Common Computer Imaging Terms


8 bit display: A computer display device capable of displaying 256 (28) colors or shades of gray. Additional colors are simulated using a process called dithering.

24 bit display: A computers display system capable of displaying over 16 million (224) colors. 24 bit is also called true color.

32 bit color system: A computer system or software capable of displaying and processing 24 bit color with 8 extra bits of information used for transparency effects or masking.

Additive primaries: A color system in which wavelengths of light of the additive primaries (red, green, blue) are mixed to form other colors. A mix of all three primaries produces white light.

Adjustment Layer: A layer in Photoshop that contains a Levels, Curves, Hue/Saturation or other image adjustment. This differs significantly from merely running a Levels, etc. command on a picture because the original image on the Background layer is not permanently affected by the adjustment. It is a non-destructive adjustment and the picture may be exactly restored to it's original state if needed.

Alpha Channel: An 8 bit channel reserved by some imaging processing applications for transparency masking or additional color information.

Analogue: Representation of data in any non-digital form.

Anti-aliasing: Visual smoothing of the edges of a digital drawing or image by filling the gaps between pixel "stairsteps" with intermediate shades of color.

ASCII: Near-universal data format pronounced "ask-key." A document saved in ASCII text can be read by almost any word processing program regardless of computer type or program used. On the Macintosh, ASCII text is sometimes called "plain text" which refers to its inability to contain formatting options (i.e. italic, bold, typeface, etc.).

Baud: The measure of speed at which a modem can transmit data over a phone line. Roughly equivalent to bits per second.

Binary: Refers to a data format comprised of only ones and zeros. Sending Postscript data to a printer in binary format shortens data transmission times and results in faster printing.

Bit resolution: or bit depth: the number of bits of data stored per pixel. Common values of this resolution of ranges are 1 to 32 bits per pixel. A one-bit image is comprised of either black or white pixels -- no shades of gray.

Bit depth: The measure of a display's capability to display different colors simultaneously. A monochrome monitor has a bit depth of one. A display with a bit depth of four can display 16 different colors. The number of colors possible can be calculated by raising 2 to the power of the bit depth, i.e. 28 = 256 for an 8 bit display. The term can be confusing because some sources identify bit depth per color channel (i.e. red, blue, green) and others specify the total bits for the picture. For example, a 24-bit picture is comprised of three 8-bit channels.

Bitmap image: Images comprised of a grid of picture elements (pixels) that make up a picture. Used in digital representations of photographs and illustrations. Photoshop erroneously characterizes bitmaps as 1-bit black and white images, but all scanned images from b/w line art through scanned color photos are proper bitmaps.

Bridge: A companion program that shipped initially with Photoshop CS2. It is a file ingestion, camera RAW host, and file organizing program.

Buffers: A storage area, device, or memory area in which date can be placed temporarily.

Cache: A portion of RAM or disk space used to keep often used data for quicker access.

Camera RAW: A file made by a digital camera encoded to include raw sensor data together with metadata (usually the time, date, aperture, shutter speed, and other information about the shot). RAW files are the equivalent of digital negatives and contain more information than a standard JPG file making it possible to more easily salvage poorly exposed frames.

CCD: (Charged Coupled Device) a light sensitive chip used in scanner and cameras to convert light into an electrical charge. Although these devices struggle with excessive infrared sensitivity, back-scatter light, and sharpness problems, they are widely used because of favorable cost.

CD-ROM: Compact Disk-Read Only Memory (a universal name for a variety of disk formats)

CDI: Compact Disk Interactive -- a specific CD-ROM format

CF card: Compact Flash memory card for digital cameras. They are flat rectangular devices roughly 1.68 inches wide x 1.43 inches long and 0.15 inches thick. Capacities in megabytes or gigabytes can vary depending on cost.

Channel: Analogous to a plate in the printing process, a channel is part of an image in Photoshop and other image programs. There are color channels (red, green, blue for RGB pictures, or cyan, magenta, yellow, black for CMYK pictures, or grayscale channels in every image, and the user can add additional alpha channels to the file. Usually alpha channels are good places to store image selections (Lasso tool, etc.) that may be needed again in the creative process.

CIE: (Commission International l'Eclirage) an international group that has developed a set of color definition standards. Used by Adobe in PostScript Level 2 in an effort to standardize how color is rendered on different printers.

Closed architecture: Computers that are proprietary or restricted by language to a specific platform or application.

CLUT: (Color Look-Up Table) a color indexing system used by some computers to reference color if their systems do not support a high enough bit depth to represent all colors.

CMOS: A type of light-sensitive image sensor used in some cameras. Said to have lower noise than a CCD sensor chip. CMOS image sensors draw much less power than CCDs and permit longer battery life in digital cameras. They are also cheaper to make than CCD chips.

CMYK: Cyan, Magenta, Yellow, and Black (called Key), the colors used in most printing processes. Also called subtractive colors.

Color Management System: Interrelated software that is designed to correct for differences between different color displays and different printers. The goal of a color management system is to present a similar picture as a file is displayed on different monitors and printed to different printers. In the eighties, the term WYSIWYG (What You See Is What You Get) was used to describe the goal of matching color. Because of inherent color differences (especially differences in color gamut) between hardware devices, the match is never entirely perfect but is perceived to be good. Photoshop software beginning with version 5 has had the ability to color manage pictures.

Color Profile: A computer file containing information about a physical device that displays color pictures (monitors, printers, etc.). A color management system uses to information about the device to adjust color on the fly to closely match the color and tone information in a picture file.

Continuous-tone image: An image containing intermediate tones in addition to black to white. Examples are dye-sublimation printers and slide setters. These devices do not have to use halftone dots to reproduce a photograph.

CPU: (Central Processing Unit) The chip in a computer that performs most of the computations.

CT: (Continuous Tone) A file format used for exchanging high-level scan information. Scitex is the company that uses this format with their ultra-expensive scanning, printing and imagesetting machines.

DCS: (Desktop Color Separation) an image file format comprised of five sets of data for each color image (one Post Script file for each color (CMYK) and one PICT preview file with controller data).

Despeckle: To remove or reduce speckles or dust spots introduced during scanning or image processing.

Digital: Information represented by integers rather than by analog data.

Digitizing tablet: A mouse replacement comprised of a "pen" and flat panel wired to the computer. Pen movements on the tablet are reproduced on the computer screen and pressing the tip of the pen against the tablet mimics pressing the mouse button. Some tablets may be pressure-sensitive in illustration programs like Photoshop -- a harder pressure makes a thicker line. Wacom brand pressure-sensitive tablets are favored over the mouse by many graphic artists.

Dithering: The technique of making adjacent pixels different colors to give the illusion of a third color. Dithering can simulate the effect of shades of gray on a black and white screen display, or more colors than only 256 on an 8-bit color display.

Dot gain: An unavoidable increase in the size of half-tone dots used in the printing process due to characteristics of ink, paper, press. This increase in dot size increases the density of ink on the page and thus makes printed tones darker. This is accounted for in a color-managed printing system.

DPI: (dots per inch) a measure of the resolution of printers, image setters and other output devices.

Dye-sub: A technology for printing images using CMYK colored ribbon and a receiver sheet. It is characterized by continuous tone, photographic-looking images with no halftone dots. In operation, the ribbon is in intimate contact with the receiver sheet when it passes over a print head where minuscule spots of the ribbon are heated to vaporize the dye. The dye vapor deposits onto the receiver sheet to produce a spot of color. After all color components of the ribbon have been transferred, the image is complete. Dye sub printers are expensive and slow (~5 min. per page), but produce outstanding results. Carefully handled, most dye sub prints will last as long as a color photograph.

EPS: (Encapsulated PostScript) a file format used to transfer PostScript image information from one program to another. The file includes PostScript code for the actual image and a low resolution representation of the image for display purposes. EPS files require a Postscript-language printer to properly render the image they contain.

Firewire: A computer interface made popular by Apple on Macintosh computers in the late '90s and often offered as an option on PC computers. It offers fast transmission speed and uses a simple cable. Some peripherals like flash memory card readers and scanners make use of the Firewire interface.

Flash Memory: Computer memory in a form that doesn't require continuous electrical current to retain data. Used in digital cameras, Apple's iPod Shuffle, and memory keys made by a host of manufacturers.

Four-color process: The use of cyan, magenta, yellow, and black in printing to simulate a wide variety of other colors.

GCR: Grey Component Replacement, a technique for reducing the amount of cyan, magenta and yellow in an area and replacing them with the appropriate level of black.

GIF: (Graphic Interchange Format) A compressed picture file format popularized by CompuServe and widely used on the World Wide Web. Characterized by compact file size, a GIF file is limited to 256 colors or shades of gray.

Halftone screen: A pattern of dots, lines, or other patterns printed on paper that simulate shades of gray of photos or illustrations. Later developments in the process extended its usage to cyan, magenta, yellow, and black ink printing (CMYK) to simulate the multitude of colors found on the printed page using just four inks. It is a printing process invented in the late 19th century (about 50 years after photography was invented) as a way to simulate a real photograph on the printed page.

High density disk: Floppy disk that contains about 1400K (1,400,000 bytes) of data.

Histogram: A graphic representation of the number of pixels with given color values. A histogram shows the breakdown of colors in an image.

HSL: Hue, Saturation and Brightness (sometimes HSB)

HSV: A color model based on three co-ordinates: hue, saturation and value

HUE: The wavelength of light of a color in its purest state( without the addition of white or black.)

ImageReady: A companion program that comes with later versions of Photoshop that prepares pictures for web distribution and for special effects like animation.

Indexed color image: A single channel image, with 8 bits of color information per pixel. The index is a color lookup table containing up to 256 colors.

Ink jet: A common type of desktop printer that uses liquid ink in a traversing printhead. The printhead sprays a pattern of colored dots onto paper to form an image. The ink colors are usually CMYK, with some low-end printers having only CMY. The resulting images from most desktop inkjet printers are prone to rapid fading if kept in brightly lit, humid areas if the inks are dyes. Some versions of inkjet printers use pigments that have a very long life, but the printers for pigmented inks tend to cost more.

Photo-quality versions of inkjet printers may have extra colors in addition to CMYK. This allows them to produce smoother tones in light areas.

ISDN: (Integrated Services Digital Network) A standard of telecommunications that will allow digital information of all types to be quickly transmitted over phone lines.

JAZ: A removable-disk drive manufactured by Iomega that holds one gigabyte of information on each removable disk.

JPEG: (Joint Photographers Expert Group) a set of standards developed by this group for compressing and decompressing digitized images with computers.

k: One kilobyte or 1024 bytes of date. For easy remembering, k= approximately one thousand.

Knockout A shape or object printed by eliminating (knocking our) all background colors. contrast to Overprinting.

LAN: (Local Area Network) A group of connected computers in a relatively small area that share access to printers and other peripheral devices.

Layer: An element in Adobe Photoshop files that contains picture information. All images have at least one layer (the Background layer) as seen in the Layers palette, but a user can add more picture elements on additional layers for creative effects.

Loss less: Non lossy, a term used to describe a file compression scheme that reduces the size of a file without losing any of the information in the file.

Lossy: A term used to describe a file compression scheme that may discard information (and thus detail in an image) as it reduces the size of the file. The JPEG (Joint Photographic Experts Group) file format is the most common lossy file format. Used wisely, its effect is not noticeable.

LPI: Lines per inch, a measure of how many rows of dots exist in an inch of a halftone-printed picture. Values may range from 85 lines per inch for newsprint and laser printers, to 133 lines per inch for commercial work on glossy paper, and even higher for quality print runs.

Luminance: Lightness; the highest of the individual RGB values plus the lowest of the individual RGB values, divided by two: a component of an HSL image.

Luminosity: A value corresponding to the brightness of color.

LUT: (look-up table) The table of colors a computer can display at a given time. the computer uses the table to approximate the table to approximate the desired color from the range it has available.

Megabyte: Approximately a million bytes of data and a common indicator of file size. Listed several different ways i.e. 32 megabytes, 32 MB, 32,000,000 bytes.

Megapixel: Approximately a million pixels (picture elements) that comprise a digital picture. Increasing the number of megapixels recorded increases potential detail in a picture. A quality camera will usually have around 6 megapixels and up.

Modem: A device used to transmit date between two computers over telephones lines.

Moiré pattern: an undesirable pattern in color printing, resulting in incorrect screen angles of overprinting halftones. Moiré patterns can be minimized with the use of proper screen angles. Moiré patterns also appear when a previously halftoned image is scanned into the computer. Click here to see a large-scale example of the effect.

Open architecture: Computers that are not locked into one hardware supplier or programming language.

Pict/Pict2: A common format for defining bit-mapped or object-oriented images on the Macintosh. The more recent format (PICT2) supports 24 bit color. These are mostly obsolete formats because of the prevalence of JPG and TIF file formats.

Pixel: A single picture element on a computer display or a single information data point in a digital image.

Platform: The brand or computer type you are using, i.e., a Windows platform or MAC platform. Other commonly encountered platforms include Unix and Linux, both of which are encountered more in server applications.

Postscript: A page description language used to render text and pictures generated by computer programs. PostScript is a trade name owned by Adobe and widely licensed to manufacturers of printers to accurately display text and image on the printed page. Postscript is capable of rendering both bitmap images (scans) as well as vector illustrations from popular programs like Freehand and Illustrator.

RAM: Volatile memory or Random Access Memory. The memory used to contain data being displayed on screen. More RAM enables you to work on larger images. A workable minimum for working with Photoshop is around 16 megabytes.

RAW: Files from digital cameras that have not been processed nor compressed in-camera. They permit later adjustments to color balance, etc.  Many consider these to be the equivalent of a digital "negative", and they offer considerably more latitude in adjustment than JPG files from the same camera.

Rasterization: The process of converting mathematical and digital information into a series of dots by an image setter for the production of negative or positive film. One common example is converting vector artwork (usually created by programs like Freehand, Illustrator, or CorelDraw) into images comprised of pixels. Photoshop has the ability to rasterize EPS vector artwork into native Photoshop images.

Resample: To change the number of pixels in an image. Resampling down discards pixel information in an image; resampling up adds pixel information through interpolation.

Resize: To change the size of a digital image.

Resolution: The number of pixels per inch in an image, or the number of spots per inch used by an output device.

RGB: The additive primary colors, Red, Green, Blue. In Photoshop, editing digital pictures in RGB color space has advantages over working in CMYK color. Almost all digital cameras output  RGB files.

RIP: (Raster Image Processor) part of an output device that rasterizes information so that it may be imaged onto film or paper.

RISC: (Reduced Instruction Set Computing) a type of raster image processor capable of faster computation because it uses simpler and more regular instructions.

ROM: Non volatile memory or Read Only Memory. ROM memory retains data even when the power is turned off, and is mostly used to contain operating system data.

Saturation: Richness of color determined by the amount of gray in a color. More gray in a color means lower saturation; less gray in a color means higher saturation. Pictures that have excessively saturated colors look cartoon-like.

Scanned image: The computer picture that results when a photograph, slide, paper image, or other two or three dimensional images are converted into a digital image.

Scanner: A device that converts photographs and drawings into digital pictures visible on a computer screen. Flatbed scanners scan reflective artwork like photographic prints or drawings on paper, while film scanners read translucent art like negatives and slides. Hand scanners, once widely sold as an inexpensive substitute for flatbed scanners, read a 4-5 inch section of artwork as they are pulled along by hand. Film scanners are used exclusively for photographic film and can often perform better than scanning film on flatbed scanners.

Screen angles: The angles used to offset the different films in process color separations. Proper screen angles are critical to minimize moiré patterns.

Screen frequency: The density of dots on halftone screen, commonly measured in lines per inch. See LPI.

SCSI: Small Computer System Interface. A common way to attach peripheral components (removable drives, scanners, etc.) to a computer. While it is available on PCs via an add-in card, it is more common on the Macintosh where it is built into the machine.

Spooling: Temporarily storing information onto a hard drive (or even a remote drive) to return the control of a program to the user. It is often used in the background to feed data to a printer while the user continues to work.

Subtractive primaries: The inks (cyan, magenta, and yellow) used in printing to create different colors. In contrast to additive primaries, these produce darker colors when combined.

TIFF: Tagged Image Film Format, a file format for exchanging bit mapped images (usually scans ) between applications. It is a highly reliable file format, easily readable by most any software program.

Trap: An overlap that prevents gaps from appearing along the edges of an object in a separated image due to slight misalignment on press.

UCR: (Undercolor Removal) A technique for reducing the amount of magenta, yellow and cyan in neutral areas and replacing them with an appropriate amount of black.

USB: A computer interface that uses an inexpensive cable to transmit information to and from peripherals like scanners, card readers, printers, etc. The initial implementation of USB (version 1) was fairly slow. Newer implementations offer a much improved speed, but both the computer and the peripheral must use USB 2.0 to take advantage of the increased speed.

Vector art: A type of computer data format used for illustration. Images are comprised of graphic shapes that scale to any size without displaying the "Jaggies". Vector art is not used to display scanned photos. Freehand, CorelDraw, and Illustrator are vector art illustration programs.

VRAM: A type of computer memory often used for video purposes. Faster than normal computer RAM, the amount of VRAM determines how many colors a given video system can display.

Virtual memory: Hard drive space set aside to complement the main memory (physical random access memory). Virtual memory allows you to work on a large document without requiring you to have large amounts of RAM.

ZIP disk: A small, high-capacity (100 Megabyte) removable disk manufactured by Iomega for storage or transfer of computer files.

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