There are two ways to save information about pictures:
- Bit maps , which use a patterns of pixels.
- Object lists, which specify the picture like a diagram.
Pixel bit maps are the most convenient way for digital computers and communications systems to describe artistic pictures or photographs. The picture
is divided up into a pattern os small rectangles, the colour of each rectangle, picture element, can then be described by a number which
specifies how much red, green and blue you need to make that colour. A computer can then store a series of numbers which tell it how to 'paint' the
picture on its screen. These numbers can then be saved on discs, transmitted from one computer to another, or manipulated by a suitable computer
program.
In some older British books you may see the term 'pels' rather than 'pixels' to mean 'picture elements'. This was originally the official abreviation, but has been swamped by the word 'pixel'. Despite that, some TV engineers still use 'pel' when discussing TV broadcasting systems rather than computers.
For diagrams, we can store or transmit a series of (usually binary) numbers which describe a series of 'building block' objects like lines, rectangles,
etc. along with some numbers that record their positions, sizes, shapes and colours. Files which store graphical data in this way are often called 'vector' files. The term 'object' files, although have a more obvious meaning, is avoided as the word 'object' has come to have some special, quite different, meanings to computer scientists.
It does not matter to a computer whether the picture information is a series of pixel colours or a series of object descriptions. In either case the information
can be recorded as a pattern of high and low voltages (binary '1's and '0's).
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