Common
file terms
Bit depth – When an image is described as "x
bit" with x being some number or other, what's being talked about
is the number of colours. In bitplane or raster graphics, each pixel
has its colour described by a string of bits, and the more bits there
are per pixel the more possible colours there are. The number of colours
equals two to the power of the bit depth, so one bit (or "one
bitplane") files can have only two colours, two bit can have
four colours, three bit has eight colours and so on. The most common
depths are 8 bit (256 colours) and 24 bit (16.8 million colours).
Bit depths higher than 24 provide a wider colour "gamut",
so image manipulation software can pull out otherwise invisible detail
out of the image.
Bitmapped file – Bitmapped files, also known
as raster files, contain graphics information described as pixels,
such as photographic images. The image is built up dot by dot; if
you zoom in, the pixels get bigger and the image ends up looking like
Lego.
Compression – Data compression is not a new
concept – it's been around in one form or another for decades.
If an image format includes data compression, then generally speaking
the images will be smaller in size but take more computing power to
load, as the computer has to work out what the original data was.
In the olden days, compressed formats weren't popular because processors
were too slow to display compressed images quickly. This, plus modern
advances in compression technology, explains why older image formats
tend to be bigger.
Dithering – What do you do if you have to display
an image with lots of colours on a screen without enough to show it
properly? You do dithering. This involves mixing pixels of the colours
you have so that the end result looks more like the colour you don't
have. It's not as good as having enough colours to show the image
properly, but it's better than the "banding" that results
from doing a best-match sort of display.
Lossy compression – A form of compression in
which some data is discarded to allow much smaller file sizes. In
image compression, lossy techniques such as those uses in the JFIF
format.
Metafiles – These are files that may contain
either bitmapped or vector graphics data.
Page Description Languages – PDLs, as they're
more often called, are used to describe the layout of a printed page
of graphics and text. Two examples are Postscript and HPGL (the Hewlett
Packard version). They're used almost exclusively in desktop publishing,
most often as the file format sent from the computer to the printer.
Palette – The number of colours on screen is
not necessarily the same as its palette. The palette is used in the
same sense as a painter's palette; it holds all the colours that can
be used, from which the ones that actually are used are chosen. A
given video card might, for example, allow you to display 256 colours
at a time from a palette of 32,768.
Pixel – The smallest element of an image, and
the unit in which its resolution is expressed. The normal resolution
of VGA graphics is 640x480 pixels; such a screen has a total of 307,200
pixels. A low resolution 320x200 image has only 64,000 pixels; a high
res 1024x768 image has 786,432.
Raster file – See bitmapped file.
Vector file – Bitmapped files describe a picture
in terms of pixels, while vector files describe it in terms of geometry.
A line here, a curve here, this area filled with this colour, and
so on. Vector files are much larger for an image of a given detail
level, but they can be magnified as much as you like without turning
into giant pixels.
