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What
kinds of images exist on the net?
- Text
as image
- Icons
and Thumbnails
- Animated
image
- Photos
- Diagams/Simple
illustrations
- Image
maps
Remember the GIF and JPEG rule?
Photographic
images and images with a lot of colours should be saved as
JPEGs. While simpler images such as icons, diagrams, drawings
and buttons can be saved as GIFs. Sometimes complex images
can be saved as GIF and still look fine - however, have a
read of this site: Web
Design Group.
The
sizes of Images.
There
are three ways to identify the size of an image:
The print size width and height in cm/inches.
The width and height in pixels which indicates tha mount
of screen space it occupies.
Kilobytes/Megabytes measure the amount of memory (or bit
information) the file uses.
The
trick to keep the screen size at the size you want for your
web page but to reduce the file size to the minimum.
Ideally
Icons (small images, buttons, text graphics) should be no
bigger than 5k. Bigger images, photos etc, should try and
stay under 50K. The smaller the file size, the faster it wil
download.
The
key to making an image smaller is understanding the relationship
between the dimensions of the images and the resolution. Resolution
is usually defined as dots per inch - or dpi.
When
you have images to print you need a high resolution, depending
on the the quality and the type of image) but if you are publishing
images to the screen - you don't need a resolution over 72
dpi. All screen run at a base resolution of 72 dpi. Anything
higher than that is a waste of memory and would be lost on
most screens.
Despite
the proliferation of huge high resolution screens on the market.
Some people still prefer to set up their screen settings at
the base dimensions 640 pixels by 480 pixels. If you make
sure every web page sits within these dimensions you will
limit the amount of overflow on some screens and can better
control the layout of your page.
Resolution
Resolution
is the main control you have over graphic file sizes. It has
two componants: spatial resolution (the size and sampling
frequency) and the depth resolution (numbers of bits per pixel
which governs the number of colours).
When
scanning images the old rule of thumb was to scan at a very
high resolution and adjust it down, however, with increasingly
better printers available, scanning beyond 300 dpi is really
only applicable for those developing printed publications
on high gloss paper - or other larger print jobs such as posters.
Resolution
grows geometrically - when you double the resolution of an
image - you quadruple the size of your scanned file. For examplein
very basic terms, if you have a picture that is 6 x 4 inches
to scan. If you set the resolution at 72dpi you will have
a file that is (6x4)(72x72)x3 (the 3 is one byte for eachof
the red green and blue - or 24 bits) = 372,248 bytes in total.That's
about 390Kb.
Now
let's boost the resolution to a moderate 300dpi (6x4)(300x300)x3
= 6,480,000 bytes. That's just over 5 Mb. Visit this web site
for byte
converstions. For more information see this site about
colours
and bit depth.
Next
page: Sources of Images
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