Colourmap

At a time when only black and white monitors were available, each pixel on the screen was controlled by a single bit. Thus, the two possible values, 0 or 1, signified black or white for that pixel. In order to control each pixel on a grey-scale or colour monitor, several bits are required per pixel. The value of these bits is used as an index to a table of intensities, called the colourmap. Each entry in the colourmap is known as a colourcell. The greater the number of bits per pixel, the larger the colourmap resulting in more different colours available for displaying simultaneously. For colour monitors, each colourcell has a separate intensity value for each primary colour, red, green and blue, (RGB). Bitmapped monitors have at least one hardware colourmap, from which clients can allocate and share colourcells. Furthermore, windows in X can possess virtual colourmaps which can be swapped in and out of the hardware colourmap, as required by the application.