CIE Division 8 - TC8-03: Survey of Gamut Mapping Papers

Spaulding, Ellson & Sullivan (1995)

This algorithm is the one implemented in the Kodak ColorEase PS and Kodak XLS 8600 PS printers and the main idea is to use different kinds of mappings in different parts of colour space (as was suggested by previous authors as well). It is claimed, that the core around the neutral axis of the colour space (which includes skin tones and some other memory colours) needs to be mapped colorimetrically whereas highly saturated colours benefit from a transformation which preserves as much of their chroma as possible. Two different mapping approaches are described first:

colorimetric mapping This is very similar to one of Saraís methods (1984) (i.e. chroma is clipped along lines of constant hue and lightness)
non-colorimetric mapping This method uses 1D LUTs which map R to C, G to M and R to Y whereby the result is that the two colour gamuts are mapped exactly onto each other so that maximum possible C* is preserved. However, all colours (most notably memory colours) are distorted.

What is new in this paper is an approach called colour gamut morphing, which consists in combining different mappings for different regions of colour space with smooth transitions between them. The algorithm is made up of two stages:

(1) explicit definition of colour gamut mapping functions for subsets of the original gamut (e.g. neutrals, memory colours, saturated colours) in the form of:

D = fD(A,B,C)
E = fE(A,B,C)
F = fF(A,B,C)

(2) colour mapping for remaining colours, which uses interpolation between the existing mapped colours and should be smooth and continuous relative to them.

Extrapolation may also be needed for colours which are outside the convex hull of explicitly mapped colours.

A particular embodiment of this technique is UltraColor, which uses the colorimetric algorithm for mapping the core of the colour gamut and the non-colorimetric algorithm for mapping highly saturated colours. It was implemented in the printers mentioned above in the form of a Postscript ColorRenderingDictionary (i.e. a 3D LUT).

The framework for gamut mapping outlined in this paper emphasises the need for different mappings in different parts of colour space and suggests a way for achieving smooth transitions between the individual mappings.

Last updated: 17 August 1999 by Jan Morovic