Photoshop.CS.Bible [Electronic resources] نسخه متنی

Creating Color Separations

Unless you’re a printing professional, you’ll rarely have to print color separations directly from Photoshop. You’ll more likely import the image to QuarkXPress, PageMaker, InDesign, or a similar application before printing separations. It’s even more likely that you’ll take the image or page-layout file to a commercial printer and have a qualified technician take care of it.

So why discuss this process? Two reasons. First, it’s always a good idea to at least peripherally understand all phases of the computer imaging process, even if you have no intention of becoming directly involved. This way, if something goes wrong on the printer’s end, you can decipher the crux of the problem and either propose a solution or strike a compromise that still works in your favor.

Second, before you import your image to another program or submit it to a commercial printer, you’ll want to convert the RGB image to the CMYK color space. (You don’t absolutely have to do this — with Photoshop’s improved color-matching functions, you can exchange RGB images with greater confidence — but it’s always a good idea to prepare your images down to the last detail, and CMYK is invariably the final destination for printed imagery.)

Outputting separations

Accurately converting to CMYK is the trickiest part of printing color separations; the other steps require barely any effort at all. So without further ado, here’s how you convert an image to the CMYK color space and print separations. Many of the steps are the same as when printing a grayscale or color composite, others are new and different.

STEPS: Printing CMYK Color Separations

Calibrate your monitor and specify the desired RGB environment. Use the techniques discussed in the following sections in Chapter 16: “The Gamma control panel,” “The Macintosh Display Calibrator Assistant,” and “Selecting the ideal working space.”

Identify the final output device. Again, follow the advice I give in Chapter 16, this time in the section “Custom CMYK Setup.” If you’re lucky, your commercial printer may provide a CMYK table that you can load. Otherwise, you have to grapple with some weird settings. The good news is that you need to complete this step only once each time you switch hardware. If you always use the same commercial printer, you can set it up and forget about it.

Convert the image to the CMYK color space. Choose Image Mode CMYK Color to convert the image from its present color mode to CMYK.

Adjust the individual color channels. Switching color modes can dramatically affect the colors in an image. To compensate for color and focus loss, you can edit the individual color channels as described in the “Color Channel Effects” section of Chapter 4.

Trap your image, if necessary. If your image features many high-contrast elements and you’re concerned that your printer might not do the best job of registering the cyan, magenta, yellow, and black color plates, you can apply Image Trap to prevent your final printout from looking like the color funnies on a bad Sunday. (When working with typical “continuous-tone” photographs, you can skip this step.)

Choose your printer. Select the printer you want to use, as described earlier in this chapter.

Turn on a few essential printer marks. Choose File Page Setup or press Ctrl+Shift+P (z -Shift-P on the Mac) to specify the size of the pages and the size and orientation of the image on the pages, as described in "The Page Setup dialog box" section, earlier in this chapter. And in the Print with Preview dialog box, also introduced earlier, be sure to select the Calibration Bars, Registration Marks, and Labels check boxes, at the very least. (You need to select the Show More Options check box and then select Output from the pop-up menu to display these options.)

Adjust the halftone and transfer functions as needed. Click the Screen and Transfer buttons in the Print with Preview dialog box to modify the halftone screen dots and map brightness values for each of the CMYK color channels, as described earlier in the “Changing the halftone screen” and “Specifying a transfer function” sections. This step is entirely optional.

Send the job to the printer. In the Print with Preview dialog box, make sure the Show More Options check box is turned on and choose Color Management from the pop-up menu. Then choose Separations from the Profile pop-up menu in the Print Space section of the dialog box. This tells Photoshop to print each color channel to a separate piece of paper or film. Finally, click the Print button to bring up the Print dialog box and initiate the print job.

Steps 1 through 4 were covered at length in Chapters 4 and 16. Steps 6 through 9 are repeats of concepts explained in previous sections of this chapter. This leaves Step 5 — trapping — which I explain in the following section.

Color trapping

If color separations misalign slightly during the reproduction process (a problem called misregistration), the final image can exhibit slight gaps between colors. Suppose an image features a 100-percent cyan chicken against a 100-percent magenta background. (Pretty attractive image idea, huh? Go ahead, you can use it if you like.) If the cyan and magenta plates don’t line up exactly, you’re left with a chicken with a white halo partially around it. Yuck.

A trap is an extra bit of color that fills in the gap. For example, if you choose Image Trap and enter 4 in the Width option box, Photoshop outlines the chicken with an extra 4 pixels of cyan and the background with an extra 4 pixels of magenta. Now the registration can be off a full 8 pixels without any halo occurring.

Continuous-tone images, such as photographs and natural-media painting, don’t need trapping because no harsh color transitions occur. In fact, trapping actually harms such images by thickening up the borders and edges, smudging detail, and generally dulling the focus.

One of the primary reasons to use the Trap command, therefore, is to trap rasterized drawings from Illustrator or FreeHand. Some state-of-the-art prepress systems trap documents by first rasterizing them to pixels and then modifying the pixels. Together, Photoshop and Illustrator (or FreeHand) constitute a more rudimentary but, nonetheless, functional trapping system. When you open an illustration in Photoshop, the program converts it into an image according to your size and resolution specifications, as described in the “Rasterizing an Illustrator or FreeHand file” section of Chapter 3. After the illustration is rasterized, you can apply ImageTrap to the image as a whole. Despite the command’s simplicity, it handles nearly all trapping scenarios, even going so far as to reduce the width of the trap incrementally as the colors of neighboring areas grow more similar.