struct</span> <span class="n">add_deref</span> <span class="p">{</span>
- <span class="k">typename</span> <span class="n">type</span><span class="p">;</span> <span class="n">where</span> <span class="n">RandomAccessNDLocatorConcept</span><span class="o"><</span><span class="n">type</span><span class="o">></span><span class="p">;</span>
- <span class="k">static</span> <span class="n">type</span> <span class="nf">make</span><span class="p">(</span><span class="k">const</span> <span class="n">Loc</span><span class="o">&</span> <span class="n">loc</span><span class="p">,</span> <span class="k">const</span> <span class="n">Deref</span><span class="o">&</span> <span class="n">deref</span><span class="p">);</span>
- <span class="p">};</span>
- <span class="n">Loc</span><span class="o">&</span> <span class="k">operator</span><span class="o">+=</span><span class="p">(</span><span class="n">Loc</span><span class="o">&</span><span class="p">,</span> <span class="k">const</span> <span class="n">difference_type</span><span class="o">&</span><span class="p">);</span>
- <span class="n">Loc</span><span class="o">&</span> <span class="k">operator</span><span class="o">-=</span><span class="p">(</span><span class="n">Loc</span><span class="o">&</span><span class="p">,</span> <span class="k">const</span> <span class="n">difference_type</span><span class="o">&</span><span class="p">);</span>
- <span class="n">Loc</span> <span class="k">operator</span><span class="o">+</span><span class="p">(</span><span class="k">const</span> <span class="n">Loc</span><span class="o">&</span><span class="p">,</span> <span class="k">const</span> <span class="n">difference_type</span><span class="o">&</span><span class="p">);</span>
- <span class="n">Loc</span> <span class="k">operator</span><span class="o">-</span><span class="p">(</span><span class="k">const</span> <span class="n">Loc</span><span class="o">&</span><span class="p">,</span> <span class="k">const</span> <span class="n">difference_type</span><span class="o">&</span><span class="p">);</span>
- <span class="n">reference</span> <span class="k">operator</span><span class="o">*</span><span class="p">(</span><span class="k">const</span> <span class="n">Loc</span><span class="o">&</span><span class="p">);</span>
- <span class="n">reference</span> <span class="k">operator</span><span class="p">[](</span><span class="k">const</span> <span class="n">Loc</span><span class="o">&</span><span class="p">,</span> <span class="k">const</span> <span class="n">difference_type</span><span class="o">&</span><span class="p">);</span>
- <span class="c1">// Storing relative location for faster repeated access and accessing it</span>
- <span class="n">cached_location_t</span> <span class="n">Loc</span><span class="o">::</span><span class="n">cache_location</span><span class="p">(</span><span class="k">const</span> <span class="n">difference_type</span><span class="o">&</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
- <span class="n">reference</span> <span class="k">operator</span><span class="p">[](</span><span class="k">const</span> <span class="n">Loc</span><span class="o">&</span><span class="p">,</span><span class="k">const</span> <span class="n">cached_location_t</span><span class="o">&</span><span class="p">);</span>
- <span class="c1">// Accessing iterators along a given dimension at the current location or at a given offset</span>
- <span class="k">template</span> <span class="o"><</span><span class="kt">size_t</span> <span class="n">D</span><span class="o">></span> <span class="n">axis</span><span class="o"><</span><span class="n">D</span><span class="o">>::</span><span class="n">iterator</span><span class="o">&</span> <span class="n">Loc</span><span class="o">::</span><span class="n">axis_iterator</span><span class="p">();</span>
- <span class="k">template</span> <span class="o"><</span><span class="kt">size_t</span> <span class="n">D</span><span class="o">></span> <span class="n">axis</span><span class="o"><</span><span class="n">D</span><span class="o">>::</span><span class="n">iterator</span> <span class="k">const</span><span class="o">&</span> <span class="n">Loc</span><span class="o">::</span><span class="n">axis_iterator</span><span class="p">()</span> <span class="k">const</span><span class="p">;</span>
- <span class="k">template</span> <span class="o"><</span><span class="kt">size_t</span> <span class="n">D</span><span class="o">></span> <span class="n">axis</span><span class="o"><</span><span class="n">D</span><span class="o">>::</span><span class="n">iterator</span> <span class="n">Loc</span><span class="o">::</span><span class="n">axis_iterator</span><span class="p">(</span><span class="k">const</span> <span class="n">difference_type</span><span class="o">&</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"><</span><span class="k">typename</span> <span class="n">Loc</span><span class="o">></span>
- <span class="n">concept</span> <span class="nl">MutableRandomAccessNDLocatorConcept</span>
- <span class="p">:</span> <span class="n">RandomAccessNDLocatorConcept</span><span class="o"><</span><span class="n">Loc</span><span class="o">></span>
- <span class="p">{</span>
- <span class="n">where</span> <span class="n">Mutable</span><span class="o"><</span><span class="n">reference</span><span class="o">></span><span class="p">;</span>
- <span class="p">};</span>
- </pre></div>
- </div>
- <p>Two-dimensional locators have additional requirements:</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">RandomAccess2DLocatorConcept</span><span class="o"><</span><span class="n">RandomAccessNDLocatorConcept</span> <span class="n">Loc</span><span class="o">></span>
- <span class="p">{</span>
- <span class="n">where</span> <span class="n">num_dimensions</span><span class="o">==</span><span class="mi">2</span><span class="p">;</span>
- <span class="n">where</span> <span class="n">Point2DConcept</span><span class="o"><</span><span class="n">point_t</span><span class="o">></span><span class="p">;</span>
- <span class="k">typename</span> <span class="n">x_iterator</span> <span class="o">=</span> <span class="n">axis</span><span class="o"><</span><span class="mi">0</span><span class="o">>::</span><span class="n">iterator</span><span class="p">;</span>
- <span class="k">typename</span> <span class="n">y_iterator</span> <span class="o">=</span> <span class="n">axis</span><span class="o"><</span><span class="mi">1</span><span class="o">>::</span><span class="n">iterator</span><span class="p">;</span>
- <span class="k">typename</span> <span class="n">x_coord_t</span> <span class="o">=</span> <span class="n">axis</span><span class="o"><</span><span class="mi">0</span><span class="o">>::</span><span class="n">coord_t</span><span class="p">;</span>
- <span class="k">typename</span> <span class="n">y_coord_t</span> <span class="o">=</span> <span class="n">axis</span><span class="o"><</span><span class="mi">1</span><span class="o">>::</span><span class="n">coord_t</span><span class="p">;</span>
- <span class="c1">// Only available to locators that have dynamic step in Y</span>
- <span class="c1">//Loc::Loc(const Loc& loc, y_coord_t);</span>
- <span class="c1">// Only available to locators that have dynamic step in X and Y</span>
- <span class="c1">//Loc::Loc(const Loc& loc, x_coord_t, y_coord_t, bool transposed=false);</span>
- <span class="n">x_iterator</span><span class="o">&</span> <span class="n">Loc</span><span class="o">::</span><span class="n">x</span><span class="p">();</span>
- <span class="n">x_iterator</span> <span class="k">const</span><span class="o">&</span> <span class="n">Loc</span><span class="o">::</span><span class="n">x</span><span class="p">()</span> <span class="k">const</span><span class="p">;</span>
- <span class="n">y_iterator</span><span class="o">&</span> <span class="n">Loc</span><span class="o">::</span><span class="n">y</span><span class="p">();</span>
- <span class="n">y_iterator</span> <span class="k">const</span><span class="o">&</span> <span class="n">Loc</span><span class="o">::</span><span class="n">y</span><span class="p">()</span> <span class="k">const</span><span class="p">;</span>
- <span class="n">x_iterator</span> <span class="n">Loc</span><span class="o">::</span><span class="n">x_at</span><span class="p">(</span><span class="k">const</span> <span class="n">difference_type</span><span class="o">&</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
- <span class="n">y_iterator</span> <span class="n">Loc</span><span class="o">::</span><span class="n">y_at</span><span class="p">(</span><span class="k">const</span> <span class="n">difference_type</span><span class="o">&</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
- <span class="n">Loc</span> <span class="n">Loc</span><span class="o">::</span><span class="n">xy_at</span><span class="p">(</span><span class="k">const</span> <span class="n">difference_type</span><span class="o">&</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
- <span class="c1">// x/y versions of all methods that can take difference type</span>
- <span class="n">x_iterator</span> <span class="n">Loc</span><span class="o">::</span><span class="n">x_at</span><span class="p">(</span><span class="n">x_coord_t</span><span class="p">,</span> <span class="n">y_coord_t</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
- <span class="n">y_iterator</span> <span class="n">Loc</span><span class="o">::</span><span class="n">y_at</span><span class="p">(</span><span class="n">x_coord_t</span><span class="p">,</span> <span class="n">y_coord_t</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
- <span class="n">Loc</span> <span class="n">Loc</span><span class="o">::</span><span class="n">xy_at</span><span class="p">(</span><span class="n">x_coord_t</span><span class="p">,</span> <span class="n">y_coord_t</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
- <span class="n">reference</span> <span class="nf">operator</span><span class="p">()(</span><span class="k">const</span> <span class="n">Loc</span><span class="o">&</span><span class="p">,</span> <span class="n">x_coord_t</span><span class="p">,</span> <span class="n">y_coord_t</span><span class="p">);</span>
- <span class="n">cached_location_t</span> <span class="n">Loc</span><span class="o">::</span><span class="n">cache_location</span><span class="p">(</span><span class="n">x_coord_t</span><span class="p">,</span> <span class="n">y_coord_t</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
- <span class="kt">bool</span> <span class="n">Loc</span><span class="o">::</span><span class="n">is_1d_traversable</span><span class="p">(</span><span class="n">x_coord_t</span> <span class="n">width</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
- <span class="n">y_coord_t</span> <span class="n">Loc</span><span class="o">::</span><span class="n">y_distance_to</span><span class="p">(</span><span class="k">const</span> <span class="n">Loc</span><span class="o">&</span> <span class="n">loc2</span><span class="p">,</span> <span class="n">x_coord_t</span> <span class="n">x_diff</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
- <span class="p">};</span>
- <span class="n">concept</span> <span class="n">MutableRandomAccess2DLocatorConcept</span><span class="o"><</span><span class="n">RandomAccess2DLocatorConcept</span> <span class="n">Loc</span><span class="o">></span>
- <span class="o">:</span> <span class="n">MutableRandomAccessNDLocatorConcept</span><span class="o"><</span><span class="n">Loc</span><span class="o">></span> <span class="p">{};</span>
- </pre></div>
- </div>
- <p>2D locators can have a dynamic step not just horizontally, but
- vertically. This gives rise to the Y equivalent of
- <code class="docutils literal"><span class="pre">HasDynamicXStepTypeConcept</span></code>:</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">HasDynamicYStepTypeConcept</span><span class="o"><</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span>
- <span class="p">{</span>
- <span class="k">typename</span> <span class="n">dynamic_y_step_type</span><span class="o"><</span><span class="n">T</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">dynamic_y_step_type</span><span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="o">></span><span class="p">;</span>
- <span class="p">};</span>
- </pre></div>
- </div>
- <p>All locators and image views that GIL provides model
- <code class="docutils literal"><span class="pre">HasDynamicYStepTypeConcept</span></code>.</p>
- <p>Sometimes it is necessary to swap the meaning of X and Y for a given locator
- or image view type (for example, GIL provides a function to transpose an image
- view). Such locators and views must be transposable:</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">HasTransposedTypeConcept</span><span class="o"><</span><span class="k">typename</span> <span class="n">T</span><span class="o">></span>
- <span class="p">{</span>
- <span class="k">typename</span> <span class="n">transposed_type</span><span class="o"><</span><span class="n">T</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">transposed_type</span><span class="o"><</span><span class="n">T</span><span class="o">></span> <span class="o">></span><span class="p">;</span>
- <span class="p">};</span>
- </pre></div>
- </div>
- <p>All GIL provided locators and views model <code class="docutils literal"><span class="pre">HasTransposedTypeConcept</span></code>.</p>
- <p>The locators GIL uses operate over models of <code class="docutils literal"><span class="pre">PixelConcept</span></code> and their x and
- y dimension types are the same. They model the following concept:</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="n">concept</span> <span class="n">PixelLocatorConcept</span><span class="o"><</span><span class="n">RandomAccess2DLocatorConcept</span> <span class="n">Loc</span><span class="o">></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>
- <span class="n">where</span> <span class="n">PixelIteratorConcept</span><span class="o"><</span><span class="n">x_iterator</span><span class="o">></span><span class="p">;</span>
- <span class="n">where</span> <span class="n">PixelIteratorConcept</span><span class="o"><</span><span class="n">y_iterator</span><span class="o">></span><span class="p">;</span>
- <span class="n">where</span> <span class="n">x_coord_t</span> <span class="o">==</span> <span class="n">y_coord_t</span><span class="p">;</span>
- <span class="k">typename</span> <span class="n">coord_t</span> <span class="o">=</span> <span class="n">x_coord_t</span><span class="p">;</span>
- <span class="p">};</span>
- <span class="n">concept</span> <span class="n">MutablePixelLocatorConcept</span><span class="o"><</span><span class="n">PixelLocatorConcept</span> <span class="n">Loc</span><span class="o">></span> <span class="o">:</span> <span class="n">MutableRandomAccess2DLocatorConcept</span><span class="o"><</span><span class="n">Loc</span><span class="o">></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_has_dynamic_y_step_type_concept.html">HasDynamicYStepTypeConcept<T></a></li>
- <li><a class="reference external" href="reference/structboost_1_1gil_1_1_has_transposed_type_concept.html">HasTransposedTypeConcept<T></a></li>
- <li><a class="reference external" href="reference/structboost_1_1gil_1_1_random_access_n_d_locator_concept.html">RandomAccessNDLocatorConcept<Locator></a></li>
- <li><a class="reference external" href="reference/structboost_1_1gil_1_1_mutable_random_access_n_d_locator_concept.html">MutableRandomAccessNDLocatorConcept<Locator></a></li>
- <li><a class="reference external" href="reference/structboost_1_1gil_1_1_random_access2_d_locator_concept.html">RandomAccess2DLocatorConcept<Locator></a></li>
- <li><a class="reference external" href="reference/structboost_1_1gil_1_1_mutable_random_access2_d_locator_concept.html">MutableRandomAccess2DLocatorConcept<Locator></a></li>
- <li><a class="reference external" href="reference/structboost_1_1gil_1_1_pixel_locator_concept.html">PixelLocatorConcept<Locator></a></li>
- <li><a class="reference external" href="reference/structboost_1_1gil_1_1_mutable_pixel_locator_concept.html">MutablePixelLocatorConcept<Locator></a></li>
- </ul>
- </div>
- </div>
- <div class="section" id="models">
- <h2><a class="toc-backref" href="#id2">Models</a></h2>
- <p>GIL provides two models of <code class="docutils literal"><span class="pre">PixelLocatorConcept</span></code> - a memory-based locator,
- <code class="docutils literal"><span class="pre">memory_based_2d_locator</span></code> and a virtual locator <code class="docutils literal"><span class="pre">virtual_2d_locator</span></code>.</p>
- <p>The <code class="docutils literal"><span class="pre">memory_based_2d_locator</span></code> is a locator over planar or interleaved images
- that have their pixels in memory. It takes a model of <code class="docutils literal"><span class="pre">StepIteratorConcept</span></code>
- over pixels as a template parameter. (When instantiated with a model of
- <code class="docutils literal"><span class="pre">MutableStepIteratorConcept</span></code>, it models <code class="docutils literal"><span class="pre">MutablePixelLocatorConcept</span></code>).</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="c1">// StepIterator models StepIteratorConcept, MemoryBasedIteratorConcept</span>
- <span class="k">template</span> <span class="o"><</span><span class="k">typename</span> <span class="n">StepIterator</span><span class="o">></span>
- <span class="k">class</span> <span class="nc">memory_based_2d_locator</span><span class="p">;</span>
- </pre></div>
- </div>
- <p>The step of <code class="docutils literal"><span class="pre">StepIterator</span></code> must be the number of memory units (bytes or
- bits) per row (thus it must be memunit advanceable). The class
- <code class="docutils literal"><span class="pre">memory_based_2d_locator</span></code> is a wrapper around <code class="docutils literal"><span class="pre">StepIterator</span></code> and uses it
- to navigate vertically, while its base iterator is used to navigate
- horizontally.</p>
- <p>Combining fundamental iterator and step iterator allows us to create locators
- that describe complex pixel memory organizations. First, we have a choice of
- iterator to use for horizontal direction, i.e. for iterating over the pixels
- on the same row. Using the fundamental and step iterators gives us four
- choices:</p>
- <ul class="simple">
- <li><code class="docutils literal"><span class="pre">pixel<T,C>*</span></code> - for interleaved images</li>
- <li><code class="docutils literal"><span class="pre">planar_pixel_iterator<T*,C></span></code> - for planar images</li>
- <li><code class="docutils literal"><span class="pre">memory_based_step_iterator<pixel<T,C>*></span></code> - for interleaved images with
- non-standard step)</li>
- <li><code class="docutils literal"><span class="pre">memory_based_step_iterator<planar_pixel_iterator<T*,C></span> <span class="pre">></span></code> - for planar
- images with non-standard step</li>
- </ul>
- <p>Of course, one could provide their own custom x-iterator. One such example
- described later is an iterator adaptor that performs color conversion when
- dereferenced.</p>
- <p>Given a horizontal iterator <code class="docutils literal"><span class="pre">XIterator</span></code>, we could choose the <code class="docutils literal"><span class="pre">y-iterator</span></code>,
- the iterator that moves along a column, as
- <code class="docutils literal"><span class="pre">memory_based_step_iterator<XIterator></span></code> with a step equal to the number of
- memory units (bytes or bits) per row. Again, one is free to provide their own
- y-iterator.</p>
- <p>Then we can instantiate
- <code class="docutils literal"><span class="pre">memory_based_2d_locator<memory_based_step_iterator<XIterator></span> <span class="pre">></span></code> to obtain
- a 2D pixel locator, as the diagram indicates:</p>
- <img alt="../_images/pixel_locator.gif" src="../_images/pixel_locator.gif" />
- <p>The <code class="docutils literal"><span class="pre">memory_based_2d_locator</span></code> also offers <cite>cached_location_t</cite> as mechanism
- to store relative locations for optimized repeated access of neighborhood
- pixels. The 2D coordinates of relative locations are cached as 1-dimensional
- raw byte offsets. This provides efficient access if a neighboring locations
- relative to a given locator are read or written frequently (e.g. in filters).</p>
- <p>The <code class="docutils literal"><span class="pre">virtual_2d_locator</span></code> is a locator that is instantiated with a function
- object invoked upon dereferencing a pixel. It returns the value of a pixel
- given its X,Y coordinates. Virtual locators can be used to implement virtual
- image views that can model any user-defined function. See the GIL tutorial for
- an example of using virtual locators to create a view of the Mandelbrot set.</p>
- <p>Both the virtual and the memory-based locators subclass from
- <code class="docutils literal"><span class="pre">pixel_2d_locator_base</span></code>, a base class that provides most of the interface
- required by <code class="docutils literal"><span class="pre">PixelLocatorConcept</span></code>. Users may find this base class useful if
- they need to provide other models of <code class="docutils literal"><span class="pre">PixelLocatorConcept</span></code>.</p>
- <p>Here is some sample code using locators:</p>
- <div class="highlight-cpp"><div class="highlight"><pre><span class="n">loc</span><span class="o">=</span><span class="n">img</span><span class="p">.</span><span class="n">xy_at</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span><span class="mi">10</span><span class="p">);</span> <span class="c1">// start at pixel (x=10,y=10)</span>
- <span class="n">above</span><span class="o">=</span><span class="n">loc</span><span class="p">.</span><span class="n">cache_location</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="o">-</span><span class="mi">1</span><span class="p">);</span> <span class="c1">// remember relative locations of neighbors above and below</span>
- <span class="n">below</span><span class="o">=</span><span class="n">loc</span><span class="p">.</span><span class="n">cache_location</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">1</span><span class="p">);</span>
- <span class="o">++</span><span class="n">loc</span><span class="p">.</span><span class="n">x</span><span class="p">();</span> <span class="c1">// move to (11,10)</span>
- <span class="n">loc</span><span class="p">.</span><span class="n">y</span><span class="p">()</span><span class="o">+=</span><span class="mi">15</span><span class="p">;</span> <span class="c1">// move to (11,25)</span>
- <span class="n">loc</span><span class="o">-=</span><span class="n">point</span><span class="o"><</span><span class="n">std</span><span class="o">::</span><span class="kt">ptrdiff_t</span><span class="o">></span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="mi">1</span><span class="p">);</span><span class="c1">// move to (10,24)</span>
- <span class="o">*</span><span class="n">loc</span><span class="o">=</span><span class="p">(</span><span class="n">loc</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="o">-</span><span class="mi">1</span><span class="p">)</span><span class="o">+</span><span class="n">loc</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="mi">1</span><span class="p">))</span><span class="o">/</span><span class="mi">2</span><span class="p">;</span> <span class="c1">// set pixel (10,24) to the average of (10,23) and (10,25) (grayscale pixels only)</span>
- <span class="o">*</span><span class="n">loc</span><span class="o">=</span><span class="p">(</span><span class="n">loc</span><span class="p">[</span><span class="n">above</span><span class="p">]</span><span class="o">+</span><span class="n">loc</span><span class="p">[</span><span class="n">below</span><span class="p">])</span><span class="o">/</span><span class="mi">2</span><span class="p">;</span> <span class="c1">// the same, but faster using cached relative neighbor locations</span>
- </pre></div>
- </div>
- <p>The standard GIL locators are fast and lightweight objects. For example, the
- locator for a simple interleaved image consists of one raw pointer to the
- pixel location plus one integer for the row size in bytes, for a total of
- 8 bytes. <code class="docutils literal"><span class="pre">++loc.x()</span></code> amounts to incrementing a raw pointer (or N pointers
- for planar images). Computing 2D offsets is slower as it requires
- multiplication and addition. Filters, for example, need to access the same
- neighbors for every pixel in the image, in which case the relative positions
- can be cached into a raw byte difference using <code class="docutils literal"><span class="pre">cache_location</span></code>.
- In the above example <code class="docutils literal"><span class="pre">loc[above]</span></code> for simple interleaved images amounts to a
- raw array index operator.</p>
- </div>
- <div class="section" id="iterator-over-2d-image">
- <h2><a class="toc-backref" href="#id3">Iterator over 2D image</a></h2>
- <p>Sometimes we want to perform the same, location-independent operation
- over all pixels of an image. In such a case it is useful to represent
- the pixels as a one-dimensional array. GIL’s <code class="docutils literal"><span class="pre">iterator_from_2d</span></code> is a
- random access traversal iterator that visits all pixels in an image in
- the natural memory-friendly order left-to-right inside
- top-to-bottom. It takes a locator, the width of the image and the
- current X position. This is sufficient information for it to determine
- when to do a “carriage return”. Synopsis:</p>
- <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">Locator</span><span class="o">></span> <span class="c1">// Models PixelLocatorConcept</span>
- <span class="k">class</span> <span class="nc">iterator_from_2d</span>
- <span class="p">{</span>
- <span class="k">public</span><span class="o">:</span>
- <span class="n">iterator_from_2d</span><span class="p">(</span><span class="k">const</span> <span class="n">Locator</span><span class="o">&</span> <span class="n">loc</span><span class="p">,</span> <span class="kt">int</span> <span class="n">x</span><span class="p">,</span> <span class="kt">int</span> <span class="n">width</span><span class="p">);</span>
- <span class="n">iterator_from_2d</span><span class="o">&</span> <span class="k">operator</span><span class="o">++</span><span class="p">();</span> <span class="c1">// if (++_x<_width) ++_p.x(); else _p+=point_t(-_width,1);</span>
- <span class="p">...</span>
- <span class="k">private</span><span class="o">:</span>
- <span class="kt">int</span> <span class="n">_x</span><span class="p">,</span> <span class="n">_width</span><span class="p">;</span>
- <span class="n">Locator</span> <span class="n">_p</span><span class="p">;</span>
- <span class="p">};</span>
- </pre></div>
- </div>
- <p>Iterating through the pixels in an image using <code class="docutils literal"><span class="pre">iterator_from_2d</span></code> is slower
- than going through all rows and using the x-iterator at each row. This is
- because two comparisons are done per iteration step - one for the end
- condition of the loop using the iterators, and one inside
- <code class="docutils literal"><span class="pre">iterator_from_2d::operator++</span></code> to determine whether we are at the end of a
- row. For fast operations, such as pixel copy, this second check adds about
- 15% performance delay (measured for interleaved images on Intel platform).
- GIL overrides some STL algorithms, such as <code class="docutils literal"><span class="pre">std::copy</span></code> and <code class="docutils literal"><span class="pre">std::fill</span></code>,
- when invoked with <code class="docutils literal"><span class="pre">iterator_from_2d</span></code>-s, to go through each row using their
- base x-iterators, and, if the image has no padding (i.e.
- <code class="docutils literal"><span class="pre">iterator_from_2d::is_1d_traversable()</span></code> returns true) to simply iterate
- using the x-iterators directly.</p>
- </div>
- </div>
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