Posterization

Posterization or posterisation of an image entails conversion of a continuous gradation of tone to several regions of fewer tones, with abrupt changes from one tone to another. This was originally done with photographic processes to create posters. It can now be done photographically or with digital image processing, and may be deliberate or an unintended artifact of color quantization.

The effect may be created deliberately, or happen accidentally. For artistic effect, most image editing programs provide a posterization feature, or photographic processes may be used.

Unwanted posterization, also known as banding, may occur when the color depth, sometimes called bit depth, is insufficient to accurately sample a continuous gradation of color tone. As a result, a continuous gradient appears as a series of discrete steps or bands of color — hence the name. When discussing fixed pixel displays, such as LCD and plasma televisions, this effect is referred to as false contouring. Additionally, compression in image formats such as JPEG can also result in posterization when a smooth gradient of colour or luminosity is compressed into discrete quantized blocks with stepped gradients. The result may be compounded further by an optical illusion, called the Mach band illusion, in which each band appears to have an intensity gradient in the direction opposing the overall gradient. This problem may be resolved, in part, with dithering.

Posterization is a process in photograph development which converts normal photographs into an image consisting of distinct, but flat, areas of different tones or colors. A posterized image often has the same general appearance, but portions of the original image that presented gradual transitions are replaced by abrupt changes in shading and gradation from one area of tone to another. Printing posterization from black and white requires density separations, which one then prints on the same piece of paper to create the whole image. Separations may be made by density or color, using different exposures. Density Separations may be created by printing three prints of the same picture, each at a different exposure time that will be combined for the final image.

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