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Appropriate Image Formats for Print

How you should save your raster images is governed largely by how you intend to use them. Often, you will be placing images into InDesign or Illustrator, so you’re limited to the formats supported by those applications. The application may be willing to let you place a wide variety of file formats, but that doesn’t necessarily serve as an endorsement of file format wonderfulness. In the olden days, the most commonly used image formats were TIFF and EPS. However, native Photoshop files (PSD) and Photoshop PDF files are much more flexible, and both formats are supported by InDesign and Illustrator. So, there’s not much reason to use other formats unless you’re handing off your images to users of other applications, such as Microsoft PowerPoint or Word.

TIFF

If you need to blindly send an image out into the world, TIFF (tagged image file format) is one of the most widely supported image file formats. It’s happy being imported into Illustrator, InDesign, Microsoft Word, and even some text editors—almost any application that accepts images. The TIFF image format supports multiple layers as well as RGB and CMYK color spaces, and even allows an image to contain spot-color channels (although some applications, such as Word, do not support such nontraditional contents in a TIFF).

Photoshop EPS

Some equate the acronym EPS (Encapsulated PostScript) with vector artwork, but the encapsulated part of the format’s name gives a hint about the flexibility of the format. It’s a container for artwork, and it can transport vector art, raster images, or a combination of raster and vector content. EPS is, as the name implies, PostScript in a bag (see the sidebar, “EPS: Raster or Vector?”). The historic reasons for saving an image as a Photoshop EPS were to preserve the special function of a PostScript-based vector clipping path used to silhouette an image or to preserve an image set up to image as a duotone. If you’re using InDesign and Illustrator, that’s no longer necessary.

As applications and RIPs have progressed, you’re no longer required to save such images as Photoshop EPS. Pixel for pixel, a Photoshop native PSD is a smaller file than an equivalent EPS and offers support for clipping paths as well as duotone definitions.

This doesn’t mean you need to hunt down your legacy Photoshop EPS files and resave them as PSD (unless you’re terribly bored). Just know that unless you need to accommodate someone else’s requirements, there’s no advantage to saving as Photoshop EPS now.

Photoshop Native (PSD)

In ancient times, the native PSD (Photoshop document) format was used solely for working files in Photoshop. Copies of those working files were flattened and saved in TIFF or EPS formats for placement in a page-layout program. While PageMaker allowed placement of native Photoshop files (yes, really—although it did not honor transparency), QuarkXPress required TIFF or EPS instead. Old habits die hard, and TIFF and EPS have long been the standard of the industry. Not that there’s anything truly wrong with that.

However, Illustrator and InDesign can take advantage of the layers and transparency in Photoshop native files, eliminating the need to go back through two generations of an image to make corrections to an original file. Today, there’s no need to maintain two separate images: the working image and the finished file are now the same file.

Photoshop PDF

A Photoshop PDF (Portable Document Format) contains the same pixels as a garden-variety PSD, but those pixels are encased in a PDF wrapper—it’s like the chocolate-covered cherry of file formats. A Photoshop PDF comes in handy on special occasions, because it can contain vector and type elements without rasterizing the vector content, and it allows nondestructive round-trip editing in Photoshop.

A Photoshop EPS can contain vectors and text, but the vector content will be converted to pixels if the file is reopened in Photoshop, losing the crisp vector edge—so you lose the ability to edit text or vector content. A native Photoshop PSD can contain vector components, but page-layout programs rasterize the content. However, Photoshop PDFs preserve vector content when placed in other applications (see Table 4.1 for a feature comparison of common image formats).

Table 4.1 Image format features

Supported Feature

TIFF

EPS

PSD

JPEG

PDF

RGB color space

X

X

X

X

X

CMYK color space

X

X

X

X

X

Grayscale

X

X

X

X

X

ICC profiles

X

X

X

X

X

Clipping paths

X

X

X

X

X

Layers

X

X

X

Alpha channels

X

X

X

Spot color channels

X

1

X

X

Duotones

X

X

X

Bitmap (bi-level content)

X

X

X

X

Vector data

2

3

X

Transparency

X

X

X

1 If saved as DCS 2.0 (a variant of the EPS format)

2 EPSs cannot be reopened in Photoshop with vector content intact

3 Page-layout applications rasterize vector content in PSDs

Moving to Native PSD and PDF

Is there any compelling reason to continue using old-fashioned TIFFs and EPSs? It may seem adventurous to use such new-fangled files, but workflow is changing. The demarcation between photo-compositing and page layout is blurring, and designers demand more power and flexibility from software. RIPs are more robust than ever, networks are faster, and hard drives are huge. It’s still important to know the imaging challenges posed by using native files (such as transparency), and it’s wise to communicate with your printer before you embark on the all-native path. You’re still at the mercy of the equipment and processes used by the printer, and if they’re lagging a bit behind the latest software and hardware developments, you may be limited by their capabilities.

Bitmap Images

Also called “line art images,” bitmap images contain only black and white pixels, with no intermediate shades of gray. If you need to scan a signature to add to an editorial page or scan a pen and ink sketch, a bitmap scan can provide a sharp, clean image. Because of the compact nature of bitmap scans, they can be very high resolution (usually 600–1200 ppi) but still produce small file sizes (Figure 4.5).

Figure 4.5

Figure 4.5 This 1200 ppi bitmap scan prints nearly as sharply as vector art. It weighs in at less than 1 MB; a grayscale image of this size and resolution would be nearly 10 MB. Magnified to 300 percent, it may look a bit rough, but at 100 percent it’s crisp and clean.

Special Case: Screen Captures

If you’re creating software documentation for print, or you want to show an image of a Web page in your project, you may need to include screen captures of software interface components such as menus or panels in your page layouts. Screen captures are easy to make using a system utility or dedicated screen-capture software, but they require some special handling to print clearly. When they’re part of software documentation or instructional materials, it’s important that the details are as sharply rendered as possible.

You should understand this about screen captures: Whether you take them by using your system’s built-in screen-capture functionality or a third-party screen-capture application, you are merely intercepting information that eventually becomes pixels on your monitor. Regardless of your current monitor resolution, there is a one-to-one relationship between the fixed number of pixels that an application (and your system) uses to render panels and menus and the number of pixels you see on your screen, even if you use a zoom utility. Of course, the size of the overall image you see is a function of your current monitor resolution, but the pixel dimensions of panels, menus, and tools will be identical, regardless of resolution. (Figure 4.6)

Figure 4.6

Figure 4.6 The resolution setting of your monitor has no effect on the number of pixels used by panels and menus. Although this panel was captured at three different monitor resolutions, the three captures are identical, each consisting of exactly the same number of pixels.

An application panel that measures 244 pixels by 117 pixels appears larger when your screen resolution is set to 800 by 600, and it’s almost unreadably small when your monitor is set to 1920 by 1200. However, the panel is made of exactly the same number of pixels in both instances. So it doesn’t matter what resolution your monitor is using, or how large the panels may appear onscreen, or whether you use a utility to zoom in. The captured image of a panel or menu will be the same in terms of pixel dimensions, regardless of the monitor resolution setting, and the resulting image will be 72 ppi.

Since it’s been drilled into you that 300 ppi is the Holy Grail of image resolution, it’s tempting to try to improve screen captures by increasing the resolution. Unfortunately, this usually makes them look worse by softening small details during interpolation.

If you plan to use a screen capture at 100 percent enlargement, just leave it at 72 ppi (go ahead and freak out). Yes, the print service provider’s prepress department will raise a flag, but the examples below show why screen captures are not improved by increasing their resolution.

As you can see in Figure 4.7, the original 72 ppi screen capture seems a bit coarse, but it’s readable. Increasing the resolution to 300 ppi in Photoshop may sound like a good idea, but the interpolation will soften detail in the image.

Figure 4.7

Figure 4.7 Image A is the original 72 ppi screen shot. Image B is the result of increasing the resolution to 300 ppi, using the default Bicubic method: Note blurry text and softened edges. Image C is the result of increasing the resolution to 288 ppi, using Nearest Neighbor.

If you do feel compelled to increase the resolution of a screen capture, choose Image > Image Size in Photoshop, and then set the resolution to an even multiple of the original resolution; for example, resample a 72 ppi screen shot to 288 ppi. In that same dialog box, set the Resample Image option to Nearest Neighbor. This avoids interpolation by simply repeating pixels rather than attempting to create pixels. It’s not an appropriate approach when scaling images of a photographic nature, but it’s a helpful solution for screen captures, because of their special nature.

Converting Screen Captures to CMYK

Because screen captures are generated as RGB images, they must usually be converted to CMYK for print. When performing that conversion, a special approach is recommended to maintain the best rendering of black type. The default conversion of RGB to CMYK in Photoshop will render black as a four-color mix (Figure 4.8), with the possibility that slight misregistration on press will turn tiny details to mush.

Figure 4.8

Figure 4.8 A conventional conversion from RGB to CMYK produces four-color equivalents of the gray and black parts of a screen capture. Press misregistration will turn text and other black or gray elements to an out-of-focus rainbow. Festive, but hard to read.

To simplify printing of screen captures, use a color-separation recipe that ensures that all neutral black or gray areas of the image will print only in black ink during the RGB-to-CMYK conversion. Neutral areas in an RGB image are those areas in which the RGB values are equal; for example, R128–G128–B128 would constitute a midtone gray.

To create this custom screen-capture conversion recipe in Photoshop, choose Edit > Color Settings to access the color-separation controls. Under Working Spaces, choose Custom for the CMYK setting (Figure 4.9).

Figure 4.9

Figure 4.9 In the Color Settings dialog box, select Custom CMYK from the CMYK menu.

In the Custom CMYK dialog box, select Maximum Black Generation (Figure 4.10). The curve you see may seem odd, but it merely indicates that all equivalent RGB values are being replaced with black. The appearance of color elements won’t be compromised.

Figure 4.10

Figure 4.10 In the Custom CMYK dialog box, select the Maximum Black Generation setting. This consolidates all gray-equivalent values to the black channel, minimizing issues with registration.

Color elements will be composed of four colors in the final CMYK image. But black and gray elements will be rendered only in black (Figure 4.11). While this may look odd, it results in cleaner printing of the screen capture, because there aren’t four colors piling up in most of the image.

Figure 4.11

Figure 4.11 All the color components appear on the cyan, magenta, and yellow plates. Black and gray areas appear only on the black plate. This special treatment ensures that screen shots print cleanly.

RGB vs. CMYK

Since the dawn of desktop publishing, it’s been unquestioned that Thou Shalt Convert to CMYK. Those who submitted RGB files were considered uninformed, even uncivilized.

The rules are changing, though, because of the increased use of digital printing. Although these devices may use inks or toners named cyan, magenta, yellow, and black, those inks and toners have a different pigment makeup than the namesake inks used on offset presses, and they have a wider color gamut than offset inks. Inkjet devices such as large-format printers utilize additional inks such as light cyan, pink, light yellow, orange, and green, further extending the range of colors that they can print.

This seems like a good time to open a can of multicolored worms. After you’ve been told by printers for years that you should convert your images to CMYK before submitting, I’m now going to tell you that you might not have to do so. That’s because many digital devices happily digest RGB and can provide more vibrant output by rendering RGB content.

When you convert to CMYK, ranges of colors outside the CMYK gamut are remapped to fall within the CMYK printable gamut, and some of your most vibrant colors are lost forever.

If you happen to have some very colorful RGB images (tropical birds would do the trick), try this little experiment:

  1. Open the RGB image in Photoshop, and maybe make it even more vibrant by using Hue/Saturation or Vibrance. Get carried away; this is for science, after all, not for art.
  2. Choose Edit > Color Settings. At the top of the dialog box, choose North America Prepress 2 from the menu and click OK.
  3. Choose View > Proof Colors. The difference in appearance may not be huge, but try toggling Proof Colors on and off quickly by using the keyboard shortcut (PC: Ctrl-Y; Mac: Cmd-Y) and watch for differences in bright blues and greens. Neon greens provide a particularly noticeable difference.
  4. Choose View > Gamut Warning. The gray areas are areas whose current RGB color will be remapped (and probably become duller) when converted to CMYK, because of the smaller color gamut of CMYK.

This gives you an idea of the color range that you’ll lose when you convert to CMYK—and much of that color range can be imaged on many digital devices. Of course, ask the print service provider before you submit your work to ensure that you’re sending what they want. Just don’t be surprised if they say “RGB is OK.”

RGB as a Working Format

Because the RGB gamut is larger than that of CMYK, it’s often preferable to perform color corrections and compositing with RGB files, converting to CMYK (if necessary) as late in the process as possible. If you are participating in a fully color-managed workflow, you will keep your images as RGB with ICC profiles. The International Color Consortium (ICC) was formed by a group of graphic arts industry vendors, with the goal of promoting the use and standardization of color management tools. ICC profiles are methods of describing the characteristics of devices such as scanners, presses, and printers for optimal results. Conversion will not take place until the job is imaged. Much of today’s software offers sophisticated support of color management. For example, when exporting a PDF or printing, InDesign will perform the same conversion of RGB to CMYK that Photoshop would (assuming you’ve synchronized your color settings across all your Creative Cloud applications).

What if the Printer Demands CMYK Images?

Some print service providers and their customers have fully adopted color-managed workflows as part of their regular operation. But many print service providers (especially in North America) expect CMYK when you submit your job, believing that it’s what Nature intended, especially when the job will be printed on an offset press (as opposed to a digital printer). Consult with your printer to see what they prefer. If you’re using digital photography or scanning your own artwork, they should be able to provide you with their preferred settings, so you can make appropriate conversions to CMYK.

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