Posterization
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Posterization 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.
As an artistic effect, posterization can be deliberately created by most photo-editing programs.
Unwanted posterization, also known as banding may occur when the 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 which appear as bands of color. This can be compounded by an optical illusion that each band appears to have an intensity gradient in the direction opposing the overall gradient. This can be addressed in part with dithering.
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[edit] Definition
Posterization is a process in photograph developing 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 replaced by abrupt changes in shading and gradation from one area of tone to another. Printing posterization from black and white requires density separations, you then print them on the same piece of paper to produce types of images on the facing page. Separations can be made by density or color through different exposures. Density Separations can be achieved by printing three prints of the same picture at different exposure times.
[edit] Applications
Posterization is usually used for tracing contour lines and vectorizing photo-realistic images. This tracing process starts with 1 bit per channel, and advances to 4 bit per channel. As the bits per channel increase, the number of levels of lightness a color can display increase.
A visual artist faced with line art that has been damaged through JPEG compression may consider posterizing the image as a first step in removing artifacts on the image's edges.
[edit] See also
- Quantization error
- Descretization error
[edit] External links and sources
- Source:' Langford, Michael. The Darkroom Handbook. New York: Dorling Kindersley Limited, 1981. 245-249.
- Source:' Jasc Software. Paint Shop Pro Help 1998.
