Posterization

Source: Wikipedia, the free encyclopedia.
For the basketball slang term about dunking over an opponent, see Posterized.
256 colors
). Posterization occurs across the image, but is most obvious in areas of subtle variation in tone.
Posterized photo of a hibiscus
Posterized photo

Posterization or posterisation of an image is the conversion of a continuous gradation of tone to several regions of fewer tones, causing abrupt changes from one tone to another.[1] 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. Posterization is often the first step in vectorization (tracing) of an image.

Cause

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

fixed pixel displays, such as LCD and plasma televisions, this effect is referred to as false contouring.[2] 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
.

Photographic process

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

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 increase, 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.

Posterizing time

Temporal posterization is the

animated GIF
often looks posterized because of its normally-low frame rate.

More formally, this is

downsampling
in the time dimension, as it is reducing the resolution (precision of the input), not the bit rate (precision of the output, as in posterization).

The resulting stop-go motion is a temporal form of jaggies; formally, a form of aliasing. This effect may be the intention, but to reduce the frame rate without introducing this effect, one may use temporal anti-aliasing, which yields motion blur.

Compare with time stretching, which adds frames.

See also

References

  1. doi:10.1111/cgf.14343
    . Retrieved 19 April 2024.
  2. ^ "HDTV World Glossary". CNET Networks. 2007. Retrieved 2007-06-06.
  • Langford, Michael. The Darkroom Handbook. New York: Dorling Kindersley Limited, 1981. 245-249.
  • Jasc Software. Paint Shop Pro Help, 1998.

External links

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