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The OCS surface in NPP space as shown in Figure
(p.
) is similar in shape to the outer surface
of the Munsell color solid, a psychological color-order system (or
color- appearance system) based on the principles of color perception
[Wyszecki \& Stiles 1982][p. 510]. This is remarkable in view of the fact
that the Munsell system is a purely psychological or ``impressionistic''
color model used especially by artists, and the NPP space is defined in an
entirely analytic way, based on neurophysiological measurements and
physical object surface properties alone. As I mentioned in
Section
(p.
), the NPP space thus
provides a tentative bridge between the psychology and the neurophysiology
of (an aspect of) color perception, which has proved difficult to obtain to
date [Boynton 1990].
A direct quantitative comparison between the NPP and Munsell spaces is not possible, because the Munsell space is not defined mathematically, but rather as a set of example color chips arranged in a certain way. We can do some qualitative comparisons however:
For comparative purposes I have made similar sections through the OCS
surface in XYZ space (Figure
) and in L*a*b* space
(Figure
).
In XYZ space, both of the sections are much more symmetrical in shape than the Munsell diagram shows, which is also true for the ``green-red'' section in L*a*b* space. The ``blue-yellow'' section in L*a*b* space is more similar to the Munsell diagram than the one in XYZ space, but not quite as similar as the NPP section is. The asymmetry of the sections through Munsell and NPP space is relevant, as it corresponds to greater saturation ranges for some colors versus others, which is a psychological property of object colors that is not reflected to the same extent in the XYZ or L*a*b* spaces.
The OCS surface in NPP color space shows some clear ``lobes'' corresponding
to the locations of blue and red, and to a lesser extent yellow and green
(Figure p.
). The
locations of white and black are also clearly marked, being respectively at
the top and bottom of the hull. These observations lend some preliminary
support to the hypothesis that there is a neurophysiological basis for the
``basicness'' of basic colors like the six primary colors just
mentioned. We will look into these matters in more detail later.