File Formats
File Formats
In this lesson, we will be looking at:
• The two main families of computer graphics –
vector and bitmap
• Different types of bitmap files, and which is
best to use
• Other types of file formats – e.g. sound, text
and data
• Why it’s important to choose the right format
for your file
Bitmap Graphics
When you save bitmap graphics,
the information that makes up
the picture is saved as a grid.
This is like drawing a picture by
shading in the squares on a piece
of graph paper (only the squares are so small
that often you can't see them).
The quality of the picture is affected by the size
of the squares, and how many there are.
Disadvantages
• If you increase the size
a picture, it is still made
up of the same number of
squares, but the squares get bigger.
• It is difficult to edit a picture once you've
entered it – you can’t layer objects.
• Information is saved for every square
(pixel), even if you haven't drawn anything
in it. This means that the files you create
are always big.
Uses of Bitmaps
Bitmaps images are created by:
• Paint/Paintbrush
• PhotoEditor
• PhotoShop
• Paint Shop Pro
And are used in:
• Web pages
• CD-ROMs, etc.
Vector Graphics
When you create vector graphics, the file
stores the information needed to recreate the
shapes, for example the thickness or angle of
the lines, or the radius of a circle.
This means that the quality of your drawing is
not affected by its size. Also, each “object”
you draw is stored separately, so that you can
move them around, or take one from the back
and bring it to the front.
Advantages
• The quality stays the
same when you increase
the size.
• Only information about the “objects” you draw is
stored in the file – this means that the files tend to
be much smaller.
• Each “object” you draw is
stored separately, which
means that the picture is
easier to edit, and foreground and background
parts can be swapped around.
Uses
Vector graphics are used in:
• ClipArt and Office Drawing tools
• CorelDraw
• The Ilinc Whiteboard / Powerboard
The disadvantage is that there are some
things you can’t do, such as
• flood filling
• spraying
Bitmap Files
Various common picture formats use bitmaps:
• (Windows) bitmap - .bmp
• Graphics Interchange Format - .GIF
• Joint Photographic Expert Group - .jpg
Each supports a different range of features:
• Colour depth
• Compression
• Transparency
Colour Depth
• Different formats support different numbers
of colours.
• GIFs can only have 256 different colours in
them, which makes them more suitable for
simple pictures and diagrams
• Jpegs can contain up to 16 million
different colours, which makes them more
suitable for photographs and other images
with shading.
Transparency
Some formats (e.g. GIF) allow you to make
one colour or area of the image transparent,
so that you can see the background through
it.
Compression
Different formats use different compression
techniques to make the file smaller:
GIF (3k) "high quality" .jpeg (7k) "low quality" .jpeg (2k)
Sound Files
There are two families of sound files that are
analogous to bitmap and vector graphics:
• Wave files – such as .wav, .wma and mp3.
These are recordings of the actual sounds
- .wavs are uncompressed, and .wma and
MP3 use lossy compression
• MIDI files – like vector graphics, these contain
instructions on which notes to play - a bit like
the paper rolls in a pianola, or the
discs/cylinders in a music box – and your
soundcard makes the sounds
Other File Formats
Text can be saved in the following portable
formats:
• plain /ASCII text - .txt
• Rich Text Format - .rtf
• the new Open Document format - .odt
The most common portable format for data
(which can be opened in Excel or Access) is:
• Comma-separated variables - .csv
Why Does It Matter?
• If you are sending the files to other people (or
putting them on a web-site) it is a good idea to
use a portable format so that everyone can
open them
• If you are using the internet, or another
network, to transfer the files, then it’s a good
idea to make your files smaller so that they are
quicker to download.
• You can make files smaller by choosing a
more efficient format or by using compression.
