JPEG: JOINT PHOTOGRAPHIC EXPERTS GROUP. The original name of the committee that designed the standard image compression algorithm. JPEG is designed for compressing either full-colour or grey-scale digital images of "natural", real-world scenes. It does not work so well on non-realistic images, such as cartoons or line drawings. JPEG does not handle compression of black-and-white (1 bit-per-pixel) images or moving pictures. Standards for compressing those types of images are being worked on by other committees, named JBIG and MPEG. Filename extension: .jpg, .jpeg.
GIF: GRAPHICS INTERCHANGE FORMAT: /gif/, occasionally /jif/ (GIF, GIF 89A) A standard for digitised images compressed with the LZW algorithm, defined in 1987 by CompuServe (CIS). Graphics Interchange Format and GIF are service marks of CompuServe Incorporated. This only affects use of GIF within Compuserve, and pass-through licensing for software to access them, it doesn't affect anyone else's use of GIF. It followed from a 1994 legal action by Unisys against CIS for violating Unisys's LZW software patent. The CompuServe Vice President has stated that "CompuServe is committed to keeping the GIF 89A specification as an open, fully-supported, non-proprietary specification for the entire on-line community including the World-Wide Web". Filename extension: .gif.
GIF89a: ANIMATED GIF. The GIF89a format supports multiple "frames" that give the impression of motion when displayed in sequence, much like a flip book. The animation may repeat continuously or play once. Animated GIFs aren't supported by earlier web browsers, however the first frame of the image is still shown. There are many utilities to create animated GIFs from a sequence of individual GIF files. There are also utilities that will produce animated GIFs automatically from a piece of text or a single image. One problem with this format is the size of the files produced, as they are by definition a sequence of individual images. Apart from minimising the number of frames, the best way to decrease file size is to assist the LZW compression by using blocks of solid colour, avoid dithering, and use fewer colours. If areas of an image don't change from one frame to another, they don't need to be redrawn so make the area a transparent block in the second frame.
PNG: PORTABLE NETWORK GRAPHICS. /ping/ (PNG) An extensible file format for the lossless, portable, well-compressed storage of raster images. PNG provides a patent-free replacement for GIF and can also replace many common uses of TIFF. Indexed-colour, greyscale and truecolour images are supported, plus an optional alpha channel. Sample depths range from 1 to 16 bits. PNG is designed for on-line viewing applications, such as the World Wide Web, so it is fully streamable with a progressive display option. PNG is robust, providing both full file integrity checking and simple detection of common transmission errors. Also, PNG can store gamma correction and chromaticity data for improved colour matching on heterogeneous platforms. Filename extension: .png.
TIFF: TAGGED IMAGE FILE FORMAT. (TIFF) A file format used for still-image bitmaps, stored in tagged fields. Application programs can use the tags to accept or ignore fields, depending on their capabilities. While TIFF was designed to be extensible, it lacked a core of useful functionality, so that most useful functions (e.g. lossless 24-bit colour) requires nonstandard, often redundant, extensions. The incompatibility of extensions has led some to expand "TIFF" as "Thousands of Incompatible File Formats".
LOSSLESS: A term describing a data compression algorithm which retains all the information in the data, allowing it to be recovered perfectly by decompression. Unix compress and GNU gzip perform lossless compression.
LOSSY: A term describing a data compression algorithm which actually reduces the amount of information in the data, rather than just the number of bits used to represent that information. The lost information is usually removed because it is subjectively less important to the quality of the data (usually an image or sound) or because it can be recovered reasonably by interpolation from the remaining data. MPEG and JPEG are examples of lossy compression techniques.
EPS. ENCAPSULATED POSTSCRIPT. (EPS) An extension of the PostScript graphics file format developed by Adobe Systems. EPS is used for PostScript graphics files that are to be incorporated into other documents. An EPS file includes pragmas (special PostScript comments) giving information such as the bounding box, page number and fonts used. On some computers, EPS files include a low resolution version of the PostScript image. On the Macintosh this is in PICT format, while on the IBM PC it is in TIFF or Microsoft Windows metafile format.
POSTSCRIPT. A page description language based on work originally done by John Gaffney at Evans and Sutherland in 1976, evolving through "JaM" ("John and Martin", Martin Newell) at XEROX PARC, and finally implemented in its current form by John Warnock et al. after he and Chuck Geschke founded Adobe Systems, Inc. in 1982. PostScript is an interpreted, stack-based language (like FORTH). It was used as a page description language by the Apple LaserWriter, and now many laser printers and on-screen graphics systems. Its primary application is to describe the appearance of text, graphical shapes, and sampled images on printed or displayed pages. A program in PostScript can communicate a document description from a composition system to a printing system in a device-independent way. PostScript is an unusually powerful printer language because it is a full programming language, rather than a series of low-level escape sequences. (In this it parallels Emacs, which exploited a similar insight about editing tasks). It is also noteworthy for implementing on-the fly rasterisation, from Bezier curve descriptions, of high-quality fonts at low (e.g. 300 dpi) resolution (it was formerly believed that hand-tuned bitmap fonts were required for this task). PostScript's combination of technical merits and widespread availability made it the language of choice for graphical output until PDF appeared.
PDF: PORTABLE DOCUMENT FORMAT. (PDF) The native file format for Adobe Systems' Acrobat. PDF is the file format for representing documents in a manner that is independent of the original application software, hardware, and operating system used to create those documents. A PDF file can describe documents containing any combination of text, graphics, and images in a device-independent and resolution independent format. These documents can be one page or thousands of pages, very simple or extremely complex with a rich use of fonts, graphics, colour, and images.
Vector Graphics: A line or movement defined by its end points, or by the current position and one other point. Sometimes called "object-oriented" graphics, though it's nothing to do with object-oriented programming. The representation of separate shapes such as lines, polygons and text, and groups of such objects, as opposed to bitmaps. The advantage of vector graphics ("drawing") programs over bitmap ("paint") editors is that multiple overlapping elements can be manipulated independently without using differenet layers for each one. It is also easier to render an object at different sizes and to transform it in other ways without worrying about image resolution and pixels.
Raster Graphics: Computer graphics in which an image is composed of an array of pixels arranged in rows and columns.
Bitmap Graphics: A data file or structure which corresponds bit for bit with an image displayed on a screen, probably in the same format as it would be stored in the display's video memory or maybe as a device independent bitmap. A bitmap is characterised by the width and height of the image in pixels and the number of bits per pixel which determines the number of shades of grey or colours it can represent. A bitmap representing a coloured image (a "pixmap") will usually have pixels with between one and eight bits for each of the red, green, and blue components, though other colour encodings are also used. The green component sometimes has more bits that the other two to cater for the human eye's greater discrimination in this component.
Pixel: The smallest discrete component of an image or picture on a CRT screen (usually a colored dot); "the greater the number of pixels per inch the greater the resolution". OR The smallest resolvable rectangular area of an image, either on a screen or stored in memory. Each pixel in a monochrome image has its own brightness, from 0 for black to the maximum value (e.g. 255 for an eight-bit pixel) for white. In a colour image, each pixel has its own brightness and colour, usually represented as a triple of red, green and blue intensities (see RGB).
SVG: SCALABLE VECTOR GRAPHICS. A W3C standard for vector graphics, based on XML.
.AI: ADOBE ILLUSTRATOR DOCUMENT.
.PSD: ADOBE PHOTOSHOP DOCUMENT.
Last Updated 08/15/07
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