Refers to displays of bitmapped images, where both curved lines and diagonal straight lines appear to be jagged due to the way they are composed of square pixels (sometimes referred to as ‘staircasing’ or ‘jaggies’).

Aliasing also can result from low-resolution scanning-in or from low resolution camera images. A small file size will show ‘aliasing’ when image size increases.

A signal that represents sound or vision by electrical analogy, eg, variations in a DC voltage producing corresponding variations in luminance, or vice versa. Similarly, a silver photographic negative is an analogue representation of the subject.

A method of reducing the effect of aliasing by averaging the densities of pixels at the edges of items such as text, thereby softening their appearance.

A method of interpolation whereby to, in order to increase image resolution, the value of the new pixel is determined by averaging from all those surrounding it. It is the most accurate form of interpolation. It can be selected as the preferred 'default' setting in Photoshop.

A numerical coding system using only two digits, 0 and 1.

Two binary digits, or 'bits', can give four possible combinations:

Three bits can give 8 (2³) possible combinations:

Short for 'binary digit'. A single number having the value either zero or one, which may represent the states of 'on' or 'off'. Eight bits make up one byte.

The number of bits used to represent the colour value of each pixel.

1 bit per pixel displays 2 colours, 2 bits gives 4 colours, 3 bits gives 8 colours and so on. Continuing to 24 bit, we have 16.7 million colours.

Windows bitmap file format. This format is uncompressed which can result in very large files. .RLE is the compressed version of this format.

This refers to the predominant colour of the light source. It's measured in degrees Kelvin (^oK), which takes 'Absolute Zero' as its zero point. The redder the light source, the lower the colour temperature and conversely, the bluer the light source, the higher the colour temperature.

E.g.

Photographic tungsten ('Photoflood') light = 3,200^oK

Electronic flash = 5,500^oK

Average daylight = 5,500^oK

Note, daylight is extremely variable and can be anything from the orange/red glow of a sunset to a dull overcast day (which could be 10,000^oK or more)

'Daylight' (as opposed to 'Tungsten') colour films are balanced for a colour temperature of sunlight in mid June at midday.

High-speed memory chips, which store frequently used instructions. This is much faster that using conventional RAM.

A designated part of the computer RAM that holds the last item copied or cut from a file. An item held in the clipboard can be pasted into other files.

Loss of shadow or highlight detail due to the conversion of grey tones lighter than a certain value to white, or darker than a certain value to black.

The effective rate at which the central processing unit (CPU) in a computer communicates with the various elements within it. The processor runs at a fixed clock-speed, regulated by the pulses of a quartz or other crystal. The speed is rated in megahertz (MHz) – one megahertz representing one million instructions per second.

The selection and duplication of groups of adjacent pixels within an image. For example, an area of skin tone can be cloned from another area to hide a skin blemish. The cloning tooling in most imaging programs can be varied to select different sized areas of pixels to be cloned and the manner in which they are cloned.

CMYK = [C]yan, [M]agenta, [Y]ellow and [B]lack

The three colours and black used by printers to produce printed colour illustrations using inks or dyes.

A digital process which enables data to be stored or transmitted using less than the normal number of bits. Compression can be 'Lossy', 'Lossless' or 'Virtually Lossless'. There are many types of compression, JPEG ([J]oint [P]hotographic [E]xperts [G]roup) is one of the most commonly used, however it is 'Lossy', which means that once compressed some information is lost, this may or may not be visually apparent, depending upon the image content.

A measure of the resolution of a scanner or printer.

A type of thermal printer giving prints of photographic quality, whereby dyes are transferred onto the paper (or film) in succession.

[E]ncapsulated [P]ost[S]cript. File format widely used in desk top publishing (DTP) software.

The overall format in which any digital data file is saved.

Choosing the correct format for saving images is important so as to ensure that files are compatible with various software packages. Some formats compress the data to reduce space taken for storage. File formats include TIFF, EPS, GIF, JPG and PICT.

Generally speaking, the file size of an image can be calculated by the formula:

Total N^o of pixels x N^o bits per colour = 'X', divide by 8

This gives total N^o of bytes

E.g.

An image has 1200 x 800 pixels = 960,000 pixels and is stored as 24 bit colour

Therefore we have 960,000 x 24 = 23,040,000 divided by 8

Which gives 2,880,000 bytes or 2.8Mb

We can readily see that as the total number of pixels increases, thus improving definition, the file size will increase. Also, as 'bit depth' increases, improving colour rendering and dynamic range, the file size will, again, likewise increase.

The file size will be dependent upon the file format used to save the image.

The total number of pixels is divided by 8 because there are 8 'bits' making up one 'byte'.

Just to make life more interesting, a 4Mb file stored to different file formats becomes in:

Photoshop 4/5 = 2.1Mb

TIFF with LZW compression = 1.8Mb

JPEG settings vary from 'good' (104Kb) to 'best' (1.1Mb)

EPS = 5.6Mb.

In photographic sensitometry, gamma refers to the slope of the straight-line portion of the characteristic curve of an emulsion. In digital imaging, it is a measure of midtone image contrast, the relationship between input data from an electronic image, and output data telling the monitor how to display an image.

The range of colours (or tones) which can be displayed or printed by a particular colour system. Many programs and colour management systems have ‘out of gamut’ warnings, indicating when colours displayed on a monitor are not reproducible by the printing system in use.

[G]raphics [I]nterchange [F]ormat. Developed by Compuserve in 1987 it allows compression of 8 bit images (original intention for transmission via modems). GIF uses LZW compression and is limited to 256 colours.

[J]oint [P]hotographic [E]xperts [G]roup. A widely used 'Lossy' compression for still images. The amount of compression may be selected.

A facility in Photoshop and other image programs for overlaying successive images on the same picture. Each layer may be treated separately or as part of a group. Think of layers as transparent sheets that may be shuffled into the required order.

A single row of picture elements which traverses across the image projected by a lens (in a digital camera) or across a print or transparency during 'scanning'. The process converts the analogue picture information into digital data.

Tri-linear arrays have three lines of picture elements each coated with a primary colour filter i.e. red, green and blue.

Resolution is quoted in ppi or ppc (pixels per inch or cm).

[L]empel-[Z]iv-[W]elch

A ‘Lossless’ compression routine developed in the 1970s, and incorporated into the TIFF and GIF file formats.

If you wish to impress your friends, this stands for microreciprocal degree and represents 1 million divided by the Kelvin colour temperature of the light source. It's used as a reference number by manufactures of colour correction filters. Daylight @ 5,550 ^oK corresponds to 182 Mired.

A socket in a computer most often used for connection to a printer. Parallel ports have 8 parallel wires which send 8 bits (1 byte) of information simultaneously, in the same amount of time it takes a serial port to send '1bit'.

The term derived from Picture Element. The smallest area capable of resolving image detail in a pick-up device such as a CCD array. Pixels are usually square but can be rectangular, triangular or circular.

PPI  -  [P]ixels [P]er [I]nch

A standard unit of measurement for the spatial resolution of scanned images.

The three primary colours of white light are red, green and blue.

[R]andom [A]ccess [M]emory

Temporary memory created and used only when the computer is switched on. The size of images, which can be opened, is dependent on how much RAM is installed in the computer.

Changing the resolution of an image, either by discarding unwanted pixels, or by interpolating new ones.

Changing the size of an image without altering the resolution. Increasing the size will lead to a decrease in image quality.

The process of converting analogue value varying continuously with time into discrete values.

The strength or purity of a colour.

[S]mall [S]ystems [C]omputer [I]nterface

A port input in a computer that allows high-speed communication between the computer and a peripheral device. The devices are linked or 'daisy-chained' together. Each device requires an identity number (I.D.) so that data travelling along the 'bus' (a common pathway) can be recognised.

The resolution at which an original is scanned. Usually quoted in pixels per inch (ppi), samples per inch (spi), or Res (pixels per millimetre).

A multi-purpose port on a computer frequently used for connecting a 'mouse', a graphics pen tablet, a modem or other device. Data is sent 'in series' i.e.1 bit at a time and is thus 'slow' in comparison to a parallel port. However for devices such as the mouse etc, speed is not an essential requirement. Modems connected to a standard telephone line can only handle one signal at a time.

A reserved area of a hard disk used for temporarily storing a copy of an image being processed.

The process by which a solid becomes a gas without first passing through a liquid state.

[T]agged [I]mage [F]ile [F]ormat

Originated by Microsoft and Aldus in the late 1980's this is the most widely used format today for bitmapped images.

[T]echnology [W]ithout an [I]nteresting [N]ame

A cross platform interface for acquiring images with scanners and frame grabbers.

[U]niform [R]esource [L]ocator

The standard naming and addressing system on the World Wide Web. For example the URL of Kodak’s digital imaging page is:

http://www.kodak.com/digitallimaging/digitallimaging.shtml

A type of computer graphics where images (geometric shapes such as curves, arcs and lines) are defined as a series of mathematical formulae rather than a grid of pixels. Also referred to as object-oriented graphics.

The relative amounts of red, green and blue light in a light source. Digital and video cameras can be ‘white balanced’ i.e. the signal adjusted so that the light reflected from a white or neutral grey surface can be neutralised.

A compression method used to compress files for easier storage.