Core_Computer Studies : What is Dithering?

Dithering is a process used in output devices (monitor screens and some printers) to produce shades of gray or tints of colours.

The dithering process places dots on the paper or screen at calculated locations in a pseudo-random fashion. The algorithm that is used depends on the software you use. The idea is visually produce an effect on the image that is as close to the original image as possible but using less colours.

Dithering is a trick of the eye. When a computer monitor encounters a colour that it can't display natively, it attempts to match it as best it can. Monitors display elements as pixels on the screen, and native colours are displayed with all pixels as the same colour.

When an application dithers an image, it converts colours that it can not display natively into patterns of two or more colours that closely resemble the original. Sometimes the match will be close to what you intended and other times, it can be very wrong. In either case, it won't be the same as it looks on your screen.

Below are some more examples:

Diffusion dither

Noise dither

No dither

Most computers sold these days are sold with monitors that can support millions of colours. However, on a lot of these systems the default setting is at 640x480 pixels on the screen and 256 colours. Many people are not aware that they can change that setting, or how to do it.

For images which are to be displayed on web sites we have to also use Gif file formats.

These notes are written for students who undertake the Core Computer course at School of Art, Australian National University. Students of the course should feel free to contact any member of staff with comments questions and general suggestions..

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