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ADOBE PHOTOSHOP CS2
User Guide
JPEG format
Joint Photographic Experts Group (JPEG) format is commonly used to display photographs and other continuous
tone images in hypertext markup language (HTML) documents over the World Wide Web and other online services.
JPEG format supports CMYK, RGB, and Grayscale color modes, and does not support alpha channels. Unlike GIF
format, JPEG retains all color information in an RGB image but compresses file size by selectively discarding data.
A JPEG image is automatically decompressed when opened. A higher level of compression results in lower image
quality, and a lower level of compression results in better image quality. In most cases, the Maximum quality option
produces a result indistinguishable from the original.
JPEG 2000 format (Photoshop optional plug-in)
JPEG 2000 is a file format that provides more options and greater flexibility than the standard JPEG (JPG) format.
Using JPEG 2000, you can produce images with better compression and quality for both web and print publishing.
To save files in the JPEG 2000 format, you must locate the optional JPEG 2000 plug-in and install it in this location:
Adobe Photoshop CS/Plug-Ins/Adobe Photoshop Only/File Formats. You can find the plug-in on the Photoshop CS
installation CD in Goodies/Optional Plug-Ins/Photoshop Only/File Formats.
Note:
To view JPEG 2000 image files on the web, your browser must have a plug-in that enables viewing JPEG 2000 files.
Unlike traditional JPEG files, which are lossy, the JPEG 2000 format supports optional lossless compression. The
JPEG 2000 format also supports 16-bit color or grayscale files and 8-bit transparency, and it can retain alpha channels
and spot channels. Grayscale, RGB, CMYK, and Lab are the only modes supported by the JPEG 2000 format.
The JPEG 2000 format also supports using a region of interest (ROI) to minimize file size and preserve quality in
critical regions of an image. By using an alpha channel, you can specify the area (ROI) where the most detail should
be preserved, allowing greater compression and less detail in other regions.
See also
“About masks and alpha channels” on page 338
Large Document Format
The Large Document Format (PSB) supports documents up to 300,000 pixels in any dimension. All Photoshop
features, such as layers, effects, and filters, are supported. You can save high dynamic range, 32-bits-per-channel
images as PSB files. Currently, if you save a document in PSB format, it can be opened only in Photoshop CS or
Photoshop CS2. Other applications and earlier versions of Photoshop cannot open documents saved in PSB format.
Note:
Most other applications and older versions of Photoshop cannot support documents with file sizes larger than 2 GB.
The Enable Large Document Format (.psb) option must be enabled in Preferences before you can save documents
in PSB format. There is one exception: it is possible to open an
existing
PSB file and save it in PSB format even if the
Enable Large Document Format (.psb) option is not selected in Preferences.
See also
“Saving large documents” on page 655
