1. Technical Field
This invention relates generally to the creation of a bi-directional color transform between a color specification in a CIE-derived color space and device control values for extra-quaternary inking processes so that (i) colors can be predicted from a set of ink control values; and (ii) given a desired color, the device control values of a subset of inks derived from the original set of inks can be specified which achieve that desired color.
2. Background Art
As Desktop Color Publishing Systems become more ubiquitous, 4-color CMYK (cyan, magenta, yellow, black) printing has become a commodity item. Consequently the technical community is being driven by the high-end color market to develop new printing technologies to differentiate and add value to their product. The intent is to produce images of superior color, tone and detail than is obtainable with traditional processes. One approach to obtaining more colorful images is by employing more than 4 inks as process inks; in short, extra-quaternary printing processes.
Augmenting a set of CMYK inks with additional inks increases the gamut of printable colors. In any printing process, colors tend to become darker as more ink is laid down on paper because of the subtractive interaction of ink and incident light. The lighter hues of red, green and blue are consequently difficult to achieve with the traditional set of CMYK primary inks since they are produced from 2-ink overprints of the primaries. These lighter hues can be obtained by single inks properly formulated to the desired hue. Additional inks tend to increase the color gamut in the darker tonal regions as well. Their presence chromatically expands the entire gamut up and down the tonal range. The quarter tones and 3 quarter tones are especially augmented.
U.S. Pat. No. 4,812,899, which issued to Harold Kueppers on Mar. 14, 1989, discloses a scheme for creating 7-ink color separations based on an empirical algorithm which converts an additive interpretation of an RGB encoding of the source color to a set of specially formulated CMYKRGB colorants. Each color is inked with a maximum of 3 inks: all colors contain black; another ink is selected from the negative set of CMY primaries, cyan, magenta, and yellow; and the final chromatic ink is selected from the positive set of primaries RGB, red, green, and blue. In this context the terms `positive` and `negative` are being used as labels and are not meant to suggest that the subtractive colorimetry of the 2 inksets differ in any fundamental way. This approach has significant advantages for conventional halftoning technology: with just 3 inks per color, there are no moire concerns in printing. However, being empirical in nature implies that it is not colorimetrically based. There is no mechanism present to ensure that colors specified in the units of a CIE derived colorspace can be achieved with this inking scheme.