Glossary

This section will acquaint you with some basic terms used in imaging. It's important to know what they are and when they apply to an image.

Artifacts - Visual effects (usually thought of as defects) introduced into a digital image in the course of scanning or compression that do not correspond to the image scanned.

Batch - In the imaging process as we do it, the grouping of books, photographs, or other objects into an organizational unit for our reference only. Each collection has its own distinct series of batch number assignments.

Binary image - A computer image where each dot may have just two values, either black ( 0', for the absence of light) or white ( 1' for the presence of light). Suitable for printed text or line art because only the values of black and white are needed for these types of images. Synonym for bitonal image.

Bit-mapped image - An image created from a series of bits that form pixels. Each pixel can vary in color or gray-scale value.

Compression - A process that reduces the file size for processing, storage, transmission, and display. Compression may be lossless (e.g., CCITT) or lossy (JPEG). The quality of the image may be affected by the compression techniques used and the level of compression applied.

Digitizing - To convert an image into binary code. Visual images are digitized by scanning them and assigning a binary code to the resulting bit-mapped image data.

Dithering - A technique for creating an image with apparent levels of gray by grouping black and white dots into cells. According to the number of black and white dots and their arrangement in the cell, the eye perceives the cell as a single gray, not as a group of black and white dots. Commonly used by laser printers to simulate grey images. The NDLP use of diffuse dithering represents a special case in which the scanner's dot pattern is randomized.

DPI (Dots per inch)- It is a measurement of the scanning resolution of an image or the quality of an output device. It expresses the number of dots a printer can print per inch, or a monitor can display, both horizontally and vertically. A 600-dpi printer can print 360,000 (600 x 600) dots on one square inch of paper.

Dynamic range - It is the number of possible colors or shades of gray that can be included in a particular image. 8-bit images can represent as many as 256 colors; 24-bit images can represent approximately 16 million colors.

Eight-bit image - An image where each dot may have any one of either 256 possible gray values or 256 possible color values. The value is calculated by raising 2 to the 8th power.

GIF - A file format developed by Compuserv, it has gained widespread use on the internet becuase most browsers can view GIF images without a plug-in or associated viewer. GIF stands for Graphic Information File format.

Grayscale - The range of shades of gray in an image. The grayscale of scanners and terminals are determined by the number of shades between black and white that they can recognize and reproduce.

Halftone image - An image in which combinations of dots are used to create an impression of grays or colors by grouping and density. For example, the eye will see shades of gray in black dots on a white background. Where the dots are large, dense and possibly overlapping, the eye sees dark gray or black; where the dots are small and sparse, the eye sees light gray or white.

JPEG - Used to refer to a standard for still image compression developed by Joint Photographic Experts Group. Compression is achieved by dividing the picture into tiny pixel blocks, which are halved over and over until the ratio is achieved. JPEG comprssion is a "lossy" compression, meaning that the compressed information cannot be retrieved. Also, the more information available for loss, the greater the ratio of compression.

Optical Disk - Generic term for any disk read by a laser, including all CD-ROMs. In the NDLP production environment, the term is a shorthand for the write-once Panasonic disks sometimes used for contractor delivery, especially the "shuttle" disks used to transfer image files between us and the contractor.

PCX - A file format not used currently by NDLP. It previously was a format used to scan and store illustrated images. The images can either be dithered or undithered. Older collections such as Conservation, Upper Midwest, and Coolidge used this format extensively.

Pixel - The picture elements that make up an image, similar to grains in a photograph or dots in a halftone. Each pixel can represent a number of different shades or colors, depending upon how much storage space is allocated for it. Raster the grid pattern applied when an image is scanned.

Resolution (spatial) - Images measured in terms of pixels dots per inch or other linear measure. The higher the number of pixels, the higher the resolution.

Resolution (tonal) - Another way of describing the numbers of bits of information per pixel, used of grayscale or color images.

Rework - The delivery of contractor work that responds to a quality review memo sent by us. It should address all the problems indicated in the review memo.

TIFF - TIFF stands for Tagged Image/Interchange File Format, and serves as the standard file-storage format for all scanned still-image files in the NDLP. Still images stored as the TIFF files can be either compressed or uncompressed. Compressed TIFF files are usually Group IV compressed, a lossless compression that allows all compressed information to be retrieved upon opening the image. One of the format's advantages is the many ways the header information can be used.

Twenty-four bit image - A color image where each dot may have any one of 16.7 million color values. The 24 bits are typically made up of three eight-bit values, with each eight bit value representing a separate color (usually red, green, and blue). Very high tonal resolution.