Graphic Card Colour Depth

Each pixel of a screen image is displayed using a combination of three different colour signals: red, green and blue. The precise appearance of each pixel is controlled by the intensity of these three beams of light and the amount of information that is stored about a pixel determines its colour depth. The more bits that are used per pixel (bit depth), the finer the colour detail of the image.

The table below shows the colour depths in current use:

Colour depth Description No. of colours Bytes per pixel
4-bit Standard VGA 16 0.5
8-bit 256-colour mode 256 1.0
16-bit High colour 65,536 2.0
24-bit True colour 16,777,216 3.0

For a display to fool the eye into seeing full colour, 256 shades of red, green and blue are required; that is 8 bits for each of the three primary colours, hence 24 bits in total. However, some graphics cards actually require 32 bits for each pixel to display true colour, due to the way in which they use the video memory – the extra 8 bits generally being used for an alpha channel (transparencies).

High colour uses two bytes of information to store the intensity values for the three colours, using 5 bits for blue, 5 bits for red and 6 bits for green. The resulting 32 different intensities for blue and red and 64 different intensities for green results in a very slight loss of visible image quality, but with the advantages of a lower video memory requirement and faster performance.

The 256-colour mode uses a level of indirection by introducing the concept of a palette of colours, selectable from the entire range of 16.7 million colours. Each colour in the 256-colour palette is defined using the standard 3-byte colour definition used in true colour: 256 possible intensities for each of red, blue and green. Any given image can then use any colour from its associated palette.

Dithering substitutes combinations of colours that a graphics card is able to generate for colours that it cannot produce. For example, if a graphics subsystem is capable of handling 256 colours, and an image that uses 65,000 colours is displayed, colours that are not available will be substituted by colours created from combinations of colours that are available. The colour quality of a dithered image is inferior to a non-dithered image.

The palette approach was an excellent compromise solution, allowing for far greater precision in an image than would be possible by using the 8 bits available by, for example, assigning each pixel a 2-bit value for blue and 3-bit values each for green and red. Because of its relatively low demands on video memory the 256-colour mode was a widely used standard in PCs used primarily for business applications, before the advent of dedicated graphics processing chips.