C++ Boost

Pixel Iterator


Pixel iterators are random traversal iterators whose value_type models PixelValueConcept.

Fundamental Iterator

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:

concept PixelIteratorConcept<RandomAccessTraversalIteratorConcept Iterator>
    : PixelBasedConcept<Iterator>
  where PixelValueConcept<value_type>;
  typename const_iterator_type<It>::type;
      where PixelIteratorConcept<const_iterator_type<It>::type>;
  static const bool  iterator_is_mutable<It>::value;
  static const bool  is_iterator_adaptor<It>::value;   // is it an iterator adaptor

template <typename Iterator>
concept MutablePixelIteratorConcept : PixelIteratorConcept<Iterator>, MutableRandomAccessIteratorConcept<Iterator> {};


A built-in pointer to pixel, pixel<ChannelValue,Layout>*, is GIL model for pixel iterator over interleaved homogeneous pixels. Similarly, packed_pixel<PixelData,ChannelRefVec,Layout>* is GIL model for an iterator over interleaved packed pixels.

For planar homogeneous pixels, GIL provides the class planar_pixel_iterator, 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:

template <typename ChannelPtr, typename ColorSpace>
struct planar_pixel_iterator;

// GIL provided typedefs
typedef planar_pixel_iterator<const bits8*, rgb_t> rgb8c_planar_ptr_t;
typedef planar_pixel_iterator<      bits8*, rgb_t> rgb8_planar_ptr_t;

planar_pixel_iterator also models HomogeneousColorBaseConcept (it subclasses from homogeneous_color_base) 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 operator++ of planar iterators approximately like this:

template <typename T>
struct inc : public std::unary_function<T,T>
  T operator()(T x) const { return ++x; }

template <typename ChannelPtr, typename ColorSpace>
  return *this;

Since static_transform uses compile-time recursion, incrementing an instance of rgb8_planar_ptr_t amounts to three pointer increments. GIL also uses the class bit_aligned_pixel_iterator as a model for a pixel iterator over bit-aligned pixels. Internally it keeps track of the current byte and the bit offset.

Iterator Adaptor

Iterator adaptor is an iterator that wraps around another iterator. Its is_iterator_adaptor 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:

concept IteratorAdaptorConcept<RandomAccessTraversalIteratorConcept Iterator>
  where SameType<is_iterator_adaptor<Iterator>::type, mpl::true_>;

  typename iterator_adaptor_get_base<Iterator>;
      where Metafunction<iterator_adaptor_get_base<Iterator> >;
      where boost_concepts::ForwardTraversalConcept<iterator_adaptor_get_base<Iterator>::type>;

  typename another_iterator;
  typename iterator_adaptor_rebind<Iterator,another_iterator>::type;
      where boost_concepts::ForwardTraversalConcept<another_iterator>;
      where IteratorAdaptorConcept<iterator_adaptor_rebind<Iterator,another_iterator>::type>;

  const iterator_adaptor_get_base<Iterator>::type& Iterator::base() const;

template <boost_concepts::Mutable_ForwardIteratorConcept Iterator>
concept MutableIteratorAdaptorConcept : IteratorAdaptorConcept<Iterator> {};


GIL provides several models of IteratorAdaptorConcept:

  • memory_based_step_iterator<Iterator>: An iterator adaptor that changes the fundamental step of the base iterator (see design/pixel_iterator:Step Iterator)
  • dereference_iterator_adaptor<Iterator,Fn>: An iterator that applies a unary function Fn upon dereferencing. It is used, for example, for on-the-fly color conversion. It can be used to construct a shallow image “view” that pretends to have a different color space or channel depth. See Image View for more. The unary function Fn must model PixelDereferenceAdaptorConcept (see below).

Pixel Dereference Adaptor

Pixel dereference adaptor is a unary function that can be applied upon dereferencing a pixel iterator. Its argument type could be anything (usually a PixelConcept) and the result type must be convertible to PixelConcept:

template <boost::UnaryFunctionConcept D>
concept PixelDereferenceAdaptorConcept:
  typename const_t;         where PixelDereferenceAdaptorConcept<const_t>;
  typename value_type;      where PixelValueConcept<value_type>;
  typename reference;       where PixelConcept<remove_reference<reference>::type>;  // may be mutable
  typename const_reference;   // must not be mutable
  static const bool D::is_mutable;

  where Convertible<value_type, result_type>;


GIL provides several models of PixelDereferenceAdaptorConcept:

  • color_convert_deref_fn: a function object that performs color conversion
  • detail::nth_channel_deref_fn: a function object that returns a grayscale pixel corresponding to the n-th channel of a given pixel
  • deref_compose: a function object that composes two models of PixelDereferenceAdaptorConcept. Similar to std::unary_compose, except it needs to pull the additional typedefs required by PixelDereferenceAdaptorConcept

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. dereference_iterator_adaptor<Iterator,Fn> is an iterator wrapper over a pixel iterator Iterator that invokes the given dereference iterator adaptor Fn upon dereferencing.

Step Iterator

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:

  • a single-channel view of the red channel of an RGB interleaved image
  • left-to-right flipped image (step = -fundamental_step)
  • subsampled view, taking every N-th pixel (step = N*fundamental_step)
  • traversal in vertical direction (step = number of bytes per row)
  • any combination of the above (steps are multiplied)

Step iterators are forward traversal iterators that allow changing the step between adjacent values:

concept StepIteratorConcept<boost_concepts::ForwardTraversalConcept Iterator>
  template <Integral D> void Iterator::set_step(D step);

concept MutableStepIteratorConcept<boost_concepts::Mutable_ForwardIteratorConcept Iterator>
    : StepIteratorConcept<Iterator>

GIL currently provides a step iterator whose value_type models PixelValueConcept. 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.

To advance in bytes/bits, the base iterator must model MemoryBasedIteratorConcept. 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. memunit_advanced and memunit_advanced_ref have a default implementation but some iterators may supply a more efficient version:

concept MemoryBasedIteratorConcept
    boost_concepts::RandomAccessTraversalConcept Iterator
  typename byte_to_memunit<Iterator>; where metafunction<byte_to_memunit<Iterator> >;
  std::ptrdiff_t      memunit_step(const Iterator&);
  std::ptrdiff_t      memunit_distance(const Iterator& , const Iterator&);
  void                memunit_advance(Iterator&, std::ptrdiff_t diff);
  Iterator            memunit_advanced(const Iterator& p, std::ptrdiff_t diff) { Iterator tmp; memunit_advance(tmp,diff); return tmp; }
  Iterator::reference memunit_advanced_ref(const Iterator& p, std::ptrdiff_t diff) { return *memunit_advanced(p,diff); }

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:

concept HasDynamicXStepTypeConcept<typename T>
  typename dynamic_x_step_type<T>;
      where Metafunction<dynamic_x_step_type<T> >;

All models of pixel iterators, locators and image views that GIL provides support HasDynamicXStepTypeConcept.


All standard memory-based iterators GIL currently provides model MemoryBasedIteratorConcept. GIL provides the class memory_based_step_iterator which models PixelIteratorConcept, StepIteratorConcept, and MemoryBasedIteratorConcept. It takes the base iterator as a template parameter (which must model PixelIteratorConcept and MemoryBasedIteratorConcept) and allows changing the step dynamically. GIL implementation contains the base iterator and a ptrdiff_t 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:

// Iterator models MemoryBasedIteratorConcept, HasDynamicXStepTypeConcept
template <typename Iterator>
typename dynamic_x_step_type<Iterator>::type make_step_iterator(Iterator const& it, std::ptrdiff_t step);

GIL also provides a model of an iterator over a virtual array of pixels, position_iterator. 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 PixelIteratorConcept and StepIteratorConcept but not MemoryBasedIteratorConcept.