Graphic file formats

Graphic File Formats

PSD
Photoshop file format. Save your work as a PSD file until the image you are working on in Photoshop is finished.PSD is the only format that will keep Layers and Channels. Also, no information is lost when this file format is saved.

AI (Adobe Illustrator)
Vector file format.

QXD
QuarkXPress file format. According to Quark's reference manual, you can only import (using Get Picture) the following formats available to Photoshop: TIFF, PICT and EPS (Vector). Other formats can be imported, but with lesser quality. An entire Quark page can be converted to an EPS file.

JPEG (Joint Photographic Experts Group)
· Lossy compression (some information is lost every time an image is saved). JPEG trades accuracy for compression -the more the file is compressed, the more information is lost.
· Can display millions of colors. It uses 24-bit color for RGB images; 8 bits per pixel for grayscale; and 32-bit color for CMYK images for four-color printers.
Technically speaking, JPEG is a compression method, not a file format. It is used in file formats such
as JFIF (JPEG Interchange File Format) and TIFF (though universally it is referred to as JPEG).

GIF (Graphics Interchange Format)
· Lossless compression (no information is lost in compression). Not as compacting as JPEG.
· Limitation: maximum of 256 colors (8-bit or less).

GIF89a (created by CompuServe in 1989)
In Photoshop, save through FILE/EXPORT.
· Can be given a transparent background and Interlacing (Web image slowly comes into focus).
· Can be used to create Animated GIF files: multiple image blocks that are continually cycled through. Need Netscape Navigator 3.0 or Internet Explorer 3.0 or higher.

TIFF (Tagged Image File Format)
Originally developed for scanned images, providing high-quality graphics. It was developed before computers used colors (started as a grayscale format). Can be compressed by a variety of compression methods -including LZW and JPEG. Not designed for Internet use.

EPS (Encapsulated PostScript)
Vector file format.

BMP (Bit Map)
Native format for Windows and OS/2. Same as a DIB file and RLE (when compressed).

PICT (QuickDraw Picture)
Native format for the Macintosh. It combines bitmap and vector graphics data.

PNG (Portable Network Graphics -pronounced "ping")
New lossless Web format.

Web graphics file formats: All major browsers support JPEG, GIF and GIF89a formats. Though other formats can be used, they have limitations. PNG, for example, can not be viewed on Netscape, Explorer, or AOL browsers below versions 3.0. For the web, use:

JPEG for continuous tone images.
GIF for images with line art, solid color (without gradients) -logos, text, etc.
GIF89a for transparency (save through FILE/EXPORT), Interlacing, or Animated images.

Images on the web need to be compressed. A photo-realistic image is compressed at a 5:1 ratio in a GIF file, while JPEG will typically compress about 10:1 at the lowest compression level and up to 200:1 at its highest. At the medium compression level, 30:1 is the typical ratio for JPEG. There are drawbacks to both formats, however.

Using JPEG as an example, an uncompressed 3.2M file is 179K when compressed at a low level, and highly compressed at 15K.

Since JPEG and GIF files are already compressed, zipping them has little effect.