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. This was originally done with photographic processes to create

poster A poster is a large sheet that is placed either on a public space to promote something or on a wall as decoration. Typically, posters include both typography, textual and graphic elements, although a poster may be either wholly graphical or w ...

s. It can now be done photographically or with digital image processing, and may be deliberate or an unintended artifact of

color quantization In computer graphics, color quantization or color image quantization is quantization applied to color spaces; it is a process that reduces the number of distinct colors used in an image, usually with the intention that the new image should be as v ...

Cause

The effect may be created deliberately, or happen accidentally. For artistic effect, most

image editing Image editing encompasses the processes of altering images, whether they are digital photographs, traditional photo-chemical photographs, or illustrations. Traditional analog image editing is known as photo retouching, using tools such a ...

programs provide a posterization feature, or photographic processes may be used. Unwanted posterization, also known as banding, may occur when the

color depth Color depth or colour depth (see spelling differences), also known as bit depth, is either the number of bits used to indicate the color of a single pixel, or the number of bits used for each color component of a single pixel. When referring to ...

, 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 display Fixed pixel displays are display technologies such as LCD and plasma that use an unfluctuating matrix of pixels with a set number of pixels in each row and column. With such displays, adjusting ( scaling) to different aspect ratios because of di ...

s, such as LCD and plasma televisions, this effect is referred to as false contouring. Additionally,

compression Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression * Gas compression *Compression ratio, of a ...

in image formats such as

JPEG JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and imag ...

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 Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual perception, percept that arguably appears to differ from reality. Illusions come in a wide v ...

, 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 Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images. Dither is routinely used in processing of both digital audio and video data, and is often ...

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

contour lines A contour line (also isoline, isopleth, or isarithm) of a function of two variables is a curve along which the function has a constant value, so that the curve joins points of equal value. It is a plane section of the three-dimensional graph ...

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 increase, the number of levels of lightness a color can display increases. A visual artist, faced with

line art Line art or line drawing is any image that consists of distinct straight lines or curves placed against a background (usually plain), without gradations in shade (darkness) or hue (color) to represent two-dimensional or three-dimensional object ...

that has been damaged through

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

visual effect Visual effects (sometimes abbreviated VFX) is the process by which imagery is created or manipulated outside the context of a live-action shot in filmmaking and video production. The integration of live-action footage and other live-action foota ...

of reducing the number of frames of

video Video is an electronic medium for the recording, copying, playback, broadcasting, and display of moving visual media. Video was first developed for mechanical television systems, which were quickly replaced by cathode-ray tube (CRT) syste ...

, while not reducing the total time it takes the video to play. 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 Persistence of vision traditionally refers to the optical illusion that occurs when visual perception of an object does not cease for some time after the rays of light proceeding from it have ceased to enter the eye. The illusion has also been d ...

that

and

motion picture A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmosphere ...

s normally depend on). An

animated GIF The Graphics Interchange Format (GIF; or , see pronunciation) is a bitmap In computing, a bitmap is a mapping from some domain (for example, a range of integers) to bits. It is also called a bit array or bitmap index. As a noun, the ...

often looks posterized because of its normally-low frame rate. More formally, this is

downsampling In digital signal processing, downsampling, compression, and decimation are terms associated with the process of ''resampling'' in a multi-rate digital signal processing system. Both ''downsampling'' and ''decimation'' can be synonymous with ''comp ...

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 "Jaggies" is the informal name for artifacts in raster images, most frequently from aliasing, which in turn is often caused by non-linear mixing effects producing high-frequency components, or missing or poor anti-aliasing filtering prior to sampl ...

; formally, a form of

aliasing In signal processing and related disciplines, aliasing is an effect that causes different signals to become indistinguishable (or ''aliases'' of one another) when sampled. It also often refers to the distortion or artifact that results when a ...

. This effect may be the intention, but to reduce the frame rate without introducing this effect, one may use

temporal anti-aliasing Temporal anti-aliasing (TAA) is a spatial anti-aliasing technique for computer-generated video that combines information from past frames and the current frame to remove jaggies in the current frame. In TAA, each pixel is sampled once per frame but ...

, which yields

motion blur Motion blur is the apparent streaking of moving objects in a photograph or a sequence of frames, such as a film or animation. It results when the image being recorded changes during the recording of a single exposure, due to rapid movement or lo ...

. Compare with

time stretching Time stretching may refer to: * Audio time stretching and pitch scaling, in audio * Time stretching, in video * Time dilation, in physics (relativity theory) * Time stretch analog-to-digital converter The time-stretch analog-to-digital converter (T ...

, which ''adds'' frames.

References

* Langford, Michael. ''The Darkroom Handbook''. New York: Dorling Kindersley Limited, 1981. 245-249. *

Jasc Software Jasc Software, Inc. was an Eden Prairie, Minnesota-based company founded by Robert Voit, the creator of Paint Shop Pro, a popular graphics editing program. Jasc Software also created other popular software titles, including: *Image Robot *Animati ...

. ''Paint Shop Pro'' Help, 1998.

External links