To meet customer demand, the commercial printing industry requires the capability of producing spot colors accurately and consistently. Spot colors can be defined as a fixed set of colors which may be Pantone® colors, customer logo colors, colors in a customer's proprietary marked patterns, or customer defined colors in the form of an index color table. Spot colors are often used, or can be used, for large background areas, which may be the most color critical portion of a particular page. Consistent color in these areas may determine the difference between success and failure in meeting customer requirements. Customer demands for color accuracy and consistency are typically much tighter for spot colors than for colors within images.
Existing spot color editors utilize a manual approach to the adjustment of CMYK combinations of spot colors prior to raster image processing (RIPing). For example, the document creator may select a Pantone® color for application in specific areas through a user interface on a printing device or computer monitor, such as that available on the Xerox® DocuSP® Controller. The Pantone-provided CMYK combination for the selected printer is obtained from a look-up table. Prior to RIPing the document in the printer, the operator has the option of entering a spot color editor function and specifying an alternative CMYK combination to achieve the desired color. The document is then RIPed and then printed using the spot color editor combinations where specified, and Pantone combinations otherwise.
This workflow presents various problems, among which is operator error associated with manual adjustments of the CMYK combinations; modifications to the CMYK values may result in more variability from machine to machine. Also, the manually-adjusted CMYK values may require more iteration to achieve the desired color. Due to the manual adjustments it may be difficult for customers to achieve the correct CMYK combinations even after repeated trials.
For some customers, the process of manually adjusting spot colors is far too difficult or time consuming. These customers will always use the built-in static CMYK spot color dictionary and are forced to accept the potentially large accuracy errors that can occur with long term printer variation.
An automated spot color editor (ASCE) method includes determining appropriate target values for a selected color within a print job. The selected color may be described as being within a color space such as reflectance spectra, L*a*b*, XYZ, LHC, CMYK, RGB, parameters describing color, or a color number. The automated spot color editor modifies or adjusts the selected color by selecting a quality level and a maximum number of iterations. During iterations it computes the CMYK recipe for each spot color until a quality level is reached.
CMYK recipes are computed inside an algorithm module. The basic algorithm requires the use of gamut classification to find the CMYK recipe accurately and in the course of doing so save toner usage. Also, this kind of classification can help to improve the overall attainable gamut by fully utilizing the black separation.
A related problem in the implementation of an ASCE hard copy proof concept is determining whether a given color is either inside/on-boundary or outside of the printer's gamut. Colors located very near or on the gamut's boundary could have mistakenly been reported as outside the gamut by algorithms that are not accurate enough. A color that is wrongly considered as outside the gamut will be handled by the gamut mapping algorithm currently implemented in the ASCE application. Thus, this mapping algorithm will, in turn, map the colors in reference to a point in the surface of the printer's gamut. The ASCE control algorithm will iterate to seek to match the given mapped color. The consequence of this action might lead to the reproduction of a color that is slightly “off” from the original one when this could have been avoided since there was no need to map the original color.
There is thus a need for providing an accurate method to determine colors that are on the gamut's boundary.
There is also always a need for improved accuracy in the ASCE feature, and in particularly enhanced accuracy in the performance of any spot colors (arbitrary or customer specified) using inline or offline sensors.