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BOOT CAMP 189 (23/08//01)
PICTURES AND PIXELS part 3
In the final instalment of this short series on digital
imaging we'll tie up some of the loose ends involved in getting high quality
pictures into and out of your PC plus some practical tips for printing,
processing images for web pages and sending pictures as email attachments.
We'll start by talking about size. This is a slightly tricky
subject because size means several different things in computer imaging. First
there's the size of a picture as you see it, on a monitor screen or printed out
on a sheet of paper then there's the actual size or area of an image, measured
in pixels and thirdly, there's the size of an image file, in kilobytes or
megabytes.
The size of displayed and printed images throws a lot of
people. In the real world we are used to pictures and photographs having fixed
physical dimensions but in Computerworld things are very different. Photographs
in emails (as opposed to pictures sent as 'attachments') are a case in point and
they can sometimes appear so large than only a small portion of the image is
shown in the message window. It's due to something called 'scaling'. Basic
e-mail programs like Outlook Express show a picture 'as is' and make no attempt
to resize the picture to fit the screen or compensate for the Windows resolution
setting (see part 1). For example, if a picture is 1000 x 750 pixels and your
Windows display is set for 800 x 600 pixels then you will only see about three
quarters of the picture on the screen and you'll have to use the scroll bars at
the bottom and side of the window to see the rest of it.
Picture editing programs on the other hand have the facility
enlarge or reduce the size of the displayed image – usually called 'zooming' --
so an image can be scaled down to fit the screen area or blown up, to view fine
detail, and enlarged or reduced to fit onto a printed page. However, it's
important not to confuse this with a picture editing program's ability to alter
the size of the picture by changing the number of pixels. This is quite a
complicated business; if you reduce the width of an image by decreasing the
number of pixels the picture will probably end up looking squashed. That's
because in order to maintain the correct proportions the height has to be
reduced by a corresponding amount.
The relationship between the width and height of a picture is
called the Aspect Ratio and most picture editors have a facility to
automatically maintain it, it's usually on the 'Size' or 'Resize' dialogue box
on the Image menu. Sometimes it’s clearly labelled as such, i.e. 'Maintain
Aspect Ratio' though programs like Adobe Photoshop Elements insist on calling it
'Constrain Proportions'…
There's quite a bit more to resizing an image than simply
adding or taking away pixels, and picture editing programs use a variety of
exotic sounding techniques to maintain the smoothness and preserve the sharpness
of edges when enlarging or reducing an image. This comes under the general
heading of Resampling and it's not something we have the room to go into here;
fortunately it's not something you need to worry about if you're mainly
concerned with pictures captured on a 'consumer' digital still camera or scanner
but needless to say it causes a lot of arguments amongst professionals and
enthusiasts.
It's worth familiarising yourself with the resizing
facilities in your picture editing program as it is important to match the size
of an image to its medium to ensure the best possible quality and effect. In
other words there's no point trying to print or display an image that's larger
than the screen or sheet of paper can handle. In fact it can actually degrade
the image, so experiment with the sizing facilities and learn to keep an eye on
the 'statistics' display. This is usually at the bottom of the screen and shows
the actual size of the image in pixels or centimetres and the amount of
magnification (or reduction) applied to the display.
Statistics info boxes usually tell you other useful things,
like the colour depth of an image (see part 2) and the size of the image file.
This is crucially important if, for example, you want to send a picture by email
or illustrate a web page as the size of the file has a direct bearing on how
long it takes to download. File size is largely determined by the picture file
format and there are plenty to choose from though in practice most PC users will
encounter only a handful of types, namely Bitmaps, TIFFs and JPEGs.
Bitmaps (file extension *.bmp) and TIFFs (extension *.tif) are 'lossless' formats that create the
biggest files and highest quality pictures because the data describes the
position, brightness, colour and colour depth of every single pixel in the
image. Bitmaps and TIFFs are produced by scanners and some high-end digital
cameras and depending on the size and complexity of the scanned image, can be
anything from 2 to 10 megabytes or more in size. Clearly this is far too big to
send over the Internet. On a normal dial-up connection a 5Mb bitmap file could
take anything up to an hour to send or receive!
Most images sent over the Internet use the JPEG (file
extension *.jpg) file format. This is a 'lossy' format, which means that the
data is compressed to make the file size smaller. Compression ratios of 100 to 1
are possible, though for most applications a ratio of between 10 and 20 to 1
strikes a good balance between size and quality. There are no hard and fast rules abut the
size of picture files sent over the Internet or in email messages but in general
it’s best to keep them below 100Kb and preferably under 50Kb, especially if you
are going to be sending a lot of them.
Next week – Printing on a budget
JARGON FILTER
ASPECT RATIO
The shape of an image, defined by the relationship between
its height and width
JPEG
Joint Photographic Experts Group -- part of the International
Standards Organisations, responsible for devising software compression systems.
A 'Lossy' or compressed image file format.
TIFF
Tagged Image Format File -- lossless 'bitmapped' picture file
format that describes in detail the attributes of each pixel in a digital image
TOP TIP
This week's tip is actually more of a curiosity and it
concerns most versions of Windows 98. Try this: right click on the taskbar
select Properties and the familiar Taskbar Properties dialogue box opens. Close
it and this time hold down the Ctrl key and keep it pressed while you right
click on the taskbar and select Properties. The same dialogue box opens but this
time there's an extra tabbed item called Deskbar. If you click on it a new
dialogue window opens but all of the options are greyed out. It's almost
certainly an unfinished or deleted feature but what it was for or supposed to do
is one of life's little mysteries…
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