Adobe InDesign CS5 - Manual (Page 382)

376 USING INDESIGN
Graphics
Last updated 11/16/2011
JPEG (.jpg) files
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 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 Photoshop. JPEG can be
used for both online and commercially printed documents; work with your prepress service provider to preserve JPEG
quality in printing.
Bitmap (.bmp) files
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 some web browsers. BMP graphics can provide acceptable
quality when printed on low-resolution or non-PostScript printers.
Encapsulated PostScript (.eps) files
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 text, 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. 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 panel in InDesign. 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) files
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 the
Swatches panel in InDesign.) 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 preseparated, 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.)