Understanding graphic formats

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As sourced from: C:\Program Files\Adobe\Adobe InDesign CS\Help\help.html

InDesign supports a variety of graphics formats, including bitmap formats such as TIFF, GIF, JPEG, and BMP, and vector formats such as EPS. Other supported formats include DCS, PICT, WMF, EMF, PCX, PNG, and Scitex CT (.SCT).

TIFF is a flexible bitmap image format supported by virtually all paint, image-editing, and page-layout applications. Also, virtually all desktop scanners can produce TIFF images.

The TIFF format supports CMYK, RGB, grayscale, Lab, indexed-color, and bitmap files with alpha and spot-color channels. You can select an alpha channel when you place a TIFF file. (See Import options for bitmap graphics.) Spot-color channels appear in InDesign as spot colors in the Swatches palette.

You can use an image-editing program such as Photoshop to create a clipping path to create a transparent background for a TIFF image. InDesign supports clipping paths in TIFF images and recognizes encoded OPI comments.

Graphics Interchange Format (GIF) is a standard for displaying graphics on the World Wide Web and other online services. Because it compresses image data without losing detail, its compression method is called lossless. Such compression works well with graphics that use a limited number of solid colors such as logos and charts; however, GIF cannot display more than 256 colors. For this reason it is less effective for displaying photographs online (use JPEG instead) and is not recommended for commercial printing. If an imported GIF file contains transparency, the graphic interacts only where the background is transparent.

BMP is the standard Windows bitmap image format on DOS and Windows-compatible computers. However, BMP does not support CMYK, and its color support is limited to 1, 4, 8, or 24 bits. It is less than ideal for commercially printed or online documents, and it is not supported by Web browsers. BMP graphics can provide acceptable quality when printed on low-resolution or non-PostScript printers.

The Joint Photographic Experts Group (JPEG) format is commonly used to display photographs and other continuous-tone images in HTML files over the Web and in other online media. The JPEG format supports CMYK, RGB, and grayscale color modes. Unlike GIF, JPEG retains all of the color information in an RGB image.

JPEG uses an adjustable, lossy compression scheme that effectively reduces file size by identifying and discarding extra data not essential to the display of the image. A higher level of compression results in lower image quality; a lower level of compression results in better image quality, but a larger file size. In most cases, compressing an image using the Maximum quality option produces a result that is indistinguishable from the original. Opening a JPEG image automatically decompresses it.

Note: JPEG encoding, which can be performed on an EPS or DCS file in an image-editing application such as Adobe Photoshop, does not create a JPEG file. Instead, it compresses the file using the JPEG compression scheme explained above.

JPEG works well for photographs, but solid-color JPEG images (images that contain large expanses of one color) tend to lose sharpness. InDesign recognizes and supports clipping paths in JPEG files created in Adobe Photoshop. JPEG can be used for both online and commercially printed documents; work with your prepress service provider to preserve JPEG quality in printing.

The Encapsulated PostScript (EPS) file format is used to transfer PostScript language artwork between applications, and is supported by most illustration and page-layout programs. Typically, EPS files represent single illustrations or tables that are placed into your layout, but an EPS file can also represent a complete page.

Because they are based on the PostScript language, EPS files can contain both vector and bitmap graphics. Since PostScript cannot normally be displayed on-screen, InDesign creates a bitmap preview for an EPS file for on-screen display. If you print a page with an EPS file to a non-PostScript printer, only this screen-resolution preview will be printed. InDesign recognizes clipping paths in Photoshop-created EPS files.

When you import an EPS file, any spot colors it contains are added to the Swatches palette in InDesign. (See Importing swatches.) EPS allows for prepress-quality resolution, precision, and color. This format includes all of the color and image data required to color-separate DCS images embedded in the EPS graphic. EPS isn't ideal for online publishing in HTML, but it works well for online publishing in PDF.

EPS files can contain Open Prepress Interface (OPI) comments, which let you use fast, low-resolution versions (proxies) of images for positioning on a page. For final output, either InDesign or your prepress service provider can automatically replace the proxies with high-resolution versions.

Desktop Color Separations (DCS), developed by Quark, is a version of the standard EPS format. The DCS 2.0 format supports multichannel CMYK files with multiple spot channels. (These spot channels appear as spot colors in InDesign's Swatches palette.) The DCS 1.0 format supports CMYK files without spot channels. InDesign recognizes clipping paths in Photoshop-created DCS 1.0 and DCS 2.0 files.

DCS files are intended to be used in a pre-separated, host-based workflow. In most cases, color separations files associated with a DCS image are excluded when you export or print a composite to a PDF, EPS, or PostScript file. (The sole exception is made for 8-bit DCS files that were created in Photoshop and that do not contain vector graphics.)

InDesign can rebuild a composite image from DCS 2.0 or 1.0 separations files, if the files were created in Adobe Photoshop. For best results, do not include DCS 1.0 files or DCS 2.0 files created in programs other than Photoshop when you are creating high-resolution color composite proofs or separating a document in-RIP or from a composite file.

The Macintosh PICT (or Picture) format is widely used for Mac OS graphics and page-layout applications, and for transferring files between applications. The PICT format is especially effective in compressing images that contain large areas of solid color. InDesign for both Windows and Mac OS imports PICT files created from Mac OS screenshots and a variety of other applications, including clip art collections.

InDesign supports RGB PICT images with variable resolutions and embedded QuickTime images. PICT graphics do not support color separations, are device-dependent, and are not recommended for high-resolution commercial printing. The PICT format can provide acceptable quality only when printed on low-resolution or non-PostScript printers.

Windows Metafile Format (WMF) and Windows Enhanced MetafileFormat (EMF) are native Windows formats used primarily for vector graphics, such as clip art, shared between Windows applications. Metafiles may contain raster image information; InDesign recognizes the vector information and provides limited support for raster operations. Color support is limited to 16-bit RGB, and neither format supports color separations. Metafile formats are not an ideal choice for commercially printed or online documents; they provide acceptable quality only when printed on low-resolution or non-PostScript printers from a Windows desktop.

The PCX format is commonly used in Windows systems. Most Windows software supports version 5 of the PCX format.

The PCX format supports RGB, indexed-color, grayscale, and bitmap color modes, as well as the RLE compression method, which is lossless. It does not support alpha channels. Images can have a bit depth of 1, 4, 8, or 24 bits. However, PCX is not ideal for commercially printed or online documents. PCX graphics can provide acceptable quality only when printed on low-resolution or non-PostScript printers.

The Portable Network Graphics (PNG) format uses adjustable, lossless compression to display 24-bit photographs or solid-color images on the World Wide Web and in other online media. PNG was developed as a patent-free alternative to the GIF file format. It supports transparency in an alpha channel or a designated color. PNG is best used for online documents, although its color support makes it better for printed documents than GIF. However, color PNG graphics placed in an InDesign document are RGB bitmap images, so they print only as composites, not on color separations.

Understanding graphic formats

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