Colour depth, sometimes called bit depth, refers to the maximum number of colours which can be stored in an im- .age file. A 1-bit file stores two colours (usually black and white) and can be described as 1-bit deep since all the information required to specify each of the dots making up the image can be stored in a 1-bit number (0 for black or 1 for white). A 2-bit file stores four colours, a 4-bit file stores 16 colours, an 8-bit file stores 256 colours and a 24-bit file stores 16 million colours.
A greyscale image is an 8-bit file, with 254 shades of grey plus black and white. The greater the colour depth of an image, the more space it takes up on disk. A number of applications now use 32 bits to specify the colour of each pixel in an image. The extra 8 bits are used to describe the transparency of the pixel in 256 steps from completely transparent to completely opaque.
A colour mode determines the colour model (see below) used to display and print compositions. The most commonly used modes are Greyscale, for displaying black-andwhite documents, RGB, for displaying colour documents on the screen and for printing slides, transparencies, and RGB colour prints, CMYK, for printing four-colour separations and L*a”b for working with Photo CD images.
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Other modes are Bitmap, and Indexed colour. Colour mode is specified when a new painting or photo editing process is started, but can be altered midway through the task or when saving or exporting the finished work.
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If the original image has many colours, and it is converted to a lower colour depth (e.g. 24-bit RGB colour to 256 colours), the file will create a palette of colours and use combinations of these to simulate the original colour of each pixel. The colours in the palette will be derived from the colours in the original image.
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