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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 or posterisation 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] 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[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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1258 news items

PC Magazine

PC Magazine
Wed, 27 May 2015 08:44:54 -0700

Colors are a little dull, as expected for a low contrast ratio, but I didn't see any motion artifacts, posterization (colors changing suddenly where they should change gradually), or other problems. It also helps a lot that, as an LCD projector, the ...

PC Magazine

PC Magazine
Wed, 27 May 2015 13:15:00 -0700

In a few scenes, I saw posterization, the tendency for sudden shifts in color where they should be gradual, as well as a slight hatching indicative of pixelation, which is not unusual in an XGA projector. Sound from the IN2124a's single 2-watt speaker ...

PC Magazine

PC Magazine
Wed, 27 May 2015 12:07:30 -0700

I saw the slightest hint of posterization (shading changing suddenly where it should change gradually), but only in test clips that are most likely to show the problem. One pleasant surprise was the lack of rainbow artifacts in most of our video test ...

FOXSports.com

FOXSports.com
Tue, 14 Apr 2015 08:56:15 -0700

There were some pretty good candidates for dunk of the year this season, but Andrew Wiggins decided to blow all of them out of the water (including his dunk on Rudy Gobert) with this posterization of Omer Asik. true ...

Hardwood Houdini

Hardwood Houdini
Mon, 18 May 2015 20:50:37 -0700

But yet again, Chandler is faced with selecting either frontal posterization or posterization from the back, another lose-lose. Hopefully it makes sense now why his rim protection numbers have been lower than you'd think; and because he was put in ...

Tech Times

Tech Times
Fri, 15 May 2015 05:22:30 -0700

Whether it was wringing out his hat or screaming insanely, Ballmer completely lost it after Blake Griffin's posterization of San Antonio Spurs' Aron Baynes. The play was unbelievable, but Ballmer's reaction was priceless. Ballmer Gets Put Through The ...

CBSSports.com

CBSSports.com
Sun, 03 May 2015 10:57:42 -0700

An error occurred. Unable to execute Javascript. While it was no posterization, Scott threw down another powerful dunk against the Wizards in Game 1 of the second round. This time his victim was Drew Gooden.
 
NBCSports.com
Fri, 07 Nov 2014 19:00:33 -0800

Purdue steamrolled NAIA opponent Carroll College (Montana), 90-49, on Friday night in the final exhibition game for the Boilermakers. The highlight of the evening was Jon Octeus' driving by one opponent with an in-and-out move before throwing down a ...
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