| Photographic
Digital Imaging Frequently
Used Terms
& Glossary |
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Note:
If a term is underlined it links to more information
- this may be of further benefit. |
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1-bit
colour
The lowest number of colors per pixel in which a graphics file
can be stored. In 1-bit color, each pixel is either black or white. |
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8-bit
colour/greyscale
In 8-bit color, each pixel is has eight bits assigned
to it, to provide 256 colors or shades of grey, as in a greyscale image. |
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24-bit
colour
In 24-bit color, each pixel has 24 bits assigned to it, to represent
some16.7 million colors. 8 bits - or one byte - is assigned to each of the red,
green, and blue components of a pixel. |
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32-bit
colour
A display resolution setting that is often referred to as true
color and offers a color palette of over 4 billion colors. |
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| Aliasing
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. |
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Alpha
Channel
The fourth channel of an RGB image that creates transparency in
the image.
In Photoshop it's a 'saved selection'. This 'selection' can be viewed
and edited when revealed in the 'Alpha Channel' palette. |
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| Analogue (sometimes,
analog)
A signal that represents sound or vision by electrical analogy, e.g.,
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. |
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Anti-aliasing
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. |
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Baseline
When using text, this is the 'invisible' line upon which the type 'sits'. |
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Bicubic
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. |
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| Binary
A numerical coding
system using only two digits, 0 and 1. Two
binary digits, or 'bits', can give four possible combinations: 00,
01, 10, 11 Three bits can give
8 (23) possible combinations: 000,
001, 010, 011, 100, 110, 111, 101 |
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| Bit
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. |
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| Bit
depth, also known as bits per pixel
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. |
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| BMP
Windows bitmap file format. |
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Blending
Mode
A function within 'Photoshop' (and other imaging software) that changes
the behavior of a tool or layer, allowing it to interact in a variety of ways
with the underlying image. |
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CD
Compact Disk. Used for storing large amounts of data (700mb or so per
disk) |
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Cache
High-speed
memory chips, which store frequently used instructions. This is much faster that
using conventional RAM. |
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Clipboard
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 |
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Clipping
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. |
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Clock
speed
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. |
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Cloning
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. |
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CMYK
(cyan, magenta, yellow,black)
The
three colours and black used by printers to produce printed colour illustrations
using inks or dyes. |
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Colour
Temperature
This refers to the predominant colour
of the light source. It's measured in degrees Kelvin (K), 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 degrees K
Electronic flash
= 5,500 degrees K
Average daylight = 5,500 degrees K 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 degrees K or
more) 'Daylight' (as opposed to
'Tungsten') colour films are balanced for a colour temperature of sunlight in
mid June at midday. |
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Compression
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 (Joint Photographic Experts
Group) 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. |
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DCS
Desktop Colour
Separations a special version of the EPS file format. |
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DPI
Dots
per inch. A measure
of the resolution of a scanner or printer. |
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DVD
Digital Video
Disk.
Used for storing large amounts of data. Of larger capacity than CD,
4.7gb is a typical DVD size. |
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Dithering
A method of simulating a greater range of colours than a monitor can correctly
display, this is achieved by placing small dots of colour adjacent to one another.
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Dye
Sub
Dye Sublimation.
An output/printing method that produces a high
quality continuous tone by heating cyan, magenta and yellow (CMY) dyes until they
become gaseous (without going through the liquid stage) The output of a dye-sub
printer appears as a continuous tone glossy print, very similar to a photographic
print by tradition chemical process. |
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EPS
Encapsulated
Post
Script.
File format widely used in desk top publishing (DTP) software. |
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EV
A number that denotes Exposure Value. It is determined by the brightness
of the subject and sensitivity (speed) of the film/CCD array. Doubling the amount
of brightness increases the EV by 1, halving the amount of brightness decreases
the EV by 1. (See reference to 'f Stops). |
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Eye
Candy
A
set of creative plug-in filters for use with Adobe Photoshop, produced by Alien
Skin. |
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FTP
File Transfer Protocol - the most common method for
transferring files across the Internet. |
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f-Stops
A
method of controlling the amount of light reaching the film plane/sensor of a
camera by halving or doubling the amount in one-stop increment.. |
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File
Formats
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 and PICT. |
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File
size
Generally
speaking, the file size of an image can be calculated by the formula:- Total
No of pixels x No bits per colour = 'X', divide by 8 This
gives total No 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
= 2.1Mb TIFF
with LZW compression = 1.8Mb JPEG
settings vary from 'good' (104Kb) to 'best' (1.1Mb) EPS
= 5.6Mb. |
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Firewire
A
very fast external bus that supports data transfer rates of up to 400 Mbps. Firewire
was developed by Apple and falls under the IEEE 1394 standard. Other companies
follow the IEEE 1394 but have names such as Lynx and I-link.
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Frames
A method of loading multiple web pages into a single browser window. |
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Gamma
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. |
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Gamut
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. |
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GIF
Graphics
Interchange Format.
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 |
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HTML
Hyper Text Markup Language - this is the standard
language used to create web pages. |
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HTTP
Hyper Text Transfer Protocol - a method allowing
web browsers and servers to communicate. |
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Image
Map
A part of an image that has been made to be used as an HTML link or
'rollover'. |
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JPEG
Joint
Photographic Experts Group
A widely used 'Lossy' compression
for still images. The amount of compression may be selected.
See also, Compression,
File Size, Lossy |
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Java
Script
A programming language to make web pages more 'alive' and interactive.
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Jaz
drive
A removable
hard disk system using 3½" diameter disks of 1Gb capacity. |
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KPT
(Kai’s
Power
Tools)
Originated
by Kai Kreuse - a series
of plug-in filters for Adobe Photoshop. They include special effects filters,
texture generators and fractal generators. |
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Kerning
The adjustment of spacing between letters of type to create consistent
letter-spacing. |
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Layer
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. |
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Leading
Pronounced 'ledding', this is the spacing of a paragraph as measured from
one baseline to the next. Originally, in the printing industry, strips of lead
were used to increase the separation between lines of type. |
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Linear
Array
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). |
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LZW
(Lempel-Ziv-Welch)
A
‘Lossless’ compression routine developed in the 1970s, and incorporated into the
TIFF and GIF file formats. |
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Lossless
Compression
A form of compression that maintains the fidelity of an image.
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Lossy
Compression
The term used to describe any sort of compression that results
in a lower quality image. |
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Mired
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 K corresponds to 182 Mired. |
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PICT
Originally devised to create graphic images with Macintosh computers.
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PPI
Pixels
Per Inch
A
standard unit of measurement for the spatial resolution of scanned images. This
is the usual way of stating the definition of a digital image. |
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Pantone
Ink commonly used when printing with fewer than four inks or when colours
are needed that cannot be reproduced using CMYK inks (including metallic, fluorescent
vivid greens, intense blues, etc). Each Pantone colour is represented by a specific
reference number which high-end commercial printing machines recognise in this
way a high accuracy of colour is maintained. |
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Parallel
Port
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 '1 bit'. |
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Pixel
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. |
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Point
A unit for measuring text size. A 'point' is 1/72 inch (there are 12 points
to a pica and 6 picas per inch). The size is an 'indicator' as it will vary from
font style to font style. |
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Primary
colours
The three
primary colours of white light are red, green and blue (RGB). |
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Process
colours
Cyan,
magenta, yellow and black (CMYK). |
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Process
inks
The four
specific inks used in the four colour printing process, namely cyan, magenta,
and black. |
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RAM
Random
Access Memory
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. |
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RAW
This is image data that the camera 'array' records prior to internal processing
by the camera. Users can choose to process the data later using a computer program
/ RAW file reader. In this way high quality image fidelity can be maintained.
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Resampling
Changing
the resolution of an image, either by discarding unwanted pixels, or by interpolating
new ones. |
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Re-sizing
Changing
the size of an image without altering the resolution. Increasing the size will
lead to a decrease in image quality. |
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Rollover
When moving the cursor around an image/graphic, sections of the image/graphic
can be made to change when the cursor passes over. It's a feature of Java Script
that allows the original image to be (temporarily) replaced by another - such
as a 'button' appearing to be depressed. |
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Sampling
The
process of converting analogue value varying continuously with time into discrete
values. |
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Saturation
The strength
or purity of a colour. |
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SCSI
Small
ComputerSystems
Interface
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 raveling along the 'bus'
(a common pathway) can be recognised. |
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Scan
resolution
The
resolution at which an original is scanned. Usually quoted in pixels per inch
(ppi), samples per inch (spi), or Res (pixels per millimetre). |
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Serial
Port
A multipurpose
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. |
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Scratch
disk
A reserved
area of a hard disk used for temporarily storing a copy of an image being processed. |
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Slice
A square or rectangular area of an image that can be saved as a separate
file. A 'slice' may have special properties associated with it. Slices can be
used for 'rollovers' or HTML 'links' and so on. |
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Spot
Channel
A special sort of colour channel that allows an image to be made
from other inks or in addition to CMYK inks. Spot Channels are usually associated
with Pantone inks. |
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Sublimation
The
process by which a solid becomes a gas without first passing through a liquid
state. |
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TIFF
Tagged
Image
File
Format
Originated
by Microsoft and Aldus in the late 1980's this is the most widely used format
today for bitmapped images. |
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TWAIN
Technology
Without an
Interesting Name
A
cross platform interface for acquiring images with scanners and frame grabbers. |
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URL
Uniform
Resource
Locator
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 |
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USB
Universal
Serial Bus
An external input to a computer that is about 10
times faster than a typical serial connection. |
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USB2
An
improved, faster version of USB |
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Vector
graphics
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. |
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WAVE
A standard format used on 'Windows' for saving audio data ('.WAV' is the
file extension). Playback is either by 'Media Player' (Windows) or 'QuickTime
Player (Macintosh). |
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Web
Safe Colours
A special set of 216 colours that are common to both Windows
and McIntosh operating systems that will not become 'dithered' when viewed using
256-colour displays. |
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White
balance
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. |
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Zip
drive
A form
of removable hard disc storage using 3½" diameter discs of 100 Mb capacity. |
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