C++ Boost

Color Space and Layout


A color space captures the set and interpretation of channels comprising a pixel. In Boost.GIL, color space is defined as an MPL random access sequence containing the types of all elements in the color space.

Two color spaces are considered compatible if they are equal (i.e. have the same set of colors in the same order).


GIL currently provides the following color spaces:

  • gray_t
  • rgb_t
  • rgba_t
  • cmyk_t

It also provides unnamed N-channel color spaces of two to five channels:

  • devicen_t<2>
  • devicen_t<3>
  • devicen_t<4>
  • devicen_t<5>

Besides the standard layouts, it also provides:

  • bgr_layout_t
  • bgra_layout_t
  • abgr_layout_t
  • argb_layout_t

As an example, here is how GIL defines the RGBA color space:

.. code-block:: cpp
struct red_t {}; struct green_t {}; struct blue_t {}; struct alpha_t {}; rgba_t = using mpl::vector4<red_t, green_t, blue_t, alpha_t>;

The ordering of the channels in the color space definition specifies their semantic order. For example, red_t is the first semantic channel of rgba_t. While there is a unique semantic ordering of the channels in a color space, channels may vary in their physical ordering in memory

The mapping of channels is specified by ChannelMappingConcept, which is an MPL random access sequence of integral types. A color space and its associated mapping are often used together.

Thus they are grouped in GIL’s layout:

    typename ColorSpace,
    typename ChannelMapping = mpl::range_c<int, 0, mpl::size<ColorSpace>::value>
struct layout
  using color_space_t = ColorSpace;
  using channel_mapping_t = ChannelMapping;

Here is how to create layouts for the RGBA color space:

using rgba_layout_t = layout<rgba_t>; // default ordering is 0,1,2,3...
using bgra_layout_t = layout<rgba_t, mpl::vector4_c<int,2,1,0,3>>;
using argb_layout_t = layout<rgba_t, mpl::vector4_c<int,1,2,3,0>>;
using abgr_layout_t = layout<rgba_t, mpl::vector4_c<int,3,2,1,0>>;