Understanding Digital Images
With so many companies choosing
to produce their marketing and promotional material
in-house, it is important to have a full understanding
of the information you are working with. This
article gives a brief overview of digital images
and colour reproduction and their use in print
and screen output.
Digital images are composed of one of two file
types, reflecting two different technologies -
vector files and bitmap files.
Vector Files
Vector files, also known as "object-oriented"
files, plot images on a page using a point to
point system, much like join-the-dots, using a
horizontal and vertical axis to determine coordinates
(x and y axis respectively). Points can be plotted
in a straight line using two sets of coordinates
or along a bezier path using four sets of coordinates.
Bezier paths are used to plot an arc through a
series of mathematical equations, allowing curved
lines as well as straight lines. Vector files
also plot simple shapes such as squares and circles.
You can scale a vector graphic up or down and
the program recalculates the points and draws
a line in between at the highest screen or print
resolution. For this reason, vector files are
considered to be "resolution independent".
The fonts used by computers are also vector files,
as you can see when you break the letters apart
in a drawing program. Vector files have much smaller
file sizes than bitmap images as there is less
information required to be stored in the file
(vector files plot specific points, bitmaps plot
every point).
File formats
Vector file formats are generated by drawing
programs such as Adobe Illustrator, Macromedia
Freehand and CorelDRAW and include native file
formats for these applications. EPS files (which
can also be used to store bitmap files and indeed
contain a bitmap preview of the vector file for
use in on-screen placement), PICT files, WMF files
and CGM files are also vector files, amongst others.
Bitmap Files
Bitmap files are used for images that require
a full tonal range, such as photographs. There
are four characteristics to bitmap files - resolution,
dimensions, bit depth and colour model.
Resolution
Bitmap files are comprised of touching pixels
or picture elements (squares of black, white,
grey or colour) for every area of the image. The
number of pixels in an image is known as the resolution.
The resolution of the image directly affects the
file size of the image; the higher the resolution,
the larger the file sizes.
So how high should your resolution be? Screen
images cannot be reproduced by monitors at a resolution
higher than 72 dpi (dots per inch), so any resolution
higher than that offers you no increase in image
quality.
Images for print will vary in their resolution
depending on the lpi (lines per inch or the space
between printed dots on a page) at which it is
to be printed, but as a general rule of thumb
most printers use 300 dpi images to get a sharp,
clear image.
Bit Depth
Bit depth determines how many tones or colours
each pixel can have. If an image is set to a depth
of one bit, each pixel can only have two states,
black or white, zero or one. A depth of two bits
adds two grey tones to the black and white, giving
four levels in total. Eight-bit depth gives 256
levels of grey and can produce a smooth graduation
from white to black without banding. Colour images
use a series of eight bit channels to produce
colour values. For example, RGB colour usually
has a 24-bit depth (3 channels of 8 bits) and
CMYK colour uses 32-bit depth (4 channels of 8
bits).
Colour model
As we have seen in the information on bit depth,
each primary colour is produced as a grey-scale
channel which, when combined with other primary
channels, gives an approximation of full colour.
Thus an RGB file contains three channels, a CMYK
file has four channels, a greyscale file (which
contains a single black channel) has one channel,
etc. In addition to this many bitmap file formats
allow you to store an alpha channel, a single
bit channel that determines transparency in the
file. The more channels in your file, the larger
the file size will be. With this in mind it is
sometimes easier to work with a low resolution
RGB file until you are ready to print and then
swap it for an identical high resolution CMYK
file.
Scaling
If a bitmap image needs to be rescaled there
are a number of factors to take into consideration.
When the file is to be downscaled to a smaller
size, the dots of colour become smaller (using
more pixels per dot). This poses no problem with
image quality, however the file size may now be
unnecessarily high (larger file size than required).
When the image is scaled up to a larger size,
the dot of colour become larger (using more pixels
per dot) and the image takes on a stepped looked
known as aliasing or "jaggies". This
can be avoided to a certain extent using interpolation
of the pixels, where the computer determines what
the colour of each pixel should be by sampling
the neighbouring pixels and calculates an average.
Ultimately, the best way to get a clear image
at a larger scale is to rescan the original.
File formats
Bitmap file formats are generated by painting
programs such as Adobe PhotoShop and Jasc Paint
Shop Pro and include native file formats for these
applications. TIFF, JPEG, GIF, BMP, PICT and EPS
files are also bitmap files.
Colour Mode
Full colour
All colours the human eye can see are derived
from red, blue and green. On-screen colours are
created by combining these colours with varying
intensity. White occurs when the intensity of
each of these colours is equal. Black is an absence
of these colours. This is called "additive
colour". As the eye only sees in red green
and blue, the problem in printing is reflecting
these colours. To do this "subtractive colour"
is used. Cyan absorbs red light, magenta absorbs
green light and yellow absorbs blue light. White
is the absence of colour and black is an equal
mix of all three colours (though this is not a
true black and printers use an additional black
ink to add definition). Monitors use an RGB colour
space (additive colour) to produce an approximation
of full colour at a resolution of 72 dpi. Printers
use CMYK (Cyan, Magenta, Yellow and blacK) inks
(subtractive colour) at a resolution of 300 dpi
to approximate full colour.
Spot colour
Spot colour is where one or more colours are
specified for printing and can include metallic
inks and varnishes. Spot colours can be screened
and mixed to give the effect of additional colours
and where less than four colours are used is cheaper
than full colour printing. Colour matching is
achieved by specifying ink from a proprietary
colour matching system; the most common of these
is the Pantone Colour Matching System. Colours
are specified as PMS numbers and systems such
as these ensure you will get an accurate reproduction
of the colour that you want.
Monotones and Duotones
Most people think of black and white when they
think of monotone colour, but any colour can be
used instead of black. Consider sepia toned images
in a dark brown or a dark blue to add extra dimension
to the page without increasing costs. Or perhaps
a duotone (an image with two colour channels)
to add depth to the image. For more information
on duotones consult the help files of your painting
program (such as PhotoShop).
Web Images
With web images the biggest concerns are accurate
colour reproduction and download times. Different
platforms and different browsers reproduce colours
differently so to be sure that your image is going
to be reproduced accurately it is best to stick
to the palette of 216 web-safe colours. Most graphics
packages supply a palette of web safe colours
or you can download one from the internet. The
two most common file formats used for images on
the Internet are GIF images and JPEG images. GIF
images are limited to indexed colours and will
step or dither between colours to get the desired
colour effect. JPEG images are a better option
for continuous tone images such as photographs
and can be produced with varying compression to
accommodate file sizes. Keep in mind, the more
highly compressed the image the lower the image
quality.
To get the best possible result from your images,
it pays to understand how they work. This article
has hopefully given you some insight into the
basics of digital images but it really just scratches
the surface. For more information on digital images,
check out graphics books and websites such as
dtg.com, ideabook.com or allgraphicdesign.com.
Resources
An Introduction to Digital Scanning - produced
by Agfa-Gevaert Ltd
Concepts in Graphic Design, David A Holzgang and
Lesley Strother, MIS Press
http://www.dtg.com
http://www.allgraphicdesign.com
http://www.ideabook.com
