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Some Interesting Properties of the NPP Space

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:

• The vertical dimension in the Munsell space is the gray axis, ranging from black at the bottom to white at the top. This is also true for the NPP space (Figure ). For all practical purposes, we can consider the NPP gray axis to be straight (Section p. ).

  

• Hue varies as a polar-coordinate system around the gray axis in Munsell, and can be expressed the same way in NPP. The spacing of the hues around the hue circle is approximately the same in both spaces (Figure ).

  

• The overall shape of the NPP color space is remarkably similar to the Munsell space as well, e.g., when comparing planes of constant Munsell hue with planar sections through the NPP space (Figure ).

  

  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.

• Saturation is expressed as distance from the gray axis in the Munsell system, and can be expressed the same way in NPP (Figure ).

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.