About File Formats - Adobe PHOTOSHOP 5.0 User Manual

About file formats
You can use various file formats to get images into
and out of Adobe Photoshop. Graphic file formats
differ in the way they represent graphic infor-
mation—either as vector drawings or as bitmap
images. Some formats contain only vector
drawings or bitmap images, but many include
both in the same file. See Chapter 3, "Getting
Images into Photoshop," for information on
opening files and on vector and bitmap graphics;
see the beginning of this chapter for information
on saving files.
About file compression
Many image file formats use compression
techniques to reduce the storage space required by
bitmap image data. Compression techniques are
distinguished by whether they remove detail and
color from the image. Lossless techniques compress
image data without removing detail; lossy
techniques compress images by removing detail.
The following are commonly used compression
techniques:
Run Length Encoding (RLE) is a lossless
•
compression technique supported by the
Photoshop and TIFF file formats and some
common Windows file formats.
Lemple-Zif-Welch (LZW) is a lossless
•
compression technique supported by TIFF, PDF,
GIF, and PostScript language file formats. This
technique is most useful in compressing images
that contain large areas of single color, such as
screenshots or simple paint images.
Joint Photographic Experts Group (JPEG) is a
•
lossy compression technique supported by JPEG,
PDF, and PostScript language file formats. JPEG
compression provides the best results with
continuous-tone images, such as photographs.
CCITT encoding is a family of lossless
•
compression techniques for black-and-white
images that is supported by the PDF and
PostScript language file formats. (CCITT is an
abbreviation for the French spelling of Interna-
tional Telegraph and Telekeyed Consultive
Committee.)
ZIP encoding is a lossless compression
•
technique supported by the PDF file format. Like
LZW, ZIP compression is most effective for images
that contain large areas of single color.
BMP
BMP is the standard Windows image format on
DOS and Windows-compatible computers. The
BMP format supports RGB, indexed-color,
grayscale, and Bitmap color modes, and does not
support alpha channels. You can specify either
Microsoft Windows or OS/2
®
depth for the image. For 4-bit and 8-bit images
using Windows format, you can also specify RLE
compression.
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User Guide
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