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 color to be specified by the color and number of pixels with that color 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 lossy compression, lossless compression, discrete cosine transform.