Aliasing: In graphic design, aliasing occurs when an image does not have a high enough resolution to smoothly represent a graphic shape or text. An aliased image is often said to have the "jaggies". Aliasing can also be a byproduct of resizing a digital image.
Anti Aliasing: A technique used to make diagonal or curved edges appear smoother by setting pixels near the edge to intermediate colors according to where the edge crosses the underlying color.
Banding: Visible lines in a print image frequently due to print head misalignment. This also describes the broad bands that sometimes appear in large areas of graduated continuous tones. See posterization. It can also be an artifact of dithering or moiré patterns.
Dithering: A process of placing different-colored dots next to one another to create the illusion of additional colors. The eye sees the two adjacent colors, and the mind blends them into a third color.
Dot: The smallest part of an image that is identifiable. On a display monitor this is the same as an image pixel when it is viewed at 100%. For a printer, multiple dots are used to reproduce a single tone at a single image location. Also referred to as a printer spot and incorrectly referenced as a pel or pixel.
Error Diffusion: A process of dithering for bitmaps. Each cell is examined to determine any error or difference between the computed color of a given cell and the nearest color available on the device (dye). This error is passed to adjacent unprocessed pels. For example, the Floyd-Steinberg method distributes the error to neighboring cells below and to the right.
Halftone: The process of reproducing a continuous-tone image as a series of variously sized dots within a fixed grid. The finer the dot grid, the more realistic the impression conveyed.
Halftone Cell: A square area in a halftone screen which contains an array of printing dots.
Halftone Screen: In a traditional print shop, a glass or film screen with fine engraved lines was used to simulate graduated gray tones by transforming the original continuous-tone photo into a halftone cliché. Computer software is used to do the same job today. The printer driver or RIP software determines how many dots should be printed in each halftone cell.
Metameric Colors: Colors that can change their perceived hue depending on the different lighting conditions and viewing angles.
Moiré: Having a wavy pattern. The interference pattern you see in images of closely spaced lines or other finely detailed patterns. An undesirable artifact produced in printing when halftone screen patterns become visible. Fundamentally this is an artifact produced when two patterns of different frequencies are super imposed. The moiré pattern is a new harmonic frequency of these input frequencies. Moiré patterns are possible with digital cameras, video displays, scanners, and printers. Examples can be seen photographing herringbone fabrics, shingle roofs, and even the support cables on a suspension bridge from a far distance. This is a product of under-sampling where the target resolution and the source resolution are unequal.
Pixel: Short for the term Picture Element. This is the most fundamental element of a digital image. A digital image is made up of rows and columns representing points of light. Each indivisible point of light is called a pixel. Thus a pixel is a discrete and atomic piece of image information.
Pel: Short for the term Print Element. This is the most fundamental (atomic) element of a printed image. For halftone printers such as inkjets, this would be the halftone cell. Unfortunately, this term has fallen into disuse.
Pixelation: Visible square pixels in an image. This is usually the effect of re-sampling a low resolution image to a much higher resolution. For example viewing an image on the screen at 400% magnification.
Posterization: The stepped, graphic (poster) effect that may occur when an image or print has insufficient color depth. The lack of tonal range is especially noticeable in graduated tones. See also banding.
Screen Angle: Angles at which multiple halftone screens are placed with relation to one another to avoid undesirable moiré patterns. This is also used to soften the metameric effects of different dye colors. Some common angles used for printing are black at 45°, magenta at 75°, yellow at 90° and cyan at 105°.
Rags Int., Inc.
204 Trailwood Drive
Euless, TX 76039
April 19, 2004
This page last updated on: Thursday April 12 2007