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Colour Depth

Another attribute of bitmaps that can affect their quality and appearance is their colour depth.

Pixels can be assigned different lengths of binary values, for example, 8 binary digits (bits) could be assigned to each pixel. The number of bits assigned to a pixel is known as the colour depth, which is also known as the bit depth.

The bit depth determines the amount of values and hence colours in the colour palette that are available to the pixel. The higher the bit depth means more colours. This is why the term 'colour' depth is used. The amount of colours assigned to are as follows:

Typical working colour schemes
Bit Depth Number of Colours
1 bit 2 colours (usually black and white)
2 bits 4 colours
4 bits 16 colours
8 bit greyscale 256 shades of grey
8 bit colour 256 colours
16 bits 65, 536 colours (known as 'high' colour)
24 bits 16.7 million colours (known as 'true' colour)
32 bits 16.7million colours plus greyscale mask (alpha channel)

Table extracted from PC Multimedia & Web Handbook - technology & techniques (Dick 2002)

8, 16 and 24 bit are the most common colour depths. These are also known as 256 colours for 8 bit (28), thousands of colours for 16 bit (216) and millions of colours for 24 bit (224). So many bits are assigned to red, so many to green and so many to blue. In 24 bit graphics 8 bits are assigned to each colour component, therefore red, green and blue will each have a range of 256 colours (0-255).

"The 16-bit system is often described as 'high colour' while the 24 bit system is described as 'true colour'." (Dick 2002)

Graphics, eg buttons, icons, that are going to be used on the web commonly use 8 bit, whereas photo-realistic images use 16 or 24 bit. Graphics software applications allow the colour depth of a bitmap to be adjusted.

Next: Dithering