Color Book Production Using TEX

When typesetting a color book the goal is to produce a separate printer plate for each of the colors. The process of splitting the printed output into separate plates is called color separation. There are two color separation methods commonly used. Custom color separation selects colors from a standard pallet. A different plate is created for each color in the book. Process color separation separates the colors into the subtractive color components cyan, magenta and yellow, and creates a separate black plate by a process called undercolor removal. These four plates are used by a printing press to mix the colors on paper when the book is printed. Color separation is a more involved process than simply assigning RGB values to a desktop color printer. This article addresses the issues of professional color separation, and demonstrates how TEX with a suitable dvi driver can be used to produce quality custom and process color books. Introduction printer2 is two sets of negative film or camera-ready There has been recent interest in using color with TEX. This is evident by macro packages such as FoilTEX (Hafner 1992), the discussions about color on NTS-L (New Typesetting System List), and the new standard color support in LATEX^^ (Goossens, Mittelbach, and Sarnarin 1994). This interest was most likely initiated by the availability of low cost desktop color printers, and the desire to make use of these printers1 with TEX. Foil-TEX, W X color styles, and other macro packages provide an easy way for the owners of desktop color-printers to use color with TEX. Their goal is a simple method, using macros and specials, to select color output on a desktop printer. This is different from what is required in color book production. When typesetting a color book the goal is to produce separate plates for each color used by the printing press. An example Mrlll help to clarify this. Imagine that you are typesetting a book that will have red section headings. The final product required by the ' There are unfortunately 3 potential uses of the word "printer" in this article. To avoid confusion, I wdl use the term desktop printer to refer to a low resolution device, color or otherwise, which is used to print files. The term printer will refer to a person or corporation that prepares printing press plates and uses those plates to print books. Finally, imagesetter will be used to describe a high resolution printer that images to photographic paper or film. An imagesetter is also called a typesetter, but I will use that term for a person who sets type (electronically or otherwise). copy.3 The first set will be the "black" film, which contains only the black text. The second set wdl be the "red" film and will contain only the red section headings. But, both sets of film will be printed on a black and whte imagesetter because it is the responsibility o f the printer to provide the correct color ink to the printing press. The process of dividing the pages into separate printer plates is called color separation. Color separation is a more involved process than assigning RGB values to a desktop color-printer. The question addressed by this article is: "can color separation be done with TEX?", and the answer is: "yes, with an appropriate dvi driver." At Publication Services we have been typesetting color books in TEX since 1987. In 1993 we typeset our frrst process color book. This was done using a collection of specials that provide information about the current color, its type (process or custom) and its marking model, or how the color interacts with other colors placed on the paper underneath it. The person who makes plates and runs the printing press (see footnote 1). Film is clear acetate which is used to expose plates for an offset printing press. Fllm is printed with a negative image, that is transferred once to produce a positive plate. Camera ready copy, or CRC, is photograplc film exposed by an imagesetter. Before transferring to a plate, a negative must be made of CRC. Because of the loss of quahty when shooting the negative from CRC, negative film is usually requested for color books. TUGboat, Volume 15 (1994), No. 3 -Proceedings of the 1994 Annual Meeting Color Book Production Using TEX This article will explain the color separation process and describe how TEX can be used to support electronic color separation. It will concentrate on the PostScript color model, and the quality control steps necessary to ensure good color reproduction. Some of the common mistakes made by authors attempting their own color separation wdl be discussed, so pay attention. The PostScript Color Model The PostScript page description language (Adobe Systems 1990) has become a de facto standard in the publishing world. For this reason I d l be discussing color separation assuming the PostScript imaging model. The general principles, however, apply to any color separation, and we have typeset custom color books using Cora (Linotype 1988). An important element of the PostScript imaging model is that all marlung is opaque. That is, any mark placed on the page will completely cover (remove) any existing marks it overlaps. This applies equally for solid characters and rules, and for tinted regions. Not all imaging devices work this way, and any attempt to color separate non-Postscript output must take into account the page description languages imaging model.4 Adobe defined a series of commands for setting color in PostScript. Some of these commands are now built into level 2 PostScript, but others are conventions defining how to interpret other commands. If you are preparing PostScript files for color separation you should familiarize yourself with these commands and conventions. They are listed in Adobe Systems (1989), and updated (and simplified) in Adobe Systems (1990). Custom versus process color separation. There are two methods of color separation used in book production. These wdl be referred to here as custom color separation and process color separation, although the terminology used by other typesetters and by printers may vary. Custom color separation5 is the process described in the introductory example. Each element of the book design is set in a specified color. Each of these colors is printed on a separate piece of film. The colors themselves are assigned based on standard color references similar to those used to select house paints. One common reference for custom colors is the Pantone system (PanThe imagesetter language Cora, for example, assigns priorities to overlapping tinted regions. The final marks are those of the region with the highest priority. Cora also allows the specification of different tint and pattern values for the intersection of regions. Custom colors are often called "spot" colors in desktop publishulg and drawing programs. tone 1991, Pantone 1991). Pantone sells standard approved color charts, and inks. A book design, for example, may ask that Pantone 231 (a light red) be used for all section heads. The typesetter's task in this case is to provide two negatives for each page, the printers job is to prepare plates from these negatives and select an approved Pantone 231 mk for the color plate. If late in the typesetting process the the publisher changes the design to use Pantone 292 (light blue) instead of Pantone 231, the printer can supply a Pantone 292 approved ink. The typeset negatives and the prepared plates d l not have to be changed. A small sample of Pantone colors can be found on color plate 1.33 of Foley, vanDam, Feiner, and Hughes (1990). If you look up plate 1.33 you d note that the actual names are obscured since the colors reproduced in that book are not Pantone colors. With process color separation each color is separated into cyan, magenta, yellow, which are the subtractive color components. Black is supplied by a process called undercolor removal which removes equal amounts of black from cyan, magenta and yel10w.~ Thls is done to provide a better, well registered black. (Imagine how a book would look if all the text was composed of three layers of ink.) The final result is referred to as CMYK color. Process separation is a more difficult process because of the need to have correctly cahbrated colors. With custom colors the printer is responsible for supplying the correct ink. With process colors, on the other hand, the typesetter's job is more difficult because correct color balance wdl depend on the quality of negatives supplied to the printer, as well as the quality of ink provided by the printer. The final typeset output of a process color book is four negatives. Each negative represents one of the cyan, magenta, yellow or black components of the book. The ink will be mixed on the paper by the printing press. Process color is also called four-color, and the two terms are synonymous in this paper. In order to provide the correct color mixture the negatives will have screened regions corresponding to page elements. A screen is an area of shachng that provides a percentage of the required color. For example, an orange-red can be printed using 0% cyan, 30% yellow, 70% magenta and 0% black. The four negatives must reflect these percentages. In the case of the cyan and black negatives, the color region will be black because no cyan or black mk is required. The yellow film however, will have a screened area that is approximately 30% filled, and the magenta film wrll have a screened area approximately 70% filled. This method of undercolor removal can flatten colors and result in too much black on the final print. In practice color balance must be checked and adjusted as needed. TUGboat, Volume 15 (1994), No. 3 -Proceedings of the 1994 Annual Meeting