The reduction of the number of bits used to define an image. Many techniques are available. For example, run-length encoding allows a set of pixels having the same colour to be specified by the colour and number of pixels with that colour in a sequence. Other techniques include fractal image compression and wavelet image compression. Compression can produce an approximation of the image, in which case it is not possible to decompress the image and retrieve the original form. See also discrete cosine transform, lossless compression, lossy compression.