Common Graphic File Formats

	Imaging 1
h	Common Graphic File Formats
Preamble	This document gives a basic introduction to various file types for saving digital images.

Background	During the early years of computer graphics development and the race for dominance by both hardware and software manufacturers, many proprietary formats were designed.  Every graphics program saved files in its proprietary—or native—format, and therefore, any file saved from a specific application and used exclusively by that application was—and still is—known as a native file. Unfortunately, these native files weren’t always readable by other applications ... especially when the native file was originated in a Mac-based application and the target application was PC-based or vice versa. With greater demands for inter-application compatibility and cross-platform compatibility, many developers and manufacturers realized that survival was closely linked to meeting those demands. Today, most major graphics applications can save and open both in their native formats and in a number of other formats that can be used to transfer images from one application to another or from one platform to another.
TIFF File (.tif)	TIFF—or Tag Image File Format—was developed by Aldus Corporation in 1986, specifically for saving images from scanners, frame grabbers, and paint/photo-retouching programs. Today, it is probably the most versatile, reliable, and widely supported bit-mapped format. It is capable of describing bilevel, grayscale, palette-color, and full-color image data in several color spaces. It includes a number of compression schemes and is not tied to specific scanners, printers, or computer display hardware. The TIFF format does have several variations, however, which means that occasionally an application may have trouble opening a TIFF file created by another application or on a different platform.
PICT File	The PICT format—which is not an acronym—is native to the Macintosh. It first appeared in 1984 with the introduction of MacDraw software. Since then, it has been used by many applications, especially when images are designed for screen previews. It is great for presentations, screen displays, and video work. The PICT format can contain both bit-mapped and object-oriented graphics. It is a standard format for graphics that are cut or copied to the Clipboard and for drawings that will be output on raster printers.

BMP file (.bmp)	BMP is the standard Windows bitmap image format on DOS and Windows-compatible computers. When saving an image in this format, you can specify either Microsoft Windows or OS/2 format and a 1-bit to 24-bit depth for the image. For 4-bit and 8-bit images, you can also choose to use Run-Length-Encoding (RLE) compression; this compression scheme is lossless, that is, it does not discard detail from the image.
EPS File (.eps)	EPS—or Encapsulated PostScript—files are the standard format for storing high-resolution PostScript illustrations. The EPS format—which was introduced in the mid-’80s allows both Mac and Windows users to save bit-mapped screen representations of screen images.  These previews, however, don’t travel well across platforms. An EPS file generally has two parts: a PostScript (text) description that tells a PostScript printer how to output the resolution-independent image, and (optionally) a bit-mapped PICT or TIF image for on-screen previews. A drawing saved in EPS format can be imported into other documents and scaled and cropped, but its contents are often no longer editable, even by the program that created it (Adobe Illustrator files are the exception).
GIF File (.gif)	GIF—or Graphics Interchange Format—files define a protocol intended for the on-line transmission and interchange of raster graphic data in a way that is independent of the hardware used in their creation or display. The GIF format was developed in 1987 by CompuServe— one of the world’s most successful bulletin board services—for compressing eight-bit images that could be telecommunicated through their service and exchanged among users. The GIF file is defined in terms of blocks and sub-blocks which contain relevant parameters and data used in the reproduction of a graphic. A GIF data stream is a sequence of protocol blocks and sub-blocks representing a collection of graphics.
JPEG File (.jpg)	JPEG is a standardized image compression mechanism. The name derives from the Joint Photographic Experts Group, the original name of the committee that wrote the standard. In reality, JPEG is not a file format, but rather a method of data encoding used to reduce the size of a data file. It is most commonly used within file formats such as JFIF and TIFF. JPEG File Interchange Format (JFIF) is a minimal file format which enables JPEG bitstreams to be exchanged between a wide variety of platforms and applications. This minimal format does not include any of the advanced features found in the TIFF JPEG specification or any application specific file format. JPEG is designed for compressing either full-color or grayscale images of natural, real-world scenes. It works well on photographs, naturalistic artwork, and similar material, but not so well on lettering or simple line art. It is also commonly used for on-line display/transmission; such as on web sites. A 24-bit image saved in JPEG format can be reduced to about one-twentieth of its original size. Because it discards data, the JPEG compression scheme is referred to as lossy. This means that once an image has been compressed and then decompressed, it will not be identical to the original image. A higher level of compression results in lower image quality, while a lower level of compression results in better image quality. In most cases, compressing an image using the Maximum quality option produces a result that is indistinguishable from the original. When saving in JPEG format, you can specify an image quality and compression level for the file. To specify image quality, enter a value between 0 and 10 or choose an option for Quality; to specify the amount of compression, drag the slider. A trade-off exists between the image quality and the amount of compression; an image compressed using Maximum quality is less compressed (and thus takes up more disk space) than an image compressed using the Low quality option. In addition, you can select a format option for the JPEG file. To optimize the color quality of the image, select Baseline Optimized. To save the file as a progressive JPEG, select Progressive. This option displays the image gradually as it is downloaded from a Web browser, using a series of scans to show increasingly detailed versions of the entire image until all of the data has finished downloading. However, progressive JPEG images require more RAM for viewing and are not supported by all Web browsers. When you select Progressive, you can also specify the number of progressive scans.
JPEG 2000(.jp2)	JPEG 2000 is a wavelet-based image compression standard. It was created by the Joint Photographic Experts Group committee in the year 2000 with the intention of superseding their original discrete cosine transform-based JPEG standard (created 1992). The standardized filename extension is .jp2 for ISO/IEC 15444-1 conforming files and .jpx for the extended part-2 specifications, published as ISO/IEC 15444-2, while the MIME type is image/jp2.
PNG File (.png)	The PNG format was developed as an alternative to the GIF format and, like GIF, is used for displaying images on the World Wide Web and other online services. PNG preserves all color information and alpha channels in an image and uses  lossless compression to reduce file size. When saving an image in PNG format, you can choose to display the image in gradually increasing detail as it is downloaded. To do this, select Adam7 for Interlace. You can also select a filtering algorithm, which is used to prepare the image data for compression.
PDF	The PDF format is used by Adobe Acrobat, Adobe’s electronic publishing software for Macintosh, Windows, UNIX, and DOS. You can view PDF files using the Acrobat Reader software included on your Adobe Photoshop CD-ROM. Based on the PostScript Level 2 language, PDF can represent both vector and bitmap graphics. For the purposes of representing pages, PDF pages are identical to PostScript pages, but PDF files can also contain electronic document search and navigation features. PDF files, for example, can contain hypertext links and an electronic table of contents For more information on PDF and Adobe Acrobat see the Electronic Publishing Guide included on the Adobe Photoshop Tutorial CD-ROM.
Photoshop File (.psd)	A Photoshop file is the native file format for Adobe Photoshop. A file saved in this manner can only be opened and edited in Photoshop. However, the user has the option to save the file in a variety of other formats that are readable in both the Macintosh and PC environment. The major advantage of the Photoshop format becomes apparent when working on documents with layers. For example, a background can be created on one layer, then graphics can be added on a second layer, a drop-shadow on a third layer and text on yet another layer. Each layer is independent of the others and can be edited separately without affecting the contents of the other layers. Photoshop supports the preservation of layer information, so that the layers can be preserved for additional editing.