Understanding PostScript
Understanding the difference between composites and separations
Determining when to use process color instead of spot color separations
Printing separations out of Illustrator
Understanding line screens
Printing separations from other programs
Understanding trapping
Using Pathfinder Trap
Trapping after you create an image in Illustrator
Until the mid-1980s, computer graphics were, well, crusty. Blocky. Jagged. Rough. If you looked at graphics that were done on computers in 1981 and printed to a black-and-white printer, you'd laugh so hard you couldn't breathe, stopping the laughter only when you realized that you actually could not breathe. Of course, in 1981, the world was gaga over the capabilities of computers and computer graphics. Those same pictures were admired, and the average person was generally amazed. The average designer, on the other hand, shuddered and prayed that this whole computer thing wouldn't catch on.
Desktop publishing was pushed to a level of professionalism in 1985 by a cute little software package called PageMaker. With PageMaker, you could do typesetting and layout on the computer screen, seeing everything onscreen just as it would eventually be printed. Well, almost. Aldus was the company that created PageMaker. In 1994, Adobe swallowed Aldus.
Problems aside, PageMaker would not have been a success if the laser printer hadn't handily arrived on the scene. Even so, there were problems inherent with laser printers too.
At 300 dots per inch (dpi), there were 90,000 dots in every square inch. A typical 8½-inch × 11-inch page of type had 8.5 million dots to put down. Computers were finally powerful enough to handle this huge number of dots, but the time it took to print made computers pretty much useless for any real work.
Several systems were developed to improve the printing process, and the one standout was PostScript from Adobe Systems. Apple licensed PostScript from Adobe for use on its first LaserWriter, and a star was born. Installed on every laser printer from Apple were two things from Adobe: the PostScript page description language and the Adobe base fonts, which included Times, Helvetica, Courier, and Symbol.
PostScript became fundamental to Apple computers and laser printers and became the standard. To use PostScript, Apple had to pay licensing fees to Adobe for every laser printer it sold. Fonts were PostScript, and if there ever was a standard in graphics, the closest thing to it was PostScript (commonly called EPS, for Encapsulated PostScript).
Today, the majority of fonts for both Mac and Windows systems are OpenType fonts, and many of the typefaces used in professional work are OpenType. However, almost all graphics and desktoppublishing software can read PostScript in some form (especially since OpenType is a derivative of PostScript).
You can print Illustrator documents in two ways: as a composite, which is a single printout that contains all the colors and tints used, or as a series of color separations, with a printout for each color. Color separations are necessary for illustrations that are printed on a printing press.
A typical graphic object in painting software is based on a certain number of pixels that are a certain color. If you make that graphic larger, the pixels become larger, giving a rough, jagged effect to the art. To prevent these jaggies, two things can be done: Ensure that enough dots per inch are in the image so that when the image is enlarged, the dots are too small to appear jagged. Or define graphics by mathematical equations instead of by dots.
PostScript is a mathematical solution to high-resolution imaging. Areas, or shapes, are defined and then these shapes are either filled or stroked with a percentage of color. The shapes are made up of paths, and the paths are defined by a number of points along the path (anchor points) and controls off those points (control handles, sometimes called curve handles or direction points) that control the shape of the curve. Those paths fill up with dots at print time, but the higher number of dots per inch (commonly referred to as dpi) give the illusion of a perfectly smooth edge.
Because the anchor points and control handles have real locations on a page, mathematical processes can be used to create the shapes based on these points. The mathematical equation for Bézier curves is quite detailed (at least for someone who, like me, fears math).
PostScript is not just math though. It's actually a programming language and, more specifically, a page description language. Like BASIC, Pascal, Forth, SmallTalk, and C, PostScript is made of lines of code that are used to describe artwork.
Fortunately, the average user never has to deal directly with PostScript code; instead, the average user uses a simplified interface, such as Illustrator. Software that has the capability to save files in PostScript or to print to a PostScript printer writes this PostScript code for you. Printers that are equipped with PostScript then take that PostScript code and convert it to dots on a printed page.
That most applications can handle EPS files and that most printers can print PostScript are of great benefit to users, but the strength of PostScript is not really in its widespread use.
If you create a 1-inch closed path in pixel-based drawing software and then enlarge that same path in any application, the path begins to lose detail. A 300-dpi path at twice its original size becomes 150 dpi. Those jagged edges become more apparent than ever.
If you create a 1-inch circle in Illustrator, you can enlarge it to any size possible without losing one iota of resolution. The Illustrator circle stays perfectly smooth, even enlarged to 200%, because the circle's resolution depends on the laser printer or imagesetter that prints it. Therefore, a perfect 1-inch circle has the potential to be a perfect 2-foot circle (providing you can find a printer or imagesetter that can print a 2-foot-diameter circle).
But scaling objects is only the beginning. You can distort, stretch, rotate, skew, and flip objects created in Illustrator to your heart's content, and the object still prints to the resolution of the output device.
Here's an example: A company wants its tiny logo on a 3-foot-wide poster. If you use raster methods, the edges become fuzzy and gross-looking — pretty much unacceptable to your client. Your other conventional option is to redraw the logo at a larger size or to trace the blown-up version — a time-consuming proposition either way.
Illustrator's solution is to scan the logo, trace it either in another software tracing program or with the Live Trace tool, and then allow you to touch it up and build your design around it. Afterward, output the illustration to a printer that can handle that size poster. There's no loss of quality; instead, the enlarged version from Illustrator often looks better than the scanned original.
Before you start the printing process, you may need to change or adjust a few items. For example, you may need to change the page size and orientation or set how certain colors should separate. This section deals with the issues you should be aware of before you press Ctrl+P (
Clicking the Artboard tool in the Tools panel allows you to specify the size of the Artboard in your document. If the Artboard is smaller than the printable page, then anything entirely outside the edges of the Artboard is cropped off when you print the illustration through Illustrator. Any objects that are partially on the Artboard print. Figure 18.1 shows how you can use the Artboard tool to change the Artboard size.
A composite printout looks very much like the image that appears on the screen. If you have a color printer, the image appears in color; otherwise, the colors are replaced by gray tints.
Note
Objects that are hidden or that exist on layers that are currently hidden don't print. Objects that exist on layers that have the printing check box deselected in the Layer Options dialog box also don't print.
When you're ready to print your document, choose File
The Print dialog box has a number of areas. You display each area by choosing an item from the list that appears along the left side of the dialog box.
Figure 18.2. The Print dialog box provides many options for controlling how your Illustrator documents print.
The General pane offers these options:
Copies: The number that you type here determines how many copies of each page print.
Pages: If you click the All radio button, all the pages that have art on them print. If you click the Range radio button and type numbers in the text field, only the pages that those numbers refer to print.
Media: This handles the Size, Width, Height, and Orientation of the document.
Options: This section determines how to print the layers and includes scaling options. In the Print Layers dropdown list (popup menu), choose from Visible & Printable Layers, Visible Layers, and All Layers. The Scaling options are Do Not Scale, Fit to Page, and Custom Scale (type a width and height in percentages).
The Setup (Page Setup on the Mac) button offers these options:
Crop Artwork to: Choose from Artboard, Artwork Bounding Box, or Crop Area.
Placement: Choose where you want the printing origin to start from relative to the edge of the paper.
Tiling: This relates to paging. You can print Single Full Page, Tile Full Pages, or Tile Imageable Areas.
The Marks & Bleed pane offers these options:
Marks: This lets you click or deselect the following options: All Printer's Marks, Trim Marks, Registration Marks, Color Bars, and Page Information. As for Printer's Mark Type, choose from Roman or Japanese. You can also set the Trim Mark Weight and offset from the art.
Bleeds: This relates to how the art bleeds or extends off the page. This is used to ensure the art prints to the edge. Choose the top, bottom, left, and right. There's a Link button that's on by default, so if you change one, the rest change in synchrony.
The Output pane offers these options:
Mode: This controls whether the print is a Composite (all colors together) or a Separation (each color plate printed on its own page). Depending on your printer configuration, you may have an In-Rip Separation option. This option is for raster-image processors that can perform the separation.
Emulsion: This controls the positioning of the emulsion layer. Up (Right Reading) means that the layer is facing you, and you can read the text. Down (Right Reading) means that the layer is facing away from you, and the type is readable when facing away.
Image: This controls whether the print is a Negative or a Positive.
Printer Resolution: This lets you change the printer's resolution (lines per inch/dots per inch). You can go only as high in resolution as your printer allows. You can always go lower in resolution.
Convert All Spot Colors to Process & Overprint Black check boxes: These check boxes control whether all spot colors print as process or black overprints.
Document Ink Options: This controls how the ink is printed or converts a spot color to a process color.
The Graphics pane offers these options:
Paths: The Flatness setting adjusts the lines. Curved lines are defined by lots of tiny straight lines. The more accurate to the curved path, the better the quality and the slower it is to print. The lower accuracy to the path, the faster it prints, but the quality may not be as high as you might want.
Fonts: This controls how PostScript fonts are downloaded to the printer. Some fonts are stored in the printer, but others that aren't standard on your printer can either be held on the printer or your computer.
Options: The other options under Graphics are setting the PostScript language and Data format for type. You can check the Compatible Gradient and Gradient Mesh printing by converting the gradient or gradient mesh to a JPEG format. This area is also where you're informed of your Document Raster Effects Resolution (choose Effect
The Color Management pane offers these options:
Print Method: The Print Method lists the Color Handling (whether the printer orIllustrator handles the colors), Printer Profile (the color management profile that youwant to use), and Rendering Intent (the rendering intent to use when converting colors to a profile space).
The Advanced pane offers these options:
Print as Bitmap: Click this check box to have your file print as a bitmapped image. This is useful to see a quick printout without the quality.
Overprint and Transparency Flattener Options: In this area, you choose whether you want to Simulate, Preserve, or Discard Overprints. You also choose the resolution from three presets or specify a custom resolution.
The Summary area lists the summary of the whole file. All the printing options you've chosen are listed here, and any warnings are listed at the bottom.
Tip
Choosing Level I PostScript options reduces errors when printing to an older printer.
Tip
When an illustration doesn't print, always choose Print Detailed Report. That way, you can read exactly what the error was.
Tip
Always save your file before printing.
When you print a full-color illustration on a black-and-white printer, Illustrator substitutes gray values for the process colors. In this way, the program creates the illusion that each color has a separate, distinct gray value. Of course, each color can't have its own unique gray value, so the colors have to overlap at some point.
Magenta is the darkest process color, ranging from 0% to 73% gray. Therefore, the darkest magenta prints is 73% gray. Cyan is the next darkest, ranging from 0% to 57% gray. Yellow is extremely light, ranging from 0% to only 11% gray. Figure 18.3 shows a comparison of the four process colors at various settings and their printed results. The four bars show different values, indicated above the bars, for each process color. Within each bar is the percent of black that prints when you print that color at that percentage to a black-and-white printer.
Figure 18.3. This shows how colors appear when printed on a black-and-white laser printer.25% 50% 75% 100%
Different printers may produce different tints of gray. Lower-resolution printers, such as 300-dpi laser printers, don't create an accurate gray tint because they use dots that are too large to create accurate tint patterns.
After you choose File
The picture on the left side initially shows the illustration on a portrait-oriented page, even if landscape is selected in Illustrator.
The various marks shown on the page are the printer's marks defaults. The trim marks are used for cutting the image after it's printed. The registration marks are used when printing separations, and you can line up the registration marks to ensure that the print doesn't shift and all looks as you planned.
The bleeds define how much of the illustration can be outside of the Bounding Box and still print. The default for bleed is 18 points, regardless of the size of the Bounding Box. To change the bleed, type a distance in points in the Bleed text field in the Marks and Bleed area. As you type the numbers, the bleed changes dynamically.
Bleeds are useful when you want an illustration to go right up to the edge of the page. You need to account for bleed when you create an illustration in Illustrator so that the illustration is the correct size with x amount of bleed.
Illustrator uses a PostScript Printer Description (PPD) file to customize the output for your specific printer. To change the PPD, click the arrow at the right edge of the PPD dropdown list (popup menu) in the Output area of the Print dialog box. Choose the PPD file that's compatible with your printer.
Note
PPDs were created with specific printers in mind. Unpredictable and undesirable results can occur when you use a PPD for a different printer than the one for which it's intended. If you don't have a PPD for your printer and must use a substitute, always test the substitute PPD before relying on it to perform correctly.
If your printer's PPD is not included with Illustrator, you may be able to get it directly from the printer manufacturer by visiting the manufacturer's Web site.
When you choose a different PPD file, the information in the main panel changes to reflect the new selection. Certain default settings in the lists are activated at this time. You can change the settings at any time, but most of them revert to the defaults if you choose a new PPD.
In the General pane of the Print dialog box, the Media section has a dropdown list (popup menu) that shows the available page sizes for the printer whose PPD is selected. For laser printers, few page and envelope sizes are supported. For imagesetters, many sizes are supported, and an Other option allows you to specify the size of the page on which you want to print.
Imagesetters print on rolls of paper or film. Depending on the width of the roll, you may want to print the image sideways. For example, on a Linotronic 180 or 230 imagesetter, paper and film rolls are commonly 12 inches wide. For letter-size pages, you should click the Transverse check box to print the letter-size page with the short end along the length of the roll. For a tabloid page (11 inches × 17 inches), don't click the Transverse check box because you want the long edge (17 inches) of the page to be printed along the length of the roll. If you click Transverse for a tabloid-size document, 5 of the 17 inches are cropped off because the roll is not wide enough. As always, when trying something new with printing, run a test or two before sending a large job.
Note
The page size that you choose in the Size dropdown list (popup menu) determines the size of the page on the left side of the main panel. The measurements next to the name of the page size aren't the page measurements; instead, they're the measurements of the imageable area for that page size. The imageable-area dimensions are always less than the dimensions of the page so that the margin marks can fit on the page with the illustration.
The Orientation setting controls how the illustration is placed on the page. You have four choices: portrait, landscape, portrait reversed, and landscape reversed.
Choosing Portrait causes the illustration to print with the sides of the illustration along the longest sides of the page. Choosing Landscape causes the illustration to print with the top and bottom of the illustration along the longest sides of the page.
Usually, the orientation reflects the general shape of the illustration. If the illustration is taller than it is wide, you usually choose Portrait orientation. If the illustration is wider than it is tall, you usually choose Landscape orientation.
Note
It doesn't matter to Illustrator whether the illustration fits on the page in one or both of these orientations. If you can't see all four edges of the Bounding Box, chances are good that the illustration will be cropped. Orientation is quite different from Transverse. Orientation changes the orientation of the illustration on the page, but Transverse changes the way the page is put on the paper. It's a seemingly small difference but an important one to understand.
Figure 18.5 shows an illustration that's placed on a page in both portrait and landscape orientations, with and without the Transverse option selected.
Hang out around strippers (at a commercial printing company . . . get your mind out of the gutter!), and you hear them constantly talk about emulsion up and emulsion down. In printing, emulsion is a photosensitive coating that's applied, dried, exposed, and then washed off, leaving the areas to be printed open for ink to pass through. The nonprinting areas retain the emulsion to prevent the ink from passing through. If you have a piece of film from a printer lying around, look at it near a light. One side is shinier than the other side. That side is the side without emulsion. When you're burning plates for presses, the emulsion side (dull side) should always be toward the plate.
In the Output area of the Print dialog box, you use the Emulsion option to control which side the emulsion goes on. If you're printing negatives on film, choose Down (Right Reading) from the Emulsion dropdown list (popup menu). For printing on paper, just to see what the separations look like, choose Up (Right Reading). Always consult with your printer for the correct way to output film.
Tip
Although wrong reading isn't an option in the Separation Setup dialog box, you can reverse an illustration by choosing the opposite emulsion setting. In other words, Down (right reading) is also Up (wrong reading), and Up (right reading) is also Down (wrong reading).
Thinking of the emulsion as the toner in a laser printer may help you understand this concept better. If the toner is on the top of the paper, you can read it fine, as always (Up emulsion, right reading). If the toner is on the bottom of the paper and you can read the illustration only when you place the paper in front of a light, the emulsion is Down, right reading. Thinking along these lines helped me when I was new to the printing industry, and it should also help you.
You use the Image dropdown list (popup menu) to switch between printing positive and negative images. Usually, you use a negative image for printing film negatives and a positive image for printing on paper. The default for this setting, regardless of the printer chosen or PPD selected, is Positive.
At the lower right of the Output pane of the Print dialog box, the Document Ink Options list displays where you can choose different colors and then set them to print or not print and also set Custom Colors to process separately.
The list of colors contains only the colors that are used in that particular illustration. At the top of the list of separation colors are the four process colors if they or spot colors that contain those process colors are used in the illustration. Below the process colors is a list of all the spot colors in the document.
Tip
If the illustration has any guides in it, their colors are reflected in the Document Ink Options list. From looking at the preview of the illustration in the Output options in the Print dialog box, you can't easily determine that these blank separations will print. The best thing to do is clear all guides by choosing View
By default, all process colors are set to print, and all spot colors are set to convert to process colors. Clicking the Convert All Spot Colors to Process check box toggles between converting everything (selected) and retaining spot colors (deselected).
Each color in the list has its own frequency and angle. Don't change the angle or frequency for process colors because the separator has automatically created the best values for the process colors at the halftone screen you've specified. Instead, ensure that any spot colors that may print have different angles from each other so that no moiré (wavy) patterns develop from them.
As soon as you type new values or choose different options using the color list, the changes are applied.
Color separations are necessary to print a color version of an illustration on most printing presses. Each separation creates a plate that's affixed to a round drum on a printing press. Ink that's the same color as that separation is applied to the plate, which is pressed against a sheet of paper. Because the ink adheres only to the printing areas of the plate, an image is produced on paper. Some printing presses have many different drums and can print a four-color job in one run. Other printing presses have only one or two drums, so the paper has to pass through the press four or two times, respectively, to print a four-color job.
The two types of color separations are process color separations and spot color separations. Each type has its own advantages and drawbacks, and you can use either type or a combination of both types for any print job. Process color separations typically use four colors (cyan, magenta, yellow, and black) to reproduce the entire range of colors. Spot color separations use a custom-mixed ink to precisely render a specific color.
Tip
You should always determine which type of separation you want before you begin to create a job electronically.
Jobs that are printed with spot colors are often referred to as two-color or three-color jobs when two or three colors are used. Although you can use any number of colors, most spot color jobs contain only a few colors.
Spot color printing is most useful when you're using two or three distinct colors in a job. For example, if I need only black and green to create a certain illustration, I would use only black and a green custom color for all the objects in the illustration.
There are three main reasons for using spot color separations rather than process color separations:
It's cheaper. Spot color printing requires a smaller press with fewer drums. For process color separations, you usually need to use a press with four drums or run the job through a smaller press a number of times.
Spot colors are cleaner, brighter, and smoother than the same colors that you create as process colors. To obtain a green process color, for example, you need to mix both cyan and yellow on paper. Using one spot color results in a perfectly solid area of color.
You can't duplicate certain spot colors, especially fluorescent and metallic colors, with process colors.
Illustrator creates spot colors whenever you specify a spot color in a swatch. If you use six spot colors and black, you could print seven spot color separations.
Spot colors do have their limitations and disadvantages. The primary limitation of using only spot colors is that the number of colors is restricted to the number of color separations that you want to produce. Remember that the cost of a print job is directly related to the number of different inks in the job.
The cutoff point for using spot colors is usually three colors. When you use four spot colors, you limit yourself to four distinct colors and use as many colors as a process color job that can have an almost infinite number of colors. However, spot color jobs of six colors aren't unusual. Sometimes, people use more than three spot colors to keep colors distinct and clear. Each of the six colors is bright, vibrant, and distinct from its neighbors, whereas different process colors seem to fade into each other.
Note
Spot colors are often incorrectly referred to as Pantone colors. Pantone is a brand name for a color-matching system. You can choose Pantone colors as custom colors and use them in Illustrator, and you can print them as either spot colors or as process colors.
Process color separation, also known as four-color separation, creates almost any color by combining cyan, magenta, yellow, and black inks. By using various combinations of different tints of each of these colors, you can reproduce many of the colors (more than 16 million of them) that the human eye can see.
Process printing uses a subtractive process. You start with bright white paper and darken the paper with various inks. Cyan, magenta, and yellow are the subtractive primaries (unlike additive monitor colors of red, green, and blue), and black is added to create true black, a color that the primaries together don't do very well.
The use of process color separation is advisable in two situations:
When the illustration includes color photographs
When the illustration contains more than three different colors
Everyone always says that you can create as many colors as you could ever want when you're using process colors. Maybe.
In Illustrator, you can specify colors up to 1/100% accuracy. As a result, 10,000 different shades are available for each of the four process colors. So, theoretically, 10,0004, or 10,000,000,000,000,000, different colors should be available, which is 10 quadrillion or 10 million billion. Any way you look at it, you have a heck of a lot of color possibilities.
Unfortunately, most imagesetters and laser printers can produce only 256 different shades for each color. This limitation of the equipment (not PostScript) drops the number of available colors to 2564, or 4,294,967,296, which is about 4.3 billion colors — only 1 billionth of the colors that Illustrator can specify.
This limitation is fortunate for humans, however, because the estimate is that we can detect a maximum of 100 different levels of gray — probably less. As a result, we can view only 1004, or 100,000,000, different colors.
You can, though, run into a problem when you preview illustrations. An RGB monitor (which is the color format used on computers) can theoretically display up to 16.7 million colors if each red, green, and blue pixel can be varied by 256 different intensities.
Another problem is that about 30% of the colors that you can view on an RGB monitor can't be reproduced by using cyan, magenta, yellow, and black inks on white paper. You can't create these unprintable colors in Illustrator, but you can create them in most other drawing and graphics software packages. These colors are for on-screen viewing pleasure only.
The secret to process color separation is that the four colors that make up all the different colors are themselves not visible. Each color is printed as a pattern of tiny dots, angled differently from the dots of the other three colors. The angles of each color are very important. If the angles are off even slightly, a noticeable wavy pattern commonly known as a moiré emerges.
The colors are printed in a specific order — usually cyan, magenta, yellow, and then black. Although the debate continues about the best order in which to print the four colors, black is always printed last.
To see the dots for each color, use a magnifying device to look closely at something that's preprinted and in full color. Even easier, look at the Sunday comics, which have bigger dots than most other printed pieces. The different color dots in the Sunday comics are quite visible, and the only colors used are cyan, magenta, yellow, and black.
The size of the dots that produce each of these separations is also important. The smaller the dots, the smoother the colors appear. Large dots (such as those in the Sunday comics) can actually take away from the illusion of a certain unified color because the different color dots are visible.
Figure 18.6 shows how process colors are combined to create new colors. In the figure, the first four rows show very large dots. The top three rows are cyan, magenta, and yellow. The fourth row is all four process colors combined, and the bottom row shows how the illustration looks when you print it.
Process color printing is best for photographs because photographs originate from a continuous tone that's made on photographic paper from film instead of dots on a printing press.
In Illustrator, you can convert custom colors to process colors either before or during printing. To convert custom colors to process colors before printing, select any objects that have a specific custom color and tint and then click the Process Color icon that appears in the Color panel. The color is converted to its process color counterpart, and all selected objects are filled with the new process color combination.
After you click the Process Color icon, if the selected objects become filled with white and the triangles for each process color are at 0%, you've selected objects that contain different colors or tints. Undo the change immediately.
To ensure that you select only objects that have the same color, select one of the objects and then choose Select
You can convert custom colors to process colors in the Output options in the Print dialog box and in many page-layout programs.
You can couple spot colors with process colors in Illustrator simply by creating both process and named spot colors in a document.
Usually, you add spot colors to process colors for these reasons:
You're using a company logo that has a specific color. By printing that color as a spot color, you make it stand out from the other coloring. In addition, color is more accurate when it comes from a specific ink rather than from a process color combination. Often, the logo is a Pantone color that doesn't reproduce true to form when you use process color separation.
You need a color that you can't create by using process colors. Such colors are most often metallic or fluorescent, but they can be any number of Pantone colors or other colors that you can't match with process colors.
You need a varnish for certain areas of an illustration. A varnish is a glazed type of ink that results in a shiny area wherever you use the varnish. You commonly use varnishes on titles and logos and over photographs.
You need a light color over a large area. The dots that make up process colors are most noticeable in light colors, but by using a spot color to cover the area with a solid sheet of ink that has no dots, you can make the area smoother and enhance it visually.
In some circumstances, you need to use a spot color as both a spot color and a process color. Normally, you can't do both, but the following steps describe one way to circumvent this problem:
If the color doesn't exist as a swatch, create a swatch for the color.
In the Swatch Options dialog box (double-click the swatch), choose Spot Color from the dropdown list (popup menu) and then click OK.
Duplicate the swatch by dragging it on top of the New Swatch icon (the little piece of paper).
In the Swatch Options dialog box for the duplicated swatch, choose Process Color from the dropdown list (popup menu) and then click OK.
Note
You can tell which swatch is which by looking at the lower-right corner of the swatches; the spot color swatch has a white triangle with a spot in it, whereas the process swatch is solid.
Many other software programs, particularly page-layout software programs, incorporate color-separation capabilities. These programs usually allow you to import Illustrator files that have been saved as Illustrator EPS files.
When you produce color separations from other software, ensure that any custom colors in the Illustrator illustration are present and accessible in the document that the illustration is placed within. Usually, you can set the custom colors to process separately or to spot separately.
Note
You can't change the colors of an imported Illustrator EPS document in a page-layout program, so be sure that the colors are correct for the illustration while it's in Illustrator.
Trapping is one of the most important but least understood issues in all of printing. Trapping is the process of overprinting different colored areas so there won't be any gaps between them.
Traps solve alignment problems when color separations are produced. The most common problem that occurs from misalignment is the appearance of white space between different colors.
Note
Although Illustrator incorporates a trapping effect, it's not a trap-happy piece of software. For detailed illustrations, it usually isn't worth your time to set the trapping inside Illustrator; instead, you want to have your printer do the work for you.
Note
The thought of trapping scares many graphic designers, not just because they don't know how to do it but also because they aren't sure what trapping is and what purpose it serves. Understanding the concept of trapping is the hard part; trapping objects is easy (although somewhat tedious in Illustrator).
Figure 18.7 shows a spot color illustration with four colors. The top row shows each of the individual colors. The first illustration in the second row shows how the illustration prints if all the separations are aligned perfectly. The second illustration in the second row shows what happens when the colors are misaligned. The third illustration in the second row shows how the illustration looks when trapped, with black indicating where two colors overprint each other.
This example shows extreme misalignment and excessive trapping; I designed it just as a black-and-white illustration for this book. Ordinarily, the overprinting colors may appear a tiny bit darker, but they don't show as black. I used black so that you can see what parts of the illustration overlap when trapping is used. The trapping in this case is more than sufficient to cover any of the white gaps in the second illustration.
Trapping is created by spreading or choking certain colors that touch each other in an illustration. To spread a color, enlarge an object's color so that it takes up more space around the edges of the background area. To choke a color, expand the color of the background (by scaling the paths/ objects in the background or by stretching the edges with the Direct Selection tool) until it overlaps the edges of an object.
The major difference between a spread and a choke has to do with which object is considered the background and which object is the foreground. The foreground object is the object that traps. If the foreground object is spread, the color of the foreground object is spread until it overlaps the background by a certain amount. If the foreground object is choked, the color of the background around the foreground object is expanded until it overlaps the foreground object by a certain amount.
Tip
To determine whether to use a choke or a spread on an object, compare the lightness and darkness of the foreground and background objects. The general rule is that lighter colors expand or contract into darker colors.
Figure 18.8 shows the original misaligned illustration and two ways of fixing it with trapping. The second star has been spread by 1 point, and the third star has been choked by 1 point.
Figure 18.7. This spot color illustration shows individual colors (top) and aligned, misaligned, and trapped composites.
Figure 18.8. The original illustration (left), fixing the star by spreading it 1 point (middle), and fixing the star by choking it 1 point (right)
Three common reasons why color separations don't align properly are that the negatives aren't the same size, the plates on the press aren't aligned perfectly when printing, or the design gods have decided that a piece is too perfect and needs gaps between abutting colors. Trapping is required because it's a solution for covering gaps that occur when color separations don't properly align.
Negatives can be different sizes for a number of reasons. When the film was output to an imagesetter, the film may have been too near the beginning or the end of a roll or separations in the same job may have been printed from different rolls. The pull on the rollers, while fairly precise on all but top-of-the-line imagesetters — where it should be perfect — can pull more film through when there's less resistance (at the end of a roll of film) or less film when there's more resistance (at the beginning of a roll of film). The temperature of the film may be different if a new roll is put on in the middle of a job, causing the film to shrink (if it's cold) or expand (if it's warm).
The temperature of the processor may have risen or fallen a degree or two while the film was being processed. Again, cooler temperatures in the chemical bays and in the air dryer as the film exits the process have an impact on the size of the film.
Film negatives usually don't change drastically in size, but they can vary up to a few points on an 11-inch page. That distance is huge when a page has several abutting colors throughout. The change in a roll of film is almost always along the length of the roll, not along the width. The quality of the film is another factor that determines how much the film stretches or shrinks.
Most strippers are quite aware of how temperature affects the size of negatives. A common stripper trick is to walk outside with a freshly processed negative during the colder months to shrink a negative that may have enlarged slightly during processing.
Check with your service provider staff to see how long the processor is warmed before sending jobs through it. If the answer is less than an hour, the chemicals will not be at a consistent temperature, and negatives that are sent through too early will certainly change in size throughout the length of the job. Another question to ask is how often the chemicals are changed and the density checked from the imagesetter. Once a week is acceptable for a good-quality service provider, but the best ones change chemicals and check density once a day.
The plates on a press can be misaligned by either an inexperienced press operator or a faulty press. An experienced press operator knows the press and what to do to get color plates to align properly. A faulty press is one where plates move during printing or aren't positioned correctly. An experienced press operator can determine how to compensate for a faulty press.
No press is perfect, but some of the high-end presses are pretty darn close. Even on those presses, the likelihood that a job with colors that abut one another can print perfectly is not very great.
If a job doesn't have some sort of trapping in it, it probably won't print perfectly, no matter how good the negatives, press, and press operator are.
The amount of trap that you need in an illustration depends on many things, but the deciding factor is what your commercial printer tells you is the right amount.
The most important thing to consider is the quality of the press that the printer uses. Of course, only the printer knows which press your job will run on, so talking to the printer about trapping is imperative.
Other factors to consider include the colors of ink and types of stock used in the job. Certain inks soak into different stocks differently.
Traps range from 4/1000 of an inch to 6/1000 of an inch. Most traditional printers refer to traps in thousandths of inches, but Illustrator likes values in points for this sort of operation. Figure 18.9 is a chart with traps in increments of 1/1000, from 1/1000 of an inch to 10/1000 of an inch, and gives their point measurements. The trapped area is represented by black to be more visible in this example.
Remember that the greater the trap, the less chance that any white gaps will appear, but the trap may actually be visible. Visible traps of certain color pairs can look almost as bad as white space.
In Illustrator, you accomplish manual trapping by selecting a path's stroke or fill and setting it to overprint another path's stroke or fill. Overprint can be turned on and off for each object by using the Attributes panel, which is accessed by choosing Window
The most basic way to create a trap on an object is by giving it a stroke that's either the fill color of the object (to create a spread) or the fill color of the background (to create a choke).
Tip
Be sure to make the width of any stroke that you use for trapping twice as wide as the intended trap because only half the stroke (one side of the path) actually overprints a different color. In some circumstances, fixing a stroke that's initially not wide enough can be difficult.
Another way to create a trap is to use the Pathfinder panel. Follow these steps:
Select all pieces of art that are overlapping or abutting.
Choose Trap from the Pathfinder panel. The Trap button appears when you choose Options from the Pathfinder panel's popup menu.
Type the width into the Height/Width text field.
Click OK to apply the trap.
The preceding trap explanations are extremely simplified examples of trapping methods in Illustrator. In reality, objects never seem to be a solid color, and if they are, they're never on a solid background. In addition, most illustrations contain multiple overlapping objects that have their own special trapping needs.
I consider trapping to be complex when I can't just go around selecting paths and applying the trap quickly. Complex trapping involves several techniques:
Create a separate layer for trapping objects. By keeping trapping on its own layer, you make myriad options available that aren't available if the trapping is intermixed with the rest of the artwork. Place the new layer above the other layers. Lock all the layers but the trapping layer so that the original artwork is not modified. You can turn trapping on and off by hiding the entire layer or turning off the Print option in the Layers Options dialog box.
Use the round joins and ends options in the Stroke portion of the Stroke panel for all trapping strokes. Round joins and ends are much less conspicuous than the harsh corners and 90° angles of other joins and ends, and they blend smoothly into other objects.
Trap gradations by stroking them with paths that are filled with overprinting gradients. You can't fill strokes with gradients, but you can fill paths with gradients. You can make any stroke into a path by selecting it and then choosing Outline from the Pathfinder panel. After you transform the stroke into a path, fill it with the gradient and then click the Overprint Fill check box (in the Attributes panel) for that path.
Note
Whenever I start a heavy-duty trapping project, I always work on a copy of the original illustration. Wrecking the original artwork is just too easy when you add trapping.
Printing Illustrator documents can often be quite a bit more complex than simply choosing File
Illustrator can be interpreted as a good front end for the PostScript page description language.
Print separations from within Illustrator.
Choose whether to print a composite or separations from the Print dialog box.
Determine separation information in the Output section of the Print dialog box.
Prevent potential white strips that can appear when a printer isn't perfectly aligned with trapping.
Output options in the Print dialog box let you specify which colors print and at what angle and frequency they print.