Digital Images
Digital images comprise a fixed number of rows and columns of pixels with each pixel
containing information about its colour or intensity. The physical size of an image when
displayed on a computer screen or printed depends on the image size and image resolution.
Definition of terms
File formats
BITMAP
A digital (raster) image composed of a matrix of dots. When viewed at 100%,
each dot corresponds to an individual pixel on a display. In a standard bitmap
image, each dot can be assigned a different colour. Together, these dots can be
used to represent any type of rectangular picture.
BMP
Bitmap. A file format that handles graphics within the Microsoft Windows
OS. Typically, BMP files are uncompressed and therefore large; their
advantage is their simple structure and wide acceptance in Windows
programs.
CMYK
Cyan, magenta, yellow and black. The four basic colours used for printing
colour images. Unlike RGB (red, green, blue) used for web imagery, CMYK
colours get darker as you blend them together. Since RGB colours are used for
light, not pigments, the colours grow brighter as you blend them or increase
their intensity. The letter âKâ is used to avoid confusion with blue in RGB.
EPS
Encapsulated PostScript. A PostScript image file format compatible with
PostScript printers and often used for transferring files between various
graphics applications. EPS files will print identically on all PostScriptcompatible printers and will appear the same in all applications that can
read the PostScript format.
DPI
Dots Per Inch. Used to measure the resolution of an image both on screen and
in print. DPI measures how many dots fit into a linear inch (not dots per square
inch). The higher the DPI, the more detail shown in an image.
GIF
LOSSLESS
Algorithms reduce file size while preserving a perfect copy of the original
uncompressed image. Lossless compression generally, but not always, results
in larger files than lossy compression. Lossless compression should be used to
avoid accumulating stages of re-compression when editing images.
Graphics Interchange Format. Limited to an 8-bit palette, or 256 colours,
suitable for storing graphics with relatively few colours such as simple
diagrams, shapes, logos and cartoon style images. The format supports
animation and is still widely used to provide image animation effects.
Its LZW lossless compression is more effective when large areas have a
single colour, and less effective for photographic or dithered images.
JPEG/JPG
LOSSY
Algorithms preserve a representation of the original uncompressed image
that may appear to be a perfect copy, but it is not a perfect copy. Often lossy
compression is able to achieve smaller file sizes than lossless compression.
Most lossy compression algorithms allow for variable compression that trades
image quality for file size.
Joint Photographic Experts Group. A lossy compression method usually
stored in the JFIF (JPEG File Interchange Format) file format which can
result in a significant reduction of the file size. The amount of compression
affects the visual quality of the result. JPEG files suffer generational
degradation when repeatedly edited and saved.
JPEG 2000
A compression standard enabling both lossless and lossy storage different
from standard JFIF/JPEG and which improves quality and compression
ratios while requiring more computational power to process. It is used
currently in professional movie editing and distribution.
PNG
Portable Network Graphics. A file format that, unlike GIF, supports 8 bit
paletted images (with optional transparency for all palette colours) and 24
bit truecolour (16 million colours) or 48 bit truecolour with and without
alpha channel. Compared to JPEG, PNG excels when the image has \