TIFF 6.0 Specification
Final—June 3, 1992
Full Color Gamut.
Any one image or imaging device usually encounters a very limited subset of the
entire range of humanly-perceptible color. Collectively, however, these images
and devices span a much larger gamut of color. A truly versatile exchange color
space should encompass all of these colors, ideally providing support for all vis-
ible color. RGB, PhotoYCC, YCbCr, and other display spaces suffer from gamut
limitations that exclude significant regions of easily printable colors. CIELAB is
defined for all visible color.
Efficiency
A good exchange space will maximize accuracy of translations between itself and
other spaces. It will represent colors compactly for a given accuracy. These at-
tributes are provided through visual uniformity. One of the greatest disadvantages
of the classic CIE system (and RGB systems as well) is that colors within it are not
equally spaced visually. Encoding full-color images in a linear-intensity space,
such as the typical RGB space or, especially, the XYZ space, requires a very large
range (greater than 8-bits/primary) to eliminate banding artifacts. Adopting a
non-
linear RGB space improves the efficiency but not nearly to the extent as with a
perceptually uniform space where these problems are nearly eliminated. A uni-
form space is also more efficiently compressed (see below).
Public Domain / Single Standard
CIELAB maintains no preferential attachments to any private organization. Its
existence as a single standard leaves no room for ambiguity. Since 1976, CIELAB
has continually gained popularity as a widely-accepted and heavily-used standard.
Luminance/Chrominance Separation.
The advantages for image size compression made possible by having a separate
lightness or luminance channel are immense. Many such spaces exist. The degree
to which the luminance information is fully-isolated into a single channel is an
important consideration. Recent studies (Kasson and Plouffe of IBM) support
CIELAB as a leading candidate placing it above CIELUV, YIQ, YUV, YCC, and
XYZ.
Other advantages support a separate lightness or luminance channel. Tone and
contrast editing and detail enhancement are most easily accomplished with such a
channel. Conversion to a black and white representation is also easiest with this
type of space.
When the chrominance channels are encoded as opponents as with CIELAB,
there are other compression, image manipulation, and white point handling ad-
vantages.
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