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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.
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30 news items

 
Yahoo Sports (blog)
Sun, 20 Apr 2014 15:39:55 -0700

... series-opening game against the defending champion Miami Heat, forward Josh McRoberts slipped past LeBron James, took a pass from teammate Luke Ridnour, and unloaded on Heat big man Chris "Birdman" Andersen for the emphatic posterization.
 
PC Magazine
Wed, 16 Apr 2014 08:33:45 -0700

It did a good job with color quality in general and skin tones in particular, and I didn't see any motion artifacts or posterization (shading changing suddenly where it should change gradually), even in clips that tend to cause that problem. It also ...

New York Times

RedOrbit
Wed, 09 Apr 2014 03:25:28 -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 ...
 
SI.com
Wed, 09 Apr 2014 07:52:36 -0700

Although the final posterization of Chris Bosh actually wasn't that violent, the total degree of difficulty on the play was quite high. Battling the clock just before halftime, Beverley had to lose LeBron James with a strong crossover dribble on the ...

Dime Magazine

Dime Magazine
Wed, 09 Apr 2014 13:47:46 -0700

You'll find the usual suspects peppered throughout, but don't expect an incredible posterization of a hapless defender to take the top spot. This year–on top of a bevy of dunk faces–also featured a few dunk contest-worthy slams in the open court that ...
 
Boston Globe
Tue, 08 Apr 2014 00:47:37 -0700

It was 48-47 after a pair of free throws from James Young (team-high 20 points), who had a delivered a vicious posterization of UConn's Amida Brimah just 2 minutes and 27 seconds earlier, after UConn had gone up 9. The freshmen were climbing the hill ...

HDTV Test

HDTV Test
Wed, 16 Apr 2014 03:10:05 -0700

... high-def resolution. All things considered, Netflix had done a good job, keeping compression artefacts to a minimum. We saw the odd posterization here and there, but they were few and far between, and never detracted from the sumptuous presentation.
 
NESN.com
Mon, 31 Mar 2014 05:02:37 -0700

Noah to Butler to Gibson equaled a posterization for Gibson on Hump. Fourth quarter, 9:30, Bulls 80-70: Maybe the Celtics are getting their fourth-quarter swoon out of the way early. Typically the Celtics have waited until the last couple of minutes to ...
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