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Example of a photograph in JPEG format (24-bit color or 16.7 million colors) before posterization, contrasting the result of saving to GIF format (256 colors). Posterization occurs across the image, but is most obvious in areas of subtle variation in tone.
Posterized photo of a hibiscus.

Posterization 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 may be an unintended artifact of color quantization.

Cause[edit]

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.[1] 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[edit]

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[edit]

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.

Posterizing time [edit]

Temporal posterization is the visual effect of reducing the number of frames of video, while not reducing the speed at which it actually plays. This compares to regular posterization, where the number of individual color variations is reduced, while the overall range of colors is not. The motion effect is similar to the effect of a flashing strobe light, but without the contrast of bright and dark. Unlike a pulldown, the unused frames are simply discarded, and it is intended to be apparent (longer than the persistence of vision that video and motion pictures normally depend on). An 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[edit]

References[edit]

  1. ^ "HDTV World Glossary". CNET Networks. 2007. Retrieved 2007-06-06. 

External links and sources[edit]

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

Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Posterization — Please support Wikipedia.
This page uses Creative Commons Licensed content from Wikipedia. A portion of the proceeds from advertising on Digplanet goes to supporting Wikipedia.
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91 news items

 
PC Magazine
Fri, 17 Oct 2014 11:00:00 -0700

I also observed posterization—sudden shifts in color where they should be gradual—in some scenes. Related Story See How We Test Projectors. Audio from the 1-watt speaker is feeble. To hear it, you'll want to either sit very near the projector or ...
 
PC Magazine
Wed, 15 Oct 2014 07:48:45 -0700

The projector handled shadow detail (details based on shading in dark areas) well, and I saw barely a hint of posterization (shading changing suddenly where it should change gradually) in clips that tend to cause that problem. Colors are nicely ...

Tip of the Tower

Tip of the Tower
Thu, 09 Oct 2014 03:18:45 -0700

It was never more apparent than his thunderous posterization of Serge Ibaka during the Western Conference finals last year, seen in the video link above. So if his numbers have been unspectacular and he has almost been a non-factor in the league for ...

Air Alamo

Air Alamo
Fri, 10 Oct 2014 06:07:30 -0700

David has put together some highlights for you ranging from Cory Joseph's posterization of Serge Ibaka to some beautiful highlights of the Spurs system being executed to perfection. Check them out here! Our next piece we have for you is also from ...
 
Gizmodo
Sun, 12 Oct 2014 11:17:46 -0700

The opening shot of Frank Underwood (Kevin Spacey) and his wife (Robin Wright) running in a park at night is a testing sequence with lots of gradients and camera noise, and we saw minor posterization during the fade in, and around the street lamps. To ...
 
Basketball Insiders
Sat, 27 Sep 2014 12:30:00 -0700

While still the eye-popping athlete and eternal 'posterization' threat, Griffin can now attack defenders in a variety of ways. He still isn't quite the defender his athleticism and agility might lead you to expect, but Griffin has also improved on that ...
 
BT Powerhouse (blog)
Thu, 02 Oct 2014 03:07:30 -0700

Everything from the time he sat on the bench giving the choke gesture to the Hoosiers as Purdue knocked them off to his infamous posterization of Jared Sullinger. Does it even surprise you that Barlow ultimately got kicked off the team after an ...

Bright Side of the Sun

Bright Side of the Sun
Thu, 25 Sep 2014 08:00:00 -0700

I look forward to his inevitable posterization of Dwight Howard. 5. Who makes an All-Star team first, Bled or Dragic? RJM: I lean towards Dragic, but only because I see him getting more minutes due to his compatibility in any lineup. Drags is big ...
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