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Dithering

When you reduce the colour to 8 bit, this is known as indexed colour. Each colour is assigned a number or 'index' in a CLUT (Colour Look-Up Table), a palette of up to 256 colours. The Gif file format uses a CLUT, which is one of the reasons that file sizes can be small. When you index colour a photo-realistic image from eg 24 bits, this can result in a loss of colour quality. Some of the colours will have been discarded and dithering will have been applied to try and replace these colours.

"A colour not available in the palette is displayed as a mix of pixels of other colours that are included in the palette. Consider the case of pink not being included in an image's colour palette. Any pink areas could be represented by a checkerboard of white and red pixels. When viewed from a distance, these appear as pink." www.wikipedia.com

The problem with dithering is that the graphic can end up with a 'grainy' look. The photos below demonstrate the effects of dithering.

Example of dithering

Images sourced from Dither. (2007, November 7). In Wikipedia, The Free Encyclopedia. Retrieved November, 9, 2007, from http://en.wikipedia.org/wiki/Dithering#Digital_photography_and_image_processing

As with resolution, the higher colour depth means a larger file size. When you have to use higher colour depths for photos that you want to put on the web, you will have to compress the file into a format such as JPEG. The resolution and colour depth that a computer can display depends of the specification of its graphics card. For further information about colour depth, please visit:

  1. http://en.wikipedia.org/wiki/Colour_depth
  2. http://webstyleguide.com/graphics/displays.html
  3. http://en.wikipedia.org/wiki/Dithering#Digital_photography_and_image_processing

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