Spectral and color prediction for arbitrary halftone patterns: a drop-by-drop, WYSIWYG, “ink on display” print preview

Accurately previewing the appearance of a print job can make the difference between producing saleable output and wasting expensive materials and is a challenge to which a host of solutions already exist. However, what the majority of these have in common is that they base their predictions on the inputs to a printing system (e.g., continuous-tone data in ink channels) instead of its outputs (i.e., the halftone data that is then printed) and that they are only valid for a given set of choices already made in the printing system (e.g., color separation and halftoning). Alternatively, attempting to make appearance predictions using general-purpose models such as Kubelka Munk, Yule Nielsen and Neugebauer results in limited performance on systems whose behavior diverges from these models’ assumptions, such as inkjet printing. As a result of such constraints, the resulting previews either work only under limited conditions or fail to predict some artifacts while erroneously predicting others that do not materialize in print. The approach presented here takes advantage of the flexibility of the HANS framework and the insights into spectral correlation to deliver a print preview solution that can be applied to any printing system, that allows for the variation of fundamental imaging choices without the need for re-computing model parameters and that delivers ICC-profile-level accuracy. Introduction With digital print production being characterized by large numbers of short-run jobs, the bottleneck in the end-to-end process becomes operator intervention. Web-to-print workflows further increase the cost of manual overrides and heighten the need for accurate print preview before a printed product is ordered – not only for the sake of avoiding a job having to be re-printed but also as a means of providing the print-buying customer with a sense of assurance before they trigger a print order. A key challenge in any workflow where the cost of an unsaleable print is high (either due to the cost of the materials used or even just due to the delay and frustration that such an event is likely to cause), is to make the print preview closely resemble the final printed output. In the absence of a close match, the decisions made on the basis of the preview are unreliable and its inaccuracies can further aggravate the process of getting to the right print rather than improving it. The key obstacle to providing an accurate print preview has been the lack of two components: First, an accurate model of print color formation that is applicable to technologies like inkjet, where ink-media interactions are complex, highly non-linear, and not well represented by models that apply more directly to analog printing technologies. Second, a framework for applying scaling to a print preview that reflects the nature of color formation in it, instead of assuming that of displays. The approach presented here addresses both these challenges by basing the print preview on the result of using the same image processing as used for printing and by taking advantage of recent insights into spectral correlation between neighboring spectral bands. The resulting preview is immediately identifiable as being that of a print (as opposed to only of print-like colors) and displays every feature and artifact of the print it simulates, down to its level of grain and the subtle consequences of the specific halftoning algorithm used. Viewing the on-screen preview results in an experience resembling the viewing of an ink-on-substrate print, including when viewing it at different magnification levels. Background Before proceeding to an exposition of the approach introduced here for simulating the appearance of print, two areas will be reviewed first: print spectral and color models and their challenges, and the limitations of current soft-proofing solutions, since it is against their background that a new solution will be presented. The literature on the prediction of printed spectra and color is extensive, has had good review papers (Wyble and Berns, 1999) and book chapters (Bala, 2002) written about it, and spans the prediction of both ink overprinting and the effect of halftoning on the resulting stimulus. In terms of the former, the Kubelka Munk model (1931) is widely used to predict the properties of multiple layers of ink overlaid at a given location, given information about each constituent ink’s reflectance and opacity. From these, their absorption (K) and scattering (S) coefficients can be computed: