Understanding and Choosing File Formats
In this chapter
Choosing a File Format
The Final Destinations
Photoshop's File Formats
Illustrator's File Formats
File Formats for InDesign
GoLive and File Formats
Mastering the Adobe Creative Suite
In this chapter, file formats are discussed, as are the various capabilities of each format and information on which format to select for which purpose. The information you learn here will help you determine an appropriate file format based on a file's final destination. You'll find a handy chart presenting the format capabilities and limitations of the major raster image formats. The chapter also offers an extensive discussion of how the JPEG file format compresses data, and the hazards of saving as JPEG multiple times.
Photoshop's new PSB file format for large images is presented here, as is the JPEG 2000 file format.
Choosing a File Format
Selecting the proper file format for your final artwork or project can mean the difference between having a usable document and a worthless collection of zeros and ones recorded to magnetic media. The file format should be determined by the image's final destination; for example, the Web has different requirements than do page layout programs. Even the Web design or page layout program you use can make a difference in what format to choose.
Within the broad categories of Web and print, additional choices need to be made. The file content will help you determine which format to choose.
Different file formats have different capabilities. Photoshop's native PSD format supports all the program's features. Some formats restrict the number of colors that can be used in a file (GIF and PNG-8, for example), and many don't support the CMYK color mode. The capability of portraying transparency with clipping paths and/or alpha channels also varies among formats. Vector type and vector paths can be supported in only a few file formats. Illustrator, InDesign, and GoLivethe other programs of the Adobe Creative Suiteall have their own file formats. The choice of format may require that you consider such features.
Generally speaking, working in a program's native file format until your project is complete is a good idea. You can then make a copy of the original file, saving in the required format. In Photoshop, for example, saving the original, with all layers intact, in unrasterized type, and in PSD format, gives you the most flexibility for future editing or repurposing of a file. Repurposing can be considered making another copy of a file in a different format, size, or color mode for a different purpose. For example, an image prepared as part of a print advertising campaign can be repurposed as a Web graphic for the advertiser's site.
It's also important to understand that file formats differ in how they record information. You can't change a file's format simply by going to Windows Explorer or the Mac's Finder and changing the three-letter extension at the end of a file's name; instead, you must resave or re-export the file to change the format.
You might also find that several file formats are appropriate for your end use and image content. Which one you choose could depend on factors beyond your control. For example, your service bureau or print shop might have a preference, or perhaps the intended audience for your Web site cannot be expected to have the plug-in required to see a particular format. However, in some cases, there is no difference between formats, and you're free to choose as you will.
Save, Save As, Save for Web, and Export
Generally speaking, the Save command updates a file on disk. The Save As, Save for Web, and Export commands are used to save the document in a file format other than its current format.
The primary methods of assigning file formats in Photoshop are the menu commands File, Save As and File, Save for Web. Illustrator relies heavily on the Export command to produce non-PostScript file formats. InDesign can create InDesign documents and templates with Save As, and it uses Export to generate PDF, EPS, and Web file formats (HTML, SVG, and XML). GoLive can export certain types of content as QuickTime movies, and you can export site diagrams as PDF or SVG files.
Illustrator CS uses the Export command to create so-called "legacy" files. If you need to save artwork as an Illustrator file that can be properly read by prior versions of Illustrator, use the Export command to generate an Illustrator legacy file. You can specify compatibility with Illustrator versions 10, 9, 8, 3, and the Japanese version of Illustrator 3. You can also use Export to create EPS files that can be used in the same earlier versions of Illustrator.
If you create a document in Photoshop using the command File, New, the menu command Save is not available until a file format has been assigned by using Save As. Afterward, using the Save command updates the file on disk, maintaining the same format, if possible. If you've added layers or have otherwise changed the file so that it can no longer be saved in the original format, using the Save command opens the Save As dialog box. You have the option of changing to the Photoshop file format or saving the file as a copy.
In Illustrator, the Save command is available immediately after opening a new document. It simply opens the Save As dialog box.
Photoshop's Save As dialog box tells you which features force the file to be saved as a copysmall warning triangles appear next to the check boxes of features in the image.
Photoshop's Save As command can be used to create files in virtually any file format the program supports. Illustrator, in contrast, uses Save As to create only PostScript file formats and SVG. Illustrator's Export command generates other file formats supported by the program.
Photoshop uses As a Copy to prevent accidental loss of image features. If, for example, you have numerous layers in an image and attempt to save the image in a format that doesn't support layers, Photoshop will, by default, attempt to make a copy. This preserves the layers in the original for continued editing.
Any time you select a file format in the Save As dialog box that cannot support all the document's features, Photoshop defaults to saving a copy of the file. The As a Copy check box will be selected and grayed out to prevent deselection, the word copy will be appended to the filename (if the filename and format remain unchanged), and a warning message will appear at the bottom of the dialog box (see Figure 3.1).
In addition to Photoshop's native PSD file format, layers are supported in TIFF, but at some cost. The so-called "advanced" or "enhanced" TIFF features create files that might not be compatible with your page layout program. InDesign 2.0 can place layered TIFF files.
Common triggers for the As a Copy warning are layers and transparency, which are supported by only a few file formats. When you see the warning, you have the option of saving the image as a copy or selecting a different file format from the Format pop-up menu. If you select a file format that does support the features of the image being saved, the warning disappears.
Figure 3.1 The image being saved contains layers, and the Pixar file format cannot support layers.
Photoshop's Save As command enables you to create files in a wide variety of file formats, suitable for a wide range of applications (see Figure 3.2).
Save for Web
Photoshop and Illustrator both offer Save for Web, which is (as the name implies) a Web-oriented tool and, therefore, is restricted to the Web-related formats. (The difference between Web and print formats is explained in the section "Web Versus Print," later in this chapter.) Save As can also be used to convert a file to a variety of Web-friendly formats, including JPEG and GIF. You cannot create a WBMP file using Save As.
For an in-depth look at Save for Web, see Chapter 22, "Save for Web and Image Optimization."
For more information on using Photoshop to prepare images for commercial printing, see Chapter 21, "Preparing for Print."
Figure 3.2 The Large Document Format option is not available in Save As unless that option is activated in Photoshop's File Handling preferences.
Photoshop's Export command can generate paths for use with Adobe Illustrator. Using this command creates an Illustrator format file (AI). When the file is opened in Illustrator, it contains only the unstroked, unfilled paths. The paths can be used to create objects or perhaps as clipping paths to identify visible areas of an image.
Export can also create ZoomView files. ZoomView, from Viewpoint Corporation, is a sophisticated Web presentation technology that allows for quick downloading of interactive graphics. The viewer can zoom and pan within the graphic without any additional hardware. For information on obtaining a license to broadcast ZoomView files over the Internet, contact Viewpoint (http://www.viewpoint.com).
Illustrator uses the Export command to create rasterized versions of artwork, in a variety of file formats, and vector-based artwork for use with Macromedia's Flash, Microsoft Office, and AutoCAD, as well as legacy Illustrator and EPS files (see Figure 3.3). You can also export type from an Illustrator document as plain text (TXT).
Figure 3.3 These and other file format choices are explained individually later in this chapter.
Photoshop's Export Transparent Image Command
Another way Photoshop can assign a file format to an image is found under Photoshop's Help menu. The Export Transparent Image command walks you through the process of creating an image with a transparent background for use in a Web page or page layout document. You need answer only two questions, and an appropriate file is generated (see Figure 3.4).
Figure 3.4 If you click the I Need to Select the Area to Be Made Transparent radio button, you will be instructed to make the selection before returning to the assistant.
If the image is not already on a transparent background, you should make a selection of the area before selecting the menu command. Depending on your needs, the automated assistant generates an EPS file with a clipping path (for print) or a transparent GIF or PNG (for Web). After the file is created, the Save As dialog box opens. Do not change the file format in the Save As dialog box. The final file will have transparency defined as a matte (GIF) or by a clipping path (EPS).
Clear on the difference between transparency created with clipping paths or mattes and that generated with masks? If not, see "Hard and Variable Transparency" in the "Mastering the Adobe Creative Suite" section at the end of this chapter.
After you've saved the file created by the assistant, you'll be presented with a final dialog box. After clicking Finish, you can close the Photoshop file generated during the process. There's no need to save the file.
Photoshop's File-Handling Preferences
Several file-saving options can be set in Photoshop's Preferences dialog box. The top section of the File Handling pane, shown in Figure 3.5, enables you to determine what preview (if any) is saved with the file as well as whether the file extensions are added to the name automatically. (Windows requires the file extension and offers the choice of upper- or lowercase; Windows limits the Image Preview options to Always, Never, and Ask When Saving.)
Figure 3.5 The file extension is required for Windows, suggested for Mac OS X, and optional for Mac OS 9. Remember that many programs in which you place or view files require file extensions.
The File Compatibility section of the File Handling pane contains important options for file sharing and compatibility. Many digital cameras list sRGB as their color space when they actually capture a much larger color gamut. This inaccurate color profile tag can cause an image to be converted to that working space when opened in Photoshop. Because sRGB is a small gamut, this can adversely affect the appearance of the photo. Ignoring the sRGB tag in the EXIF data prevents this problem and has no effect on digital camera shots with other embedded profiles. (If your camera embeds another color profile, you can ignore this option.)
Photoshop can create TIFF files that include layers, which might not be usable by other programs. To avoid compatibility problems, Photoshop offers the option of reminding you when saving a layered file as a TIFF.
In versions of Photoshop prior to Photoshop CS, the maximum image size was 30,000 pixels by 30,000 pixels. That limit still holds for the Photoshop (PSD) file format. However, you can now work with images much larger than that. The new maximum pixel dimensions are 300,000x300,000 pixels. Files over 30,000 pixels in either dimension can be saved as TIFF (up to 4GB, the maximum size the TIFF standard supports) or Photoshop Raw (not to be confused with Camera Raw). There is also the option of using the new PSB file formateffectively a large-image version of the Photoshop PSD format. You activate the PSB file format capability in the File Handling pane of the Photoshop Preferences dialog box by checking the box Enable Large Document Format (.psb).
Depending on compression, layered TIFFs can be somewhat smaller than layered Photoshop files. Otherwise, there is no real advantage to using layered TIFF files over layered PSD files.
The PSB file format is not compatible with earlier versions of Photoshop, nor with any other program. It can be used only within Photoshop CS. If you need to share an oversized image with someone who isn't working in Photoshop CS, use TIFF.
The PSB file format is discussed in greater depth later in this chapter. See "Large Document Format (.psb)."
The option to maximize Photoshop compatibility adds a flattened composite of the image, which increases file size (often substantially). The composite is used by early versions of Photoshop (4.0 and before) and by some programs that can place Photoshop files. If this option is turned off, you'll get a reminder when saving unflattened files (see Figure 3.6).
Both InDesign and Illustrator can place and print PSD files that were saved without the Maximize PSD File Compatibility option. The onscreen preview may differ slightly, but the output should be identical.
Figure 3.6 To turn off this warning, select Never in the Maximize PSD File Compatibility pop-up menu of the File Handling preferences.
Illustrator's File-Handling Preferences
The Files & Clipboard pane of the Illustrator Preferences dialog box offers a few options that affect the way files are saved (see Figure 3.7). You can elect to include the file format extension never, always, or have Illustrator present a dialog box each time you save a new file. The file extension can be forced to lowercase (for improved compatibility) using the Lower Case check box. Placed files that are linked to the Illustrator document can be updated manually, automatically, or you can have Illustrator present you with an update option when the linked file is modified. Illustrator's Files & Clipboard pane also offers the option of using a low-resolution image for linked EPS files. This option can speed screen redraw, but the proxy image won't be as detailed as the high-resolution version.
For more information on linked images, see Chapter 36, "Linking and Embedding Images and Fonts."
Figure 3.7 Guidelines for using file extensions with Illustrator are similar to those discussed for Photoshop earlier in this chapter.
Additional Considerations: Platforms and Compression
Although all the Web file formats and the major print-oriented formats are cross-platform (they can be viewed on both Macintosh and Windows computers), a number of the less common formats are primarily for use on one system or the other. In addition, such factors as filenaming conventions and media format could affect your ability to move a file from one operating system to another.
Windows requires each file to have an appropriate extensiona dot (period) and two- or three-letter suffix at the end of the filename. Lowercase extensions maximize compatibility. This extension identifies the file format and tells the operating system which program to use to open the file. Mac OS X prefers a file extension, but if a file or file type has been associated with a particular program, the file can still be opened by double-clicking. When no program is associated with a file or file type, a warning dialog box appears, and you're asked to pick an application for opening the file (see Figure 3.8).
Figure 3.8 The Mac OS X warning message is shown in this example. When you click Choose Application, the dialog box is opened for you.
Mac OS 9 requires no file extension. However, using extensions is a good idea. Even if you never plan on sharing the file(s) with anyone using a different operating system, eventually you'll upgrade to Mac OS X and want the files to still be usable.
When archiving files, consider using a compression utility rather than the JPEG file format. The size of Photoshop and TIFF files can be substantially reduced, without losing any image quality.
File compression can also be an issue. Compression schemes available in a file format are either lossy or lossless. Lossy compression reduces file size, in part, by discarding image data. When the file is reopened, the missing data is reconstructed from the surrounding image information. Lossless compression maintains all the original data. JPEG uses lossy compression, on a sliding scale, and PNG and GIF use lossless compression. Lossy compression can result in substantially smaller file sizes, but at the cost of some image quality.
Compression utilities are especially handy when you're transferring image files via email. In addition to reducing the file size, which speeds transmission and helps avoid "over limit" message rejection, compression helps protect the files from corruption in transmission.
The compression supplied by a file format should not be confused with that available through utilities. Compressed archives can be produced by such programs as StuffIt and from within Windows (see Figures 3.9 and 3.10).
Zipped, stuffed, and other archived files must be uncompressed by the same or another utility before a program can open them. In the archive, the file retains the format with which it was saved. These types of file compression can be considered losslessthat is, they do not degrade the image quality.
Figure 3.9 Utilities such as StuffIt can produce a variety of compressed files.
Figure 3.10 Windows XP can compress files, folders, and drives in a variety of ways. The inset shows the icon for a "zipped" folder.
The following are some considerations when you're naming files:
Windows filenames can be up to 256 characters long, and they are not case sensitive (upper- and lowercase letters are seen as the same letter). Several characters cannot be used in filenames: forward slashes (/), backslashes (\), colons (:), asterisks (*), question marks (?), quotation marks ("), left angle brackets (<), right angle brackets (>), and vertical slashes (|).
Macintosh filenames through OS 9 can be up to 31 characters long and are not case sensitive. The only forbidden character is the colon (:).
Under Mac OS X, you can have filenames as long as 255 characters, and the colon (:) is the only forbidden character.
Unix filenames can be 256 characters long, cannot use the slash character (/), and are case sensitive.
MS-DOS filenames can use only the 8.3 format and have the same character restrictions as Windows.
ISO 9660 for CD-ROMs uses the 8.3 filename format and allows only the 26 letters, the numbers 0 through 9, and the underscore (_). These names are not case sensitive. (This standard is designed to allow a CD-ROM to be recognized by any computer.)
When you're preparing files for the Web, including the three- character filename extension is important. Although the platform and operating system (OS) may support filenames without extensions, Web browsers require them. Because of the peculiarities of the Web, my best advice to you is to be as conservative as possible. Unless you have direct knowledge of (and control over) the server on which your files will be stored, use the lowest common denominator for filenames:
Use the 8.3 naming convention and never forget to add the filename extension.
Stick with the 26 letters, the 10 numerals, and the underscore (_).
Use only lowercase letters.