A trap is an overlap that prevents tiny gaps from appearing in the printed image, due to a slight misregistration on press. It's important that you first contact your service provider to see whether or not you should do any trapping. In most cases, your print shop will determine if trapping is needed and if so, they will tell you what values to enter in the Trap dialog box.



















Illustration of Trapping to correct misalignment with these callouts: A. Misregistration with no trap B. Misregistration with trap
Trapping to correct misalignment A. Misregistration with no trap B. Misregistration with trap

Trapping is intended to correct the misalignment of solid colors. In general, you don't need traps for continuous-tone images such as photographs. Excessive trapping may produce an outline effect. These problems may not be visible on-screen and might show up only in print. Photoshop uses standard rules for trapping:
* All colors spread under black.
* Lighter colors spread under darker colors.
* Yellow spreads under cyan, magenta, and black.
* Pure cyan and pure magenta spread under each other equally.

To create a trap:

1. Save a version of the file in RGB mode, in case you want to reconvert the image later. Then choose Image > Mode > CMYK Color to convert the image to CMYK mode.
2. Choose Image > Trap.
3. For Width, enter the trapping value provided by your print shop. Then select a unit of measurement, and click OK. Consult your print shop to determine how much misregistration to expect.

By default, the printer driver transfers binary information to PostScript printers; however, you can choose to transfer image data using JPEG or ASCII85 encoding. These options are not available to non-PostScript printers such as many inkjet models.

ASCII85 is a text encoding for binary data that is more compact than ASCII encoding.

JPEG-encoded files are smaller than binary files, and therefore require less time to print; however, using JPEG encoding decreases the image quality. Only PostScript Level 2 (or higher) printers support JPEG encoding; sending a JPEG-encoded file to a PostScript Level 1 output device may result in PostScript language errors.

Some print spooler programs, computer networks, and third-party printer drivers don't support files that are binary or JPEG-encoded, and some PostScript output devices accept binary and JPEG-encoded image data only through their AppleTalk and Ethernet ports, not their parallel or serial ports. In these situations, you can select the ASCII85 encoding method. However, ASCII85 files contain about twice as many characters and require about twice as much time to transfer as binary files.

To change the encoding method:

1. Choose File > Print with Preview.
2. Select Show More Options, and choose Output from the pop-up menu.
3. Select an option from the Encoding menu.

When preparing your image for prepress and working with CMYK images or images with spot colors, you can print each color channel as a separate page.














Each color channel printed as a separate page

Note: If you are printing an image from another application and want to print spot channels to spot color plates, you must first save the file in DCS 2.0 format. DCS 2.0 preserves spot channels and is supported by applications such as Adobe InDesign, Adobe PageMaker, and QuarkXPress®. (See Saving files in Photoshop EPS format (Photoshop).)

To print separations from Photoshop:

1. Make sure your document is in CMYK Color or Multichannel mode, and then choose File > Print with Preview.
2. Select Show More Options, and choose Color Management from the pop-up menu.
3. Choose Separations from the Profile pop-up menu.

Note: Depending on the designated printer and print drivers on your computer, these options may also appear in the Print dialog box (for setting printer options). On Windows, click the Properties button to access the printer driver options. On Mac OS, use the pop-up menu to access the printer driver options.
4. Click Print. Separations are printed for each of the colors in the image.

To prepare an image with spot channels for printing from another application:

1. If the image is a duotone, convert to Multichannel color mode.
2. Save the image in DCS 2.0 format.
3. In the DCS 2.0 Format dialog box, be sure to deselect the Include Halftone Screen and the Include Transfer Function options. (See Saving files in Photoshop EPS format (Photoshop).)
4. Open or import the image in the application you will be printing from, and set your screen angles. Make sure that you've communicated to the printer the spot color you want for each of the color plates.

Note: You can place a PSD file containing spot colors directly in InDesign CS without special preparation.

Halftone screen attributes include the screen frequency and dot shape for each screen used in the printing process. For color separations, the prepress person must also specify an angle for each of the color screens. Setting the screens at different angles ensures that the dots placed by the four screens blend to look like continuous color and do not produce moiré patterns.

Halftone screens consist of dots that control how much ink is deposited at a specific location on-press. Varying their size and density creates the illusion of variations of gray or continuous color. For a process color image, four halftone screens are used: cyan, magenta, yellow, and black--one for each ink used in the printing process.










Halftone screen with black ink










Halftone screens with process ink at different screen angles; correctly registered dots form rosettes
Halftone screens with process ink at different screen angles; correctly registered dots form rosettes

In traditional print production, a halftone is produced by placing a halftone screen between a piece of film and the image and then exposing the film. In Photoshop, you specify the halftone screen attributes just before producing the film or paper output. For best results, your output device (a PostScript imagesetter, for example) should be set to the correct density limit, and your processor should be properly calibrated; otherwise, results can be unpredictable.

Before creating your halftone screens, check with your print shop for preferred frequency, angle, and dot settings. (Use the default angle settings unless your print shop specifies changes.)

To define the screen attributes:

1. Choose File > Print with Preview.
2. Select Show More Options, choose Output from the pop-up menu, and click Screen.
3. In the Halftone Screens dialog box, choose whether to generate your own screen settings:
* Deselect Use Printer's Default Screens to choose your own screen settings.
* Select Use Printer's Default Screens to use the default halftone screen built into the printer. Photoshop then ignores the specifications in the Halftone Screens dialog box when it generates the halftone screens.
4. For a grayscale halftone, enter a screen frequency from 1 to 999.999, and choose a unit of measurement. Enter a screen angle from -180 to +180 degrees.
5. For a color separation, choose from the following options:
* To manually enter the screen frequency and angle, choose a color of the screen for Ink, and enter the frequency and angle; repeat for each color separation.
* To have Photoshop determine and enter the best frequencies and angles for each screen, click Auto. In the Auto Screens dialog box, enter the resolution of the output device and the screen frequency you intend to use, and click OK. Photoshop enters the values in the Halftone Screens dialog box. Changing these values may result in moiré patterns.
* If you are using a PostScript Level 2 (or higher) printer or an imagesetter equipped with an Emerald controller, make sure that the Use Accurate Screens option is selected in the Auto Screens dialog box (or in the Halftone Screens dialog box, if you're entering the values manually). The Use Accurate Screens option lets the program access the correct angles and halftone screen frequencies for high-resolution output. If your output device is not a PostScript Level 2 (or higher) printer or is not equipped with an Emerald controller, this option has no effect.
6. For Shape, choose the dot shape you want. If you want all four screens to have the same dot shape, select Use Same Shape For All Inks.

Choosing Custom from the Shape menu displays the Custom Spot Function dialog box. You can define your own dot shapes by entering PostScript commands--useful for printing with nonstandard halftone algorithms. For information about using PostScript language commands, see the PostScript Language Reference published by Addison-Wesley, or consult the imagesetter's manufacturer.

For optimal output on a PostScript printer, the image resolution should be 1.5 to 2 times the halftone screen frequency. If the resolution is more than 2.5 times the screen frequency, an alert message appears. (See Understanding image size and resolution.) If you are printing line art or printing to a non-PostScript printer, see your printer documentation for the appropriate image resolutions to use.
7. Click OK.

To save halftone screen settings:

In the Halftone Screens dialog box, click Save. Choose a location for the saved settings, enter a filename, and click Save.

Tip iconTo save the new settings as the default, hold down Alt (Windows) or Option (Mac OS), and click the --> Default button.

To load halftone screen settings:

In the Halftone Screens dialog box, click Load. Locate and select the settings, and click Load.

Tip iconTo return to the original default settings, hold down Alt (Windows) or Option (Mac OS), and click <--Default.

If you are preparing your images for commercial printing directly from Photoshop, you can select and preview a variety of page marks and other output options using the Print with Preview command.



Illustration of Page marks with these callouts: A. Gradient tint bar B. Label C. Registration marks D. Progressive color bar E. Corner crop mark F. Center crop mark G. Description H. Star target
Page marks A. Gradient tint bar B. Label C. Registration marks D. Progressive color bar E. Corner crop mark F. Center crop mark G. Description H. Star target

To set output options:

1. Choose File > Print with Preview.
2. Select Show More Options, and choose Output from the pop-up menu.

Note: Options not supported by the designated printer are dimmed.
3. Set one or more of the following options:

Background

Lets you select a background color to be printed on the page outside the image area. For example, a black or colored background may be desirable for slides printed to a film recorder. To use this option, click Background, and then select a color from the Color Picker dialog box. This is a printing option only; it does not affect the image itself.

Border

Lets you print a black border around an image. Type in a number and choose a unit value to specify the width of the border.

Bleed

Lets you print crop marks inside rather than outside the image. Use this option when you want to trim the image within the graphic. Type a number and choose a unit value to specify the width of the bleed.

Screen

Lets you set the screen frequency and dot shape for each screen used in the printing process. (See Selecting halftone screen attributes.)

Transfer

Lets you adjust the transfer functions, traditionally used to compensate for dot gain or dot loss that may occur when an image is transferred to film. This option is recognized only when you print directly from Photoshop, or when you save the file in EPS format and print to a PostScript printer. Generally, it's best to adjust for dot gain using the settings in the CMYK Setup dialog box. Transfer functions are useful, however, when compensating for a poorly calibrated output device. (See Compensating for dot gain in film using transfer functions.)

Interpolation

Reduces the jagged appearance of a low-resolution image by automatically resampling up while printing. However, resampling may reduce the sharpness of the image quality. (See About resampling.) Some PostScript Level 2 (or higher) printers have interpolation capability. If your printer doesn't, this option has no effect.

Calibration Bars

Prints an 11-step grayscale, a transition in density from 0 to 100% in 10% increments. With a CMYK color separation, a gradient tint bar is printed to the left of each CMY plate, and a progressive color bar to the right.

Note: Calibration bars, registration marks, crop marks, and labels will print only if the paper size is larger than the printed image dimensions.

Registration Marks

Prints registration marks on the image (including bull's-eyes and star targets). These marks are used primarily for aligning color separations.

Corner Crop Marks

Prints crop marks where the page is to be trimmed. You can print crop marks at the corners.

Center Crop Marks

Prints crop marks where the page is to be trimmed. You can print crop marks at the center of each edge.

Description

Prints any description text entered in the File Info dialog box up to about 300 characters. (See Adding file information (Photoshop).) Description text always prints as 9-point Helvetica plain type.

Labels

Prints the filename above the image.

Emulsion Down

Makes type readable when the emulsion is down--that is, when the photosensitive layer on a piece of film or photographic paper is facing away from you. Normally, images printed on paper are printed with emulsion up, with type readable when the photosensitive layer faces you. Images printed on film are often printed with emulsion down.

Negative

Prints an inverted version of the entire output, including all masks and any background color. Unlike the Invert command in the Image menu, the Negative option converts the output, not the on-screen image, to a negative. If you print separations directly to film, you probably want a negative, although in many countries film positives are common. Check with your print shop to determine which is required.

To determine the emulsion side, examine the film under a bright light after it has been developed. The dull side is the emulsion; the shiny side is the base. Check whether your print shop requires film with positive emulsion up, negative emulsion up, positive emulsion down, or negative emulsion down.