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Document purpose of cached_location_t [ci skip] (#287)

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Closes #130
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Mateusz Łoskot 2019-04-16 00:31:45 +02:00 committed by GitHub
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doc/design_guide.rst
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@ -1324,7 +1324,7 @@ GIL provides two models of ``PixelLocatorConcept`` - a memory-based
locator, ``memory_based_2d_locator`` and a virtual locator
``virtual_2d_locator``.
``memory_based_2d_locator`` is a locator over planar or interleaved
The ``memory_based_2d_locator`` is a locator over planar or interleaved
images that have their pixels in memory. It takes a model of
``StepIteratorConcept`` over pixels as a template parameter. (When
instantiated with a model of ``MutableStepIteratorConcept``, it models
@ -1368,7 +1368,13 @@ obtain a 2D pixel locator, as the diagram indicates:
.. image:: images/step_iterator.gif
``virtual_2d_locator`` is a locator that is instantiated with a
The ``memory_based_2d_locator`` also offers `cached_location_t` 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).
The ``virtual_2d_locator`` 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