.. _example_11:

(11) A 3-D RGB color cube
-------------------------

In this example we generate a series of 6 color images, arranged so that
they can be cut out and assembled into a 3-D color cube. The six faces
of the cube represent the outside of the R-G-B color space. On each face
one of the color components is fixed at either 0 or 255 and the other
two components vary smoothly across the face from 0 to 255. The cube is
configured as a right-handed coordinate system with *x-y-z* mapping
R-G-B. Hence, the 8 corners of the cube represent the primaries red,
green, and blue, plus the secondaries cyan, magenta and yellow, plus
black and white.

The 6 color faces are generated by feeding
:doc:`grdimage </grdimage>` three grids, one for each
color component (R, G, and B). In some cases the X or Y axes of a face
are reversed by specifying a negative width or height in order to change
the variation of the color value in that direction from ascending to
descending, or vice versa.

A number of rays emanating from the white and black corners indicate the
Hue value (ranging from 0 to 360). The dashed and dotted lines near the
white corner reflect saturation levels, running from 0 to 1 (in black
font). On these 3 faces the brightness is a constant value of 1. On the
other 3 faces of the cube, around the black corner, the white decimal
numbers indicate brightnesses between 0 and 1, with saturation fixed at 1.

.. literalinclude:: /_verbatim/example_11.txt
   :language: bash

.. _fig_ex11:

.. figure:: /_images/example_11.*
   :width: 500 px
   :align: center

   The RGB color cube