Posterization

Posterization (pronounced Poe-ster-ize-ation) of an image occurs when a region of an image with a continuous gradation of tone is replaced with several regions of fewer tones, resulting in an abrupt change from one tone to another. This creates an effect somewhat similar to that of a simple graphic poster. The effect may be created deliberately, or happen accidentally.

As an artistic effect, posterization may be created deliberately using most photo-editing programs.

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. [Cite web |url=http://www.cnet.com/4520-7874_1-5107912-7.html#falsecontouring |title=HDTV World Glossary |accessdate=2007-06-06 |year=2007 |publisher=CNET Networks] The result may be compounded further by an optical 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.

Definition

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.

Applications

Typically, posterization is used for tracing contour lines and vectorizing photo-realistic images. This tracing process starts with 1 bit per channel and advances to 4 bits per channel. As the bits per channel increases, the number of levels of lightness a color can display increases.

A visual artist, faced with line art that has been damaged through JPEG compression, may consider posterizing the image as a first step to remove artifacts on the edges of the image.

ee also

* Quantization error
* Discretization error

References

External links and sources

* http://desktoppub.about.com/cs/graphicstips/f/posterization.htm
* http://www.sphoto.com/techinfo/wdtech.html

* "Source:" Langford, Michael. "The Darkroom Handbook". New York: Dorling Kindersley Limited, 1981. 245-249.
* "Source:" Jasc Software. "Paint Shop Pro" Help, 1998.