11 Pixel iterators are random traversal iterators whose ``value_type
12 models`` ``PixelValueConcept``.
17 Pixel iterators provide metafunctions to determine whether they are mutable
18 (i.e. whether they allow for modifying the pixel they refer to), to get the
19 immutable (read-only) type of the iterator, and to determine whether they are
20 plain iterators or adaptors over another pixel iterator:
24 concept PixelIteratorConcept<RandomAccessTraversalIteratorConcept Iterator>
25 : PixelBasedConcept<Iterator>
27 where PixelValueConcept<value_type>;
28 typename const_iterator_type<It>::type;
29 where PixelIteratorConcept<const_iterator_type<It>::type>;
30 static const bool iterator_is_mutable<It>::value;
31 static const bool is_iterator_adaptor<It>::value; // is it an iterator adaptor
34 template <typename Iterator>
35 concept MutablePixelIteratorConcept : PixelIteratorConcept<Iterator>, MutableRandomAccessIteratorConcept<Iterator> {};
39 - `PixelIteratorConcept<Iterator> <reference/group___pixel_iterator_concept_pixel_iterator.html>`_
40 - `MutablePixelIteratorConcept<Iterator> <reference/structboost_1_1gil_1_1_mutable_pixel_iterator_concept.html>`_
45 A built-in pointer to pixel, ``pixel<ChannelValue,Layout>*``, is GIL model for
46 pixel iterator over interleaved homogeneous pixels. Similarly,
47 ``packed_pixel<PixelData,ChannelRefVec,Layout>*`` is GIL model for an iterator
48 over interleaved packed pixels.
50 For planar homogeneous pixels, GIL provides the class
51 ``planar_pixel_iterator``, templated over a channel iterator and color space.
52 Here is how the standard mutable and read-only planar RGB iterators over
53 unsigned char are defined:
57 template <typename ChannelPtr, typename ColorSpace>
58 struct planar_pixel_iterator;
60 // GIL provided typedefs
61 typedef planar_pixel_iterator<const bits8*, rgb_t> rgb8c_planar_ptr_t;
62 typedef planar_pixel_iterator< bits8*, rgb_t> rgb8_planar_ptr_t;
64 ``planar_pixel_iterator`` also models ``HomogeneousColorBaseConcept`` (it
65 subclasses from ``homogeneous_color_base``) and, as a result, all color base
66 algorithms apply to it. The element type of its color base is a channel
67 iterator. For example, GIL implements ``operator++`` of planar iterators
68 approximately like this:
73 struct inc : public std::unary_function<T,T>
75 T operator()(T x) const { return ++x; }
78 template <typename ChannelPtr, typename ColorSpace>
79 planar_pixel_iterator<ChannelPtr,ColorSpace>&
80 planar_pixel_iterator<ChannelPtr,ColorSpace>::operator++()
82 static_transform(*this,*this,inc<ChannelPtr>());
86 Since ``static_transform`` uses compile-time recursion, incrementing an
87 instance of ``rgb8_planar_ptr_t`` amounts to three pointer increments.
88 GIL also uses the class ``bit_aligned_pixel_iterator`` as a model for a pixel
89 iterator over bit-aligned pixels. Internally it keeps track of the current
90 byte and the bit offset.
95 Iterator adaptor is an iterator that wraps around another iterator. Its
96 ``is_iterator_adaptor`` metafunction must evaluate to true, and it needs to
97 provide a member method to return the base iterator, a metafunction to get its
98 type, and a metafunction to rebind to another base iterator:
102 concept IteratorAdaptorConcept<RandomAccessTraversalIteratorConcept Iterator>
104 where SameType<is_iterator_adaptor<Iterator>::type, mpl::true_>;
106 typename iterator_adaptor_get_base<Iterator>;
107 where Metafunction<iterator_adaptor_get_base<Iterator> >;
108 where boost_concepts::ForwardTraversalConcept<iterator_adaptor_get_base<Iterator>::type>;
110 typename another_iterator;
111 typename iterator_adaptor_rebind<Iterator,another_iterator>::type;
112 where boost_concepts::ForwardTraversalConcept<another_iterator>;
113 where IteratorAdaptorConcept<iterator_adaptor_rebind<Iterator,another_iterator>::type>;
115 const iterator_adaptor_get_base<Iterator>::type& Iterator::base() const;
118 template <boost_concepts::Mutable_ForwardIteratorConcept Iterator>
119 concept MutableIteratorAdaptorConcept : IteratorAdaptorConcept<Iterator> {};
123 - `IteratorAdaptorConcept<Iterator> <reference/structboost_1_1gil_1_1_iterator_adaptor_concept.html>`_
124 - `MutableIteratorAdaptorConcept<Iterator> <reference/structboost_1_1gil_1_1_mutable_iterator_adaptor_concept.html>`_
129 GIL provides several models of ``IteratorAdaptorConcept``:
131 - ``memory_based_step_iterator<Iterator>``: An iterator adaptor that changes
132 the fundamental step of the base iterator
133 (see :ref:`design/pixel_iterator:Step Iterator`)
135 - ``dereference_iterator_adaptor<Iterator,Fn>``: An iterator that applies a
136 unary function ``Fn`` upon dereferencing. It is used, for example, for
137 on-the-fly color conversion. It can be used to construct a shallow image
138 "view" that pretends to have a different color space or channel depth.
139 See :doc:`image_view` for more. The unary function ``Fn`` must
140 model ``PixelDereferenceAdaptorConcept`` (see below).
142 Pixel Dereference Adaptor
143 -------------------------
145 Pixel dereference adaptor is a unary function that can be applied upon
146 dereferencing a pixel iterator. Its argument type could be anything (usually a
147 ``PixelConcept``) and the result type must be convertible to ``PixelConcept``:
151 template <boost::UnaryFunctionConcept D>
152 concept PixelDereferenceAdaptorConcept:
153 DefaultConstructibleConcept<D>,
154 CopyConstructibleConcept<D>,
157 typename const_t; where PixelDereferenceAdaptorConcept<const_t>;
158 typename value_type; where PixelValueConcept<value_type>;
159 typename reference; where PixelConcept<remove_reference<reference>::type>; // may be mutable
160 typename const_reference; // must not be mutable
161 static const bool D::is_mutable;
163 where Convertible<value_type, result_type>;
169 GIL provides several models of ``PixelDereferenceAdaptorConcept``:
171 * ``color_convert_deref_fn``: a function object that performs color conversion
173 * ``detail::nth_channel_deref_fn``: a function object that returns a grayscale
174 pixel corresponding to the n-th channel of a given pixel
176 * ``deref_compose``: a function object that composes two models of
177 ``PixelDereferenceAdaptorConcept``. Similar to ``std::unary_compose``,
178 except it needs to pull the additional typedefs required by
179 ``PixelDereferenceAdaptorConcept``
181 GIL uses pixel dereference adaptors to implement image views that perform
182 color conversion upon dereferencing, or that return the N-th channel of the
183 underlying pixel. They can be used to model virtual image views that perform
184 an arbitrary function upon dereferencing, for example a view of the Mandelbrot
185 set. ``dereference_iterator_adaptor<Iterator,Fn>`` is an iterator wrapper over
186 a pixel iterator ``Iterator`` that invokes the given dereference iterator
187 adaptor ``Fn`` upon dereferencing.
192 Sometimes we want to traverse pixels with a unit step other than the one
193 provided by the fundamental pixel iterators. Examples where this would be
196 * a single-channel view of the red channel of an RGB interleaved image
197 * left-to-right flipped image (step = -fundamental_step)
198 * subsampled view, taking every N-th pixel (step = N*fundamental_step)
199 * traversal in vertical direction (step = number of bytes per row)
200 * any combination of the above (steps are multiplied)
202 Step iterators are forward traversal iterators that allow changing the step
203 between adjacent values:
207 concept StepIteratorConcept<boost_concepts::ForwardTraversalConcept Iterator>
209 template <Integral D> void Iterator::set_step(D step);
212 concept MutableStepIteratorConcept<boost_concepts::Mutable_ForwardIteratorConcept Iterator>
213 : StepIteratorConcept<Iterator>
216 GIL currently provides a step iterator whose ``value_type models``
217 ``PixelValueConcept``. In addition, the step is specified in memory units
218 (which are bytes or bits). This is necessary, for example, when implementing
219 an iterator navigating along a column of pixels - the size of a row of pixels
220 may sometimes not be divisible by the size of a pixel; for example rows may be
223 To advance in bytes/bits, the base iterator must model
224 ``MemoryBasedIteratorConcept``. A memory-based iterator has an inherent memory
225 unit, which is either a bit or a byte. It must supply functions returning the
226 number of bits per memory unit (1 or 8), the current step in memory units, the
227 memory-unit distance between two iterators, and a reference a given distance
228 in memunits away. It must also supply a function that advances an iterator a
229 given distance in memory units. ``memunit_advanced`` and
230 ``memunit_advanced_ref`` have a default implementation but some iterators may
231 supply a more efficient version:
235 concept MemoryBasedIteratorConcept
237 boost_concepts::RandomAccessTraversalConcept Iterator
240 typename byte_to_memunit<Iterator>; where metafunction<byte_to_memunit<Iterator> >;
241 std::ptrdiff_t memunit_step(const Iterator&);
242 std::ptrdiff_t memunit_distance(const Iterator& , const Iterator&);
243 void memunit_advance(Iterator&, std::ptrdiff_t diff);
244 Iterator memunit_advanced(const Iterator& p, std::ptrdiff_t diff) { Iterator tmp; memunit_advance(tmp,diff); return tmp; }
245 Iterator::reference memunit_advanced_ref(const Iterator& p, std::ptrdiff_t diff) { return *memunit_advanced(p,diff); }
248 It is useful to be able to construct a step iterator over another iterator.
249 More generally, given a type, we want to be able to construct an equivalent
250 type that allows for dynamically specified horizontal step:
254 concept HasDynamicXStepTypeConcept<typename T>
256 typename dynamic_x_step_type<T>;
257 where Metafunction<dynamic_x_step_type<T> >;
260 All models of pixel iterators, locators and image views that GIL provides
261 support ``HasDynamicXStepTypeConcept``.
265 - `StepIteratorConcept<Iterator> <reference/structboost_1_1gil_1_1_step_iterator_concept.html>`_
266 - `MutableStepIteratorConcept<Iterator> <reference/structboost_1_1gil_1_1_mutable_step_iterator_concept.html>`_
267 - `MemoryBasedIteratorConcept<Iterator> <reference/structboost_1_1gil_1_1_memory_based_iterator_concept.html>`_
268 - `HasDynamicXStepTypeConcept<T> <reference/structboost_1_1gil_1_1_has_dynamic_x_step_type_concept.html>`_
273 All standard memory-based iterators GIL currently provides model
274 ``MemoryBasedIteratorConcept``. GIL provides the class
275 ``memory_based_step_iterator`` which models ``PixelIteratorConcept``,
276 ``StepIteratorConcept``, and ``MemoryBasedIteratorConcept``. It takes the base
277 iterator as a template parameter (which must model ``PixelIteratorConcept``
278 and ``MemoryBasedIteratorConcept``) and allows changing the step dynamically.
279 GIL implementation contains the base iterator and a ``ptrdiff_t`` denoting the
280 number of memory units (bytes or bits) to skip for a unit step. It may also be
281 used with a negative number. GIL provides a function to create a step iterator
282 from a base iterator and a step:
286 // Iterator models MemoryBasedIteratorConcept, HasDynamicXStepTypeConcept
287 template <typename Iterator>
288 typename dynamic_x_step_type<Iterator>::type make_step_iterator(Iterator const& it, std::ptrdiff_t step);
290 GIL also provides a model of an iterator over a virtual array of pixels,
291 ``position_iterator``. It is a step iterator that keeps track of the pixel
292 position and invokes a function object to get the value of the pixel upon
293 dereferencing. It models ``PixelIteratorConcept`` and ``StepIteratorConcept``
294 but not ``MemoryBasedIteratorConcept``.