CHAPTER 4
244
Graphics
4.5.4 CIE-Based Color Spaces
Calibrated color in PDF is defined in terms of an international standard used in
the graphic arts, television, and printing industries.
CIE-based
color spaces enable
a page description to specify color values in a way that is related to human visual
perception. The goal is for the same color specification to produce consistent re-
sults on different output devices, within the limitations of each device; Plate 2 il-
lustrates the kind of variation in color reproduction that can result from the use
of uncalibrated color on different devices. PDF 1.1 supports three CIE-based col-
or space families, named
CalGray
,
CalRGB
, and
Lab
; PDF 1.3 adds a fourth, named
ICCBased
.
Note:
In PDF 1.1, a color space family named
CalCMYK
was partially defined, with
the expectation that its definition would be completed in a future version. However,
this is no longer being considered. PDF 1.3 and later versions support calibrated
four-component color spaces by means of ICC profiles (see “ICCBased Color Spaces”
CalCMYK
color space at-
tributes and render colors specified in this family as if they had been specified using
DeviceCMYK
.
The details of the CIE colorimetric system and the theory on which it is based are
beyond the scope of this book; see the Bibliography for sources of further in-
formation. The semantics of CIE-based color spaces are defined in terms of the
relationship between the space’s components and the tristimulus values
X, Y,
and
Z
of the CIE 1931
XYZ
space. The
CalRGB
and
Lab
color spaces
(PDF 1.1)
are
special cases of three-component CIE-based color spaces, known as
CIE-based
ABC
color spaces. These spaces are defined in terms of a two-stage, nonlinear
transformation of the CIE 1931
XYZ
space. The formulation of such color spaces
models a simple
zone theory
of color vision, consisting of a nonlinear trichro-
matic first stage combined with a nonlinear opponent-color second stage. This
formulation allows colors to be digitized with minimum loss of fidelity, an impor-
tant consideration in sampled images.
Color values in a CIE-based
ABC
color space have three components, arbitrarily
named
A, B,
and
C.
The first stage transforms these components by first forcing
their values to a specified range, then applying
decoding functions,
and then mul-
tiplying the results by a 3-by-3 matrix, producing three intermediate components
arbitrarily named
L, M,
and
N.
The second stage transforms these intermediate
components in a similar fashion, producing the final
X, Y,
and
Z
components of
the CIE 1931
XYZ
space (see Figure 4.14).