A Survey of Color for Computer Graphics

A Survey of Color for Computer Graphics SIGGRAPH 2001
bases. For example, an inkjet printer may create each spot with multiple drops. The more
drops used, the darker the color.
There is no simple, linear transformation that maps subtractive devices to tristimulus
space. Combining filters multiplies their effect rather than adding them. Most subtractive
devices are characterized by measuring a large number of color patches then interpolating
between them. Anything that changes the output appearance, such as changing the halftoning
algorithm in printing, will change the characterization.
Gamuts and gamut mapping
The gamut of a device creates a volume in the standard space. Only colors that fall within
this volume can be reproduced on that particular device. The slides show several gamut
representations. The scatterplots represent the gamuts as 3D plots, which is the most accurate
way to think about them. Shown are a gamut for an additive system (a monitor),
which is a regular shape similar to a cube but with parallelograms for faces. This graphically
demonstrates that the transformation from the RGB color cube to CIEXYZ is a
linear one. The Cromalin print gamut, however, is distinctly non-linear. While there is
some hint of the original cube, each face is a different size and some of the edges are distinctly
curved. The gamut is actually concave as well as bent. The gamut for the DLP
projector shows a basic additive structure plus an extrusion at the white point caused by
selectively adding white. As a result, it is linear in some regions and not in others.
Because the data used to generate the plots was absolute measurement data, the scales are
quite different. However, in color management applications, this data would be converted
to CIELAB, which normalizes it to a common white (but not necessarily black)
value. Another figure in the slides shows gamuts projected onto the CIE chromaticity
diagram. Here, linear systems project as triangles whose vertices are the chromaticity coordinates
of the red, green and blue primaries. The print gamut, in contrast, is quite
irregularly shaped.
To map from one device to another, gamut mapping is applied as transformations in this
common color space. Current practice is to project colors towards the center of the
gamut in a way that reduces saturation, and, to a lesser extent, brightness, while maintaining
hue. However, if no in-gamut color is really close, than the reproduction will look
substantially different no matter how clever the gamut mapping algorithm. Only
redesigning the image can really create a good reproduction.
Color Management Systems
There are a pair of diagrams at the start of this section (courtesy of Michael Bourgoin of
Adobe Systems) that succinctly describe the digital color management problem. Given a
set of digital input and output devices such as printers, scanners, monitors and film recorders,
how do we convert colors between them without creating a customized
transform between each pair of devices? The answer is to use a single, deviceindependent
color space, such as those defined by the CIE, as the common interchange
format.
Many companies produce color management systems, including Kodak, Agfa,
Lineotronic and EFI, just to name a few. Apple’s ColorSync is built into the Apple oper-
Maureen C. Stone 14
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