GLOSSARIES RELATED TO DIGITAL IMAGING

URLs

Among the glossaries included there are the following:

Imaging Dictionary / Northern Micrographics, Inc. 1996

Less useful is :

TERMS USED IN DIGITAL IMAGING:

BIT-DEPTH (or PIXEL DEPTH):

The number of bits used to convey tonality for each pixel. In general, the more bits per pixel the larger the file size.

1-bit or Bi-tonal - a 1-bit pixel has two possible values, black or white; the scanned image has no shading or gray.

8-bit Gray-scale - provides 256 shades of gray ranging from pure white to pure black.

24-bit Color - provides a tonal range of about 16 million different colors.

CCITT

Consultative Committee for International Telephony and Telegraphy; part of the International Telecommunications Union. CCITT sets international communications standards.

.GIF : Graphical Interchange Format.

An 8-bit lossless compression format which is well-suited for screen presentation format for black and white text documents and microfilm; it is often used for image thumbnails. As an 8-bit format it is limited to 256 colors. Among its features are Transparency, Reducibility, and Interlacing. If those features are needed, GIF is the compression format to be be used.

INTERLACING:

A description of how an image loads on the screen, either progressively or sequentially. An interlaced GIF appears to load faster than a normal GIF because the entire graphic is loaded first at a low resolution, and then "fine-tuned". Without interlacing a file loads sequentially, line by line from top to bottom. Interlacing is usually not available with JPEG unless one has Progressive JPEG which older browsers cannot deal with.

ISO:

International Organization for Standardization (often referred to as the International Standards Organization); founded in 1947 to set standards in all fields, ISO is part of the U.N. and represents more than 75 countries.

.JPEG : Joint Photographic Experts Group.

A joint committee set up by the CCITT and ISO to devise a standard for compression of continuous-tone gray- scale or colour still images. While the format they devised is actually JFIF it is universally known as JPEG.

A 24-bit compression format which is suited for both screen and print presentations, particularly of non-print materials. Options for JPEG are (the more usual) LOSSY (q.v.) or LOSSLESS (q.v.) (a trade- off between image size and image quality), and progressive or sequential (a choice of how the image emerges on the screen, significant in applications involving browsability). Cf. INTERLACING.

LOSSLESS:

No information lost. Term used to describe compression formats.

LOSSY:

Some information lost. Term used to describe compression formats. The file characteristics discarded tend to be things the human eye will not notice, e.g., shades of color, rather than those visible to the human eye, e.g., brightness.

.PDF : Adobe Acrobat Portable Document Format.

A format that allows one to preserve the page image as it was originally formatted. It is a good print presentation format for black and white text documents and microfilm and whenever you want to preserve the original layout of a document.

PIXEL:

Term standing for Pic(x)ture Element. This is the most basic measurement for expressing the amount of information stored in a digital file. (The equivalent term used in printing is Dots Per Inch (DPI)) . A document 10 inches across scanned at 600 DPI requires 6000 pixels.

PNG : Portable Network Graphic scheme.

A lossless compression format that can do whatever GIF does and is better at interlacing. It supports true color. As it is not patented it can be used without requiring licensing. It is not compabible with older browsers, however.

RASTER IMAGING:

Representation of a picture image as a matrix of dots; also known as bit mapping.

RESOLUTION:

A physical measurement described in terms of the number of horizontal and vertical pixels that make up the image though it is usually referred to in terms of DPI. It can be used to:

1. refer to the number of pixels or dots CAPTURED per inch from the original material (IMAGE RESOLUTION); or
2. describe the number of pixels per inch on computer displays and the output quality of printers (OUTPUT RESOLUTION). Output decisions have to be made on the basis of image size and needed resolution of the printer, the Web, or other end use.
3. INPUT RESOLUTION is a definition of space; it tells how many DPI a given scanner can divide your image into. Use the desired output (e.g., 300 dpi) as the fulcrum between the file size of the image in your computer (MB) and the physical size of the print (inches).
4. MONITOR RESOLUTION: Monitors have a maximum resolution that is determined by dividing the horizontal pixels (dots) by the horizontal dimension (inches)

REDUCIBILITY:

The ability, in many graphics programs, to reduce the file size by reducing the number of colors in your GIF. This often does not affect the quality of the image. Examples:

No. of bits	No. of colors	File size
28	256	9.4 K
27	128	7.9 K
26	64	6.5 K
25	32	5.3 K
24	16	4.3 K
23	8	3.2 K

.TIFF: Tagged Image File Format.

A format that provides a high level of detail combined with a small file size (?); it makes a good archival format for all types of material but it is generally not useful for viewing or printing. It can be converted to GIF or JPEG, etc., but the process is not reversible. While it is not platform-independent it is available on all platforms. It is lossless, can contain true color, has low compression, but file sizes can therefore be high.

TIFF w/CCITT Fax 4 Compression: ideally suited for black and white text documents.

TIFF w/LZW Compression: A 24-bit lossless compression format commonly used by Adobe PhotoShop, this format may be used to store color images.

THUMBNAIL:

A small image presentd with a bibliographic record to allow users to judge whether they wish to take the time to retrieve a higher quality image.

TONALITY:

Black and white, gray-scale, or color. Expressed in terms of bit-depth.

TRANSPARENCY:

The ability with GIF files to allocate a color that your browser should ignore, displaying the page background instead. This feature is important for screen display, not for scanning or print output. This ability is not available with JPEF files.

File created 6 March 1998; last updated 4 March 1999

To suggest additions or other changes contact Judith Hopkins