Page 95 - Computational Colour Science Using MATLAB

82 CHROMATIC-ADAPTATION TRANSFORMS AND COLOUR APPEARANCE
the corresponding colour under a reference illuminant for a stimulus defined
under a test illuminant. Corresponding colours are colours that have the same
appearance under different illumination.
Fairchild (1998) defines a CAM as any model that includes predictors of at
least the relative colour-appearance attributes of lightness, chroma and hue. The
attribute brightness is a visual perception according to which an area appears to
exhibit more or less light. Lightness is the brightness of an area judged relative to
the brightness of a similarly illuminated reference white. The lightness of a
sample is in the range 0–100 and is influenced by the surrounding background.
Colourfulness is that attribute of a visual sensation according to which an area
appears to exhibit more or less chromatic content. Hunt (1952) has shown that
the colourfulness of an object increases as the luminance increases so that a
typical outdoor scene appears much more colourful in bright sunlight than it
does on an overcast day (the Hunt effect). Chroma is the colourfulness of an area
judged as a proportion of the brightness of a similarly illuminated reference
white. The colourfulness of an area judged in proportion to its brightness is
called the saturation. Finally, hue is the attribute of a sensation according to
which an area appears to be similar to one, or to a proportion of two, of the
perceived colours red, yellow, green and blue.
In this chapter the basic principles that underlie CATs will be introduced and
three models (CIECAT94, CMCCAT97 and CMCCAT2000) will be described in
detail. The CIECAM97s CAM will then be described and CMCCAM2000 will
be introduced. Finally, MATLAB code will be presented for the CATs and
CAMs described in this chapter.

6.2 CATs

In psychophysical studies of chromatic adaptation it is useful to define the
concept of corresponding colours (colours that have the same appearance under
different illumination). In a typical colour-appearance experiment to determine
the corresponding colour of a grey surface or chip under a test light source (for
example, corresponding to illuminant A) observers adapt to the chip viewed
under illuminant A and are then asked to memorize the colour of the chip. The
observers are then adapted to the reference light source (often corresponding to
illuminant D65) and requested to select a chip, from a large number of different
coloured chips, that matches the memorized colour of the original chip that was
viewed under the test illumination. If the chip is a perfectly neutral grey, then it
would have the chromaticity under D65 corresponding to the D65 illuminant
itself and the chromaticity under A corresponding to illuminant A. In Figure 6.1,
these points are denoted by an asterix ( * ) and a cross (+), respectively. If the
observer is able to discount the change in illumination perfectly, then the colour
appearance of a given surface will be the same under both the test and reference