HANS - A New Color Separation and Halftoning Paradigm

Traditionally the choices made by color separation are expressed as amounts of each of the available colorants to use for each of the reproducible colors. Halftoning then deals with the spatial distribution of colorants, which also results in the nature of their overprinting. However, having a colorant space as the way for color separation to communicate with halftoning gives access only to some of the possible printed patterns that a given printing system is capable of and therefore only to a reduced range of print attributes. In the present paper a method – HANS – is proposed to gain access to all possible, printable patterns by specifying relative area coverages of a printing system’s Neugebauer primaries instead of only colorant amounts. This results in delivering prints with more optimal print attributes than were possible using existing methods, allowing for up to 34% less ink use while delivering a 10% greater color gamut on a test printing system using CMYKcm inks. Introduction Print is the result of a number of colorants of different colors being superimposed on top of a substrate. Since the majority of printing technologies only allow for a very small number of levels of ink to be deposited at a given location on a substrate, halftoning is used to obtain ink patterns that result in a given color when seen from an appropriate viewing distance. These halftone patterns also result in inks being deposited on top of or next to one another in a specific way, giving a color that relates non-linearly to the amounts of the inks used. How much of an ink to use is the result of color separation, where ink amounts are chosen for each printable color. This is preceded by color management, where a choice of color reproduction objective (e.g., accuracy or pleasingness) can be made, where differences between the color gamuts of source content and the destination printing system are dealt with and where a color characterization of a printing system is employed with the aim of accurately rendering the chosen color reproduction objective Early color separation methods for three–ink printing systems, used since the late 19th century, involved the photomechanical construction of halftone patterns by filtering a projection of an original image through a set of color filters, each determining how much of a cyan, a magenta and a yellow ink to use, and then through a halftone screen, which resulted in the formation of dots of proportional sizes on the three printing plates. Here color separation filters determined ink amounts while halftone screens resulted in corresponding per–ink patterns, which were finally superimposed. The effectiveness of such methods was relatively limited given their very indirect control over the resulting printed patterns and therefore colors. Such control was significantly increased when computational color reproduction was pioneered during the first half of the 20th century. Here Neugebauer’s model of halftone color reproduction was key, which in its simplest form states that the color of a halftone pattern is the convex combination of the colors (i.e., CIE XYZs) of the Neugebauer Primaries (NPs) used in it. Here an NP is one of the possible ink overprints, with its convex weight being the relative area covered by it (Fig. 1). Figure 1. Relationship between print materials (top), resulting Neugebauer Primaries (center) for a three–colorant, bi–level printing system and an example of how colorant amounts and Neugebauer Primary area coverages relate in a halftone (bottom). The Neugebauer model enabled much tighter control over a printing system and was used as follows: For each color to be reproduced, find the amounts of inks, which, when halftoned using a given halftoning method, match that color. This involves having a model of the halftoning method, which for given ink amounts predicts corresponding NP area coverages. Having measured the NPs and using the Neugebauer model, a prediction can then be made of the resulting color from the NPs’ colors and their area coverages. Using these two models in reverse, appropriate ink amounts can be obtained for in–gamut colors. Broadly the same principle is employed even in the most recent color separation approaches and halftoning techniques. 10–15 A key to the success of such color separation is the accuracy of the model used and there have been numerous improvements here since the Neugebauer model’s introduction in 1937. In the above approaches, color separation and halftoning communicate via an ink space where color separation determines amounts of inks to use for a given color and halftoning then constructs patterns that deliver them. However, only certain Page 1 of 6