11 A Locator allows for navigation in two or more dimensions. Locators are
12 N-dimensional iterators in spirit, but we use a different name because they
13 don't satisfy all the requirements of iterators. For example, they don't
14 supply increment and decrement operators because it is unclear which dimension
15 the operators should advance along.
16 N-dimensional locators model the following concept:
20 concept RandomAccessNDLocatorConcept<Regular Loc>
22 typename value_type; // value over which the locator navigates
23 typename reference; // result of dereferencing
24 typename difference_type; where PointNDConcept<difference_type>; // return value of operator-.
25 typename const_t; // same as Loc, but operating over immutable values
26 typename cached_location_t; // type to store relative location (for efficient repeated access)
27 typename point_t = difference_type;
29 static const size_t num_dimensions; // dimensionality of the locator
30 where num_dimensions = point_t::num_dimensions;
32 // The difference_type and iterator type along each dimension. The iterators may only differ in
33 // difference_type. Their value_type must be the same as Loc::value_type
34 template <size_t D> struct axis {
35 typename coord_t = point_t::axis<D>::coord_t;
36 typename iterator; where RandomAccessTraversalConcept<iterator>; // iterator along D-th axis.
37 where iterator::value_type == value_type;
40 // Defines the type of a locator similar to this type, except it invokes Deref upon dereferencing
41 template <PixelDereferenceAdaptorConcept Deref> struct add_deref {
42 typename type; where RandomAccessNDLocatorConcept<type>;
43 static type make(const Loc& loc, const Deref& deref);
46 Loc& operator+=(Loc&, const difference_type&);
47 Loc& operator-=(Loc&, const difference_type&);
48 Loc operator+(const Loc&, const difference_type&);
49 Loc operator-(const Loc&, const difference_type&);
51 reference operator*(const Loc&);
52 reference operator[](const Loc&, const difference_type&);
54 // Storing relative location for faster repeated access and accessing it
55 cached_location_t Loc::cache_location(const difference_type&) const;
56 reference operator[](const Loc&,const cached_location_t&);
58 // Accessing iterators along a given dimension at the current location or at a given offset
59 template <size_t D> axis<D>::iterator& Loc::axis_iterator();
60 template <size_t D> axis<D>::iterator const& Loc::axis_iterator() const;
61 template <size_t D> axis<D>::iterator Loc::axis_iterator(const difference_type&) const;
64 template <typename Loc>
65 concept MutableRandomAccessNDLocatorConcept
66 : RandomAccessNDLocatorConcept<Loc>
68 where Mutable<reference>;
71 Two-dimensional locators have additional requirements:
75 concept RandomAccess2DLocatorConcept<RandomAccessNDLocatorConcept Loc>
77 where num_dimensions==2;
78 where Point2DConcept<point_t>;
80 typename x_iterator = axis<0>::iterator;
81 typename y_iterator = axis<1>::iterator;
82 typename x_coord_t = axis<0>::coord_t;
83 typename y_coord_t = axis<1>::coord_t;
85 // Only available to locators that have dynamic step in Y
86 //Loc::Loc(const Loc& loc, y_coord_t);
88 // Only available to locators that have dynamic step in X and Y
89 //Loc::Loc(const Loc& loc, x_coord_t, y_coord_t, bool transposed=false);
92 x_iterator const& Loc::x() const;
94 y_iterator const& Loc::y() const;
96 x_iterator Loc::x_at(const difference_type&) const;
97 y_iterator Loc::y_at(const difference_type&) const;
98 Loc Loc::xy_at(const difference_type&) const;
100 // x/y versions of all methods that can take difference type
101 x_iterator Loc::x_at(x_coord_t, y_coord_t) const;
102 y_iterator Loc::y_at(x_coord_t, y_coord_t) const;
103 Loc Loc::xy_at(x_coord_t, y_coord_t) const;
104 reference operator()(const Loc&, x_coord_t, y_coord_t);
105 cached_location_t Loc::cache_location(x_coord_t, y_coord_t) const;
107 bool Loc::is_1d_traversable(x_coord_t width) const;
108 y_coord_t Loc::y_distance_to(const Loc& loc2, x_coord_t x_diff) const;
111 concept MutableRandomAccess2DLocatorConcept<RandomAccess2DLocatorConcept Loc>
112 : MutableRandomAccessNDLocatorConcept<Loc> {};
114 2D locators can have a dynamic step not just horizontally, but
115 vertically. This gives rise to the Y equivalent of
116 ``HasDynamicXStepTypeConcept``:
120 concept HasDynamicYStepTypeConcept<typename T>
122 typename dynamic_y_step_type<T>;
123 where Metafunction<dynamic_y_step_type<T> >;
126 All locators and image views that GIL provides model
127 ``HasDynamicYStepTypeConcept``.
129 Sometimes it is necessary to swap the meaning of X and Y for a given locator
130 or image view type (for example, GIL provides a function to transpose an image
131 view). Such locators and views must be transposable:
135 concept HasTransposedTypeConcept<typename T>
137 typename transposed_type<T>;
138 where Metafunction<transposed_type<T> >;
141 All GIL provided locators and views model ``HasTransposedTypeConcept``.
143 The locators GIL uses operate over models of ``PixelConcept`` and their x and
144 y dimension types are the same. They model the following concept:
148 concept PixelLocatorConcept<RandomAccess2DLocatorConcept Loc>
150 where PixelValueConcept<value_type>;
151 where PixelIteratorConcept<x_iterator>;
152 where PixelIteratorConcept<y_iterator>;
153 where x_coord_t == y_coord_t;
155 typename coord_t = x_coord_t;
158 concept MutablePixelLocatorConcept<PixelLocatorConcept Loc> : MutableRandomAccess2DLocatorConcept<Loc> {};
162 - `HasDynamicYStepTypeConcept<T> <reference/structboost_1_1gil_1_1_has_dynamic_y_step_type_concept.html>`_
163 - `HasTransposedTypeConcept<T> <reference/structboost_1_1gil_1_1_has_transposed_type_concept.html>`_
164 - `RandomAccessNDLocatorConcept<Locator> <reference/structboost_1_1gil_1_1_random_access_n_d_locator_concept.html>`_
165 - `MutableRandomAccessNDLocatorConcept<Locator> <reference/structboost_1_1gil_1_1_mutable_random_access_n_d_locator_concept.html>`_
166 - `RandomAccess2DLocatorConcept<Locator> <reference/structboost_1_1gil_1_1_random_access2_d_locator_concept.html>`_
167 - `MutableRandomAccess2DLocatorConcept<Locator> <reference/structboost_1_1gil_1_1_mutable_random_access2_d_locator_concept.html>`_
168 - `PixelLocatorConcept<Locator> <reference/structboost_1_1gil_1_1_pixel_locator_concept.html>`_
169 - `MutablePixelLocatorConcept<Locator> <reference/structboost_1_1gil_1_1_mutable_pixel_locator_concept.html>`_
174 GIL provides two models of ``PixelLocatorConcept`` - a memory-based locator,
175 ``memory_based_2d_locator`` and a virtual locator ``virtual_2d_locator``.
177 The ``memory_based_2d_locator`` is a locator over planar or interleaved images
178 that have their pixels in memory. It takes a model of ``StepIteratorConcept``
179 over pixels as a template parameter. (When instantiated with a model of
180 ``MutableStepIteratorConcept``, it models ``MutablePixelLocatorConcept``).
184 // StepIterator models StepIteratorConcept, MemoryBasedIteratorConcept
185 template <typename StepIterator>
186 class memory_based_2d_locator;
188 The step of ``StepIterator`` must be the number of memory units (bytes or
189 bits) per row (thus it must be memunit advanceable). The class
190 ``memory_based_2d_locator`` is a wrapper around ``StepIterator`` and uses it
191 to navigate vertically, while its base iterator is used to navigate
194 Combining fundamental iterator and step iterator allows us to create locators
195 that describe complex pixel memory organizations. First, we have a choice of
196 iterator to use for horizontal direction, i.e. for iterating over the pixels
197 on the same row. Using the fundamental and step iterators gives us four
200 - ``pixel<T,C>*`` - for interleaved images
201 - ``planar_pixel_iterator<T*,C>`` - for planar images
202 - ``memory_based_step_iterator<pixel<T,C>*>`` - for interleaved images with
204 - ``memory_based_step_iterator<planar_pixel_iterator<T*,C> >`` - for planar
205 images with non-standard step
207 Of course, one could provide their own custom x-iterator. One such example
208 described later is an iterator adaptor that performs color conversion when
211 Given a horizontal iterator ``XIterator``, we could choose the ``y-iterator``,
212 the iterator that moves along a column, as
213 ``memory_based_step_iterator<XIterator>`` with a step equal to the number of
214 memory units (bytes or bits) per row. Again, one is free to provide their own
217 Then we can instantiate
218 ``memory_based_2d_locator<memory_based_step_iterator<XIterator> >`` to obtain
219 a 2D pixel locator, as the diagram indicates:
221 .. image:: ../images/pixel_locator.gif
223 The ``memory_based_2d_locator`` also offers `cached_location_t` as mechanism
224 to store relative locations for optimized repeated access of neighborhood
225 pixels. The 2D coordinates of relative locations are cached as 1-dimensional
226 raw byte offsets. This provides efficient access if a neighboring locations
227 relative to a given locator are read or written frequently (e.g. in filters).
229 The ``virtual_2d_locator`` is a locator that is instantiated with a function
230 object invoked upon dereferencing a pixel. It returns the value of a pixel
231 given its X,Y coordinates. Virtual locators can be used to implement virtual
232 image views that can model any user-defined function. See the GIL tutorial for
233 an example of using virtual locators to create a view of the Mandelbrot set.
235 Both the virtual and the memory-based locators subclass from
236 ``pixel_2d_locator_base``, a base class that provides most of the interface
237 required by ``PixelLocatorConcept``. Users may find this base class useful if
238 they need to provide other models of ``PixelLocatorConcept``.
240 Here is some sample code using locators:
244 loc=img.xy_at(10,10); // start at pixel (x=10,y=10)
245 above=loc.cache_location(0,-1); // remember relative locations of neighbors above and below
246 below=loc.cache_location(0, 1);
247 ++loc.x(); // move to (11,10)
248 loc.y()+=15; // move to (11,25)
249 loc-=point<std::ptrdiff_t>(1,1);// move to (10,24)
250 *loc=(loc(0,-1)+loc(0,1))/2; // set pixel (10,24) to the average of (10,23) and (10,25) (grayscale pixels only)
251 *loc=(loc[above]+loc[below])/2; // the same, but faster using cached relative neighbor locations
253 The standard GIL locators are fast and lightweight objects. For example, the
254 locator for a simple interleaved image consists of one raw pointer to the
255 pixel location plus one integer for the row size in bytes, for a total of
256 8 bytes. ``++loc.x()`` amounts to incrementing a raw pointer (or N pointers
257 for planar images). Computing 2D offsets is slower as it requires
258 multiplication and addition. Filters, for example, need to access the same
259 neighbors for every pixel in the image, in which case the relative positions
260 can be cached into a raw byte difference using ``cache_location``.
261 In the above example ``loc[above]`` for simple interleaved images amounts to a
262 raw array index operator.
264 Iterator over 2D image
265 ----------------------
267 Sometimes we want to perform the same, location-independent operation
268 over all pixels of an image. In such a case it is useful to represent
269 the pixels as a one-dimensional array. GIL's ``iterator_from_2d`` is a
270 random access traversal iterator that visits all pixels in an image in
271 the natural memory-friendly order left-to-right inside
272 top-to-bottom. It takes a locator, the width of the image and the
273 current X position. This is sufficient information for it to determine
274 when to do a "carriage return". Synopsis:
278 template <typename Locator> // Models PixelLocatorConcept
279 class iterator_from_2d
282 iterator_from_2d(const Locator& loc, int x, int width);
284 iterator_from_2d& operator++(); // if (++_x<_width) ++_p.x(); else _p+=point_t(-_width,1);
292 Iterating through the pixels in an image using ``iterator_from_2d`` is slower
293 than going through all rows and using the x-iterator at each row. This is
294 because two comparisons are done per iteration step - one for the end
295 condition of the loop using the iterators, and one inside
296 ``iterator_from_2d::operator++`` to determine whether we are at the end of a
297 row. For fast operations, such as pixel copy, this second check adds about
298 15% performance delay (measured for interleaved images on Intel platform).
299 GIL overrides some STL algorithms, such as ``std::copy`` and ``std::fill``,
300 when invoked with ``iterator_from_2d``-s, to go through each row using their
301 base x-iterators, and, if the image has no padding (i.e.
302 ``iterator_from_2d::is_1d_traversable()`` returns true) to simply iterate
303 using the x-iterators directly.