Electronic trapping: by VectorS

Electronic trapping: computer-controlled register control by Brian P. Lawler for newsprint, and the amount of that trap are different than those needed for higher quality printing processes (see chart on page 3). The Graphic Arts Technical Foundation uses three terms to describe the difficulty of printing a job: hairline register (often called tight register), loose register, and lap register. Hairline register requires colors to abut Changing terms with no unprinted Trapping has evolved paper showinginto a term to describe through, and no the process of intentional intentionally overlapping of colors overlapping colors to (no trapping). When a compensate for register job is designed with error on the press. loose register, colored Though the term objects are spaced is used widely in the enough apart that electronic prepress register problems will industry, there may be not easily be visible. some confusion as we In other words, move forward with the because the objects integration of have enough open electronic prepress in space between them conventional printing that a little more or plants. less space will not be Conventional noticeable. Lap register printers, new to the jobs have built-in terminology of the The images on the left are in perfect register, but the images on the register compensation right have slipped out of register on the press, creating white spaces electronic prepress – trapping – making industry, may need to on parts of the image, and overprints of the combined color on the them easier to keep in others. Trapping is the process of creating intentional overlaps of become aware of the register on press. changing terminology. color that prevent the white from showing when a press goes out of This is the most register. There are many methods for trapping, and many technical Communications and considerations to take into account. common kind of education are needed register used in in both printing and prepress. commercial printing, but the cost of preparing the Whether you choose to use terms like spreads overlapping traps is significant, and estimators and chokes, shrinks and spreads, grips, lap register, or must plan on having their prepress staff build traps fatties and skinnies is unimportant. Regardless of into jobs, unless those traps are created what you call the technique, be sure to trap, for electronically at an earlier stage of production. without correct trapping, we cannot get excellent You might assume that everything would be printing – and excellent printing is our objective. easier if we just paid a higher price for printing in hairline register and forgot about trapping. It’s not Register that easy. A broad range of mechanical, In the context of trapping, register is the accurate environmental and human factors combine to positioning of two or more colors of ink on a make it very difficult or impossible to keep printed sheet. The requirements for correct register printing in absolute register. This is the simple vary from one printing process to the next. The reason for the existence of color trapping tolerance for register error on newsprint is usually techniques in prepress and design. different that the tolerance on coated sheet-fed printing papers. Therefore the need to trap colors

What is trapping?

Trapping is the intentional overlap of colors in a printed project that prevents the almost-inevitable register errors in printing from showing. It is compensation – in advance of production – for the human and mechanical errors that can result in misregister of images on a printing press.

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Black object no stroke; set to overprint

M C

Cyan object stroked with cyan: .5 pt.; stroke set to overprint

Magenta object stroked with magenta: .5 pt.; stroke set to overprint

Yellow object stroked with yellow: .5 pt.; stroke set to overprint

Trapping with illustration programs requires considerable attention to detail. Building traps manually requires planning and careful construction. In this example, the triangle is magenta, the circle is cyan, and the irregular polygon is yellow. The small circle in the center of the cyan circle is black. To build a trapped illustration from these elements, strokes must be added to some of the colors, with overprint selected on those strokes. The thickness of the strokes is also critical, depending on the printing process. The individual elements and their values as well as overprint characteristics are described on the right.

Trapping with illustration programs Using CorelDRAW! as an example (it has no automatic trapping capability), to spread an object, one chooses that object, and applies a stroke to it in the same color as the basic object. The stroke must be declared to overprint, and must be set to a line thickness that is double the thickness of the intended trap. Why double the thickness? PostScript paths are theoretical curves. When stroked with any color, half of the color extends on one side of the path while the other half extends out from the other side of the path. To establish a trap thickness of 0.003 in., you would enter a thickness of 0.006 in. If your illustration program allows only measurement in in points, divide the traps (already doubled) by 0.01389; the product is the stroke thickness in points. Both Adobe Illustrator and Macromedia FreeHand offer trapping filters in the most recent The PostScript path versions. Control over is always in trap colors, the center of the stroke, intensities and trap which is evenly spread on thickness is all both sides of the path under menu control, and is extraordinarily precise. In both applications, any open paths must first be converted to outlines before trapping will succeed.

Overprinting strokes of black A designer knowledgeable in the concepts of trapping can apply overprinting strokes to any object that will perform the task of trapping

on press. Black overprint stroking is simple, requiring only a black outline to stroke all objects printed like comic book art). By selecting all the objects in the illustration, and setting the strokes to overprint black, trapping the underlying colors is very simply accomplished.

Trapping with common colors Another successful technique for trapping is to build color palettes in designing the project that contain at least 20 percent of the adjacent color. This surprisingly simple process requires only a careful analysis of the color palette with the objective of modifying one of each pair of colors by adding the opposite color to the mix. For example, a palette of cyan, magenta, and yellow solids (which normally would require trapping) could be modified to add 20 percent magenta to the cyan, and 20 percent cyan to the yellow, and 20 percent yellow to the magenta, creating a palette of three colors that do not need to be trapped in production.

Trapping software In the past four years we have seen some impressive and important software developments for desktop computer design and production. There exist today two desktop trapping applications that can trap any EPS document, and several reasonable hardware solutions to the trapping dilemma. First of the trapping software products is Trapper, from DK&A of San Diego, California. Its vector-based technique analyzes color intersections and creates trapping paths in correct subordinate colors that suit the printing process. Any color, or the trap thickness of any object, is easily modifiable in the program. Running best on fast Power Macintosh computers, Trapper is very successful for many users. TrapWise from Luminous/Imation is the other

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Top stroke: 8 pts.; no overprint

3. Underlying stroke: 8.5 pts.; set to overprint

2. 1.

Manually-trapping strokes in an illustration requires either duplicating the strokes, or conversion of the strokes into outlines. Adobe Illustrator and Macromedia FreeHand can both convert strokes into outlines using a filter. This makes manual trapping much easier. Both programs also now offer semi-automatic trapping of illustration objects that is very sophisticated and functional. This illustrates the technique of duplicating the strokes and spreading one more than the other.

major software trapping product on the market. Running on both Power Mcintosh and Pentium computer platforms, the TrapWise product rasterizes a PostScript file before analyzing the color intersections in the job. When finished, it discards the rasterized data, and adds PostScript appendages to the file, making a file that, when imaged, includes the trap elements needed to prevent press misregister.

These systems are finding happy homes in the direct-to-plate imaging field, on machines such as the Komori PTP platemaker.

Trapping with RIP hardware Trapping with hardware products is also possible. Scitex has a range of imagesetters that feature rasterbased trapping in the imagesetter. With Scitex Full Auto Frames software running on a Scitex RIP, one

Method of printing

Substrate

Halftone frequency

Trap (in.)

Trap (mm.) Trap (pt.)*

Sheet-fed offset

Gloss coated

150 lpi 60 l/cm.

.003 in.

.08 mm.

.25 pt.

Sheet-fed offset

Uncoated

150 lpi 60 l/cm.

.003 in.

.08 mm.

.25 pt.

Web-fed offset

Gloss coated

150 lpi 60 l/cm.

.004 in.

.10 mm.

.30 pt.

Web-fed offset

Uncoated commercial

133 lpi 52 l/cm.

.005 in.

.13 mm.

.40 pt.

Web-fed offset

Newsprint

100 lpi 40 l/cm.

.006 in.

.15 mm.

.45 pt.

Flexography

Coated

133 lpi 52 l/cm.

.006 in.

.15 mm.

.45 pt.

Flexography

Newsprint

100 lpi 40 l/cm.

.008 in.

.20 mm.

.60 pt.

Flexography

Kraft (corrugated, other)

65 lpi 25 l/cm.

.010 in.

.25 mm.

.75 pt.

Screen printing (wet-on-wet)

Fabric

any

Screen printing (dried)

Paper, fabric, other

100 lpi 40 l/cm.

.006 in.

.15 mm.

.45 pt.

Gravure

Gloss coated

150 lpi 60 l/cm.

.003 in.

.08 mm.

.25 pt.

*Point values are rounded for convenience of entry. lpi = lines per inch; l/cm. = lines per centimeter.

Dalim Imaging Systems, of Bedford, Massachusetts, also offers a complete trapping software product as a part of its larger graphics software running on Silicon Graphics computer systems. Dalim’s technique also involves the rasterization of images and color analysis of the resulting rasterized file. They then create a PostScript append file that is tacked onto the end of the job to trap it at output. Contex also sells an impressive SGI-based prepress system that includes a lightning-fast RIP with trapping at the raster level (post-PostScript). © 1995-1998 Brian P. Lawler. All rights reserved.

can prepare a job for output, trap it, and then image the job onto color separations in one device. The speed of the Scitex traps is legendary, and the quality is similarly impressive. Another approach to hardware trapping is with raster image processors from RamPage, a firm that packages its trapping operations in the RIP. RamPage RIPs can be configured to run any of the popular imagesetters on the market, and provides a powerful solution to trapping in production. Adobe Systems has announced that PostScript 3, available in 1998 on a number of

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These colors must be trapped

These colors need no trapping

First color

Second color

Resulting trap color

First color

Second color

Color added to accomplish “natural” color trap

100C

100M

100M 100C

100C 20M

100M

20M

100M

100Y

100M 100Y

100M 10Y

100Y 10M

10M 10Y

100Y

100C

100C 100Y

100C 20Y

100Y

20Y

100K

100C

100C 100K

100K 20C

100C

20C

100C 100Y

100M

100C 100M 100Y

100C 100Y

100M 20Y

20Y

One of the simplest, and certainly the most elegant method for taking control of trapping is to build a color palette made-up of color that require no trapping at all. This charts shows colors as they must be trapped in conventional applications. By adding a small amount (at least 20 percent) of the contrasting color, new colors can be created that print side-by-side without trap lines being necessary. As can be easily seen, the changes to the colors are quite subtle, and the impact on productivity is tremendous.

imagesetters, will do in-RIP trapping. While I have not seen the interface for controlling such trapping, nor the output of this technology, it is an interesting promise from the inventors of the PostScript language. If this proves to be successful, then the issues of trapping can move to the RIP and off of the workstation, making production more efficient.

■ This is one of a series of essays on computers in the graphic arts. Other subjects include scanning, dot gain, black and white from color, Photo CD, calibration of desktop systems, and several essays dealing with World Wide Web graphics. Visit my web site for a complete index to the essays. They are available for free download.

Read my book (later) for the whole story

Brian P. Lawler Graphic Arts Consultant 6045 Madbury Court San Luis Obispo, California 93401 USA Fax: 805 544-8445 Telephone: 805 544-8814

For all the gory details on trapping, from the history through the philosophy to the present-day application, read the best (and only) book ever published on this scintillating subject. The Complete Guide to Trapping is, unfortunately, out of print at the moment (September, 1998). It is my intention to revise and publish a third edition of this book (but please don’t hold your breath for this event to take place). If you are interested in being on the waiting (and waiting, and waiting) list for this, please send me an email at the address at right, and I’ll keep your name on file until I publish again.

email: brian@thelawlers.com web site: www.thelawlers.com

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