Color classes
-------------

The two main color classes are rgb and hsv, which have slots red,
green, blue and hue, saturation, value respectively.  There is also an
rgb class with an alpha channel (slot alpha) called rgba.  In the rgb
class, valid slot values are from 0 to 1, while in the hsv class,
saturation and value are in the interval [0,1], but hue is in [0,360).

You can convert between rgb and hsv using rgb->hsv and hsv->rgb.  Note
that for the former, you need to specify what happens when the hue is
undefined (ie the color is gray).  By default, the hue of red (0) is
assigned.

Generic functions which find the appropriate conversion method are
available with names ->rgb and ->hsv.  Use these if you want your
functions to handle various different color representations but
eventually you need to work with a single one.



Named colors
------------

Named colors, parsed from the X11 colors file, are loaded from
colornames.lisp.  As they are constants, names are between +'s.  All
named colors are rgb.



Convex combinations
-------------------

Use hsv-combination or rgb-combination for taking convex combinations
in the respective color space.  Note that in the HSV space, you need
to specify the direction on the color wheel, the default is positive.


Example session
---------------

CL-COLORS> +blue+
#<RGB red: 0.0d0  green: 0.0d0  blue: 1.0d0>
CL-COLORS> (->hsv +blue+)
#<HSV hue: 240.0d0  saturation: 1.0d0  value: 1.0d0>
CL-COLORS> (rgb-combination +blue+ +green+ 0.5)
#<RGB red: 0.0d0  green: 0.5d0  blue: 0.5d0>
CL-COLORS> (->rgb (hsv-combination (->hsv +blue+) (->hsv +green+) 0.5))
#<RGB red: 1.0d0  green: 0.0d0  blue: 0.0d0>
CL-COLORS> (->rgb (hsv-combination (->hsv +blue+) (->hsv +green+) 0.5 nil))
#<RGB red: 0.0d0  green: 1.0d0  blue: 1.0d0>