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 ...

. 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 Image editing encompasses the processes of altering images, whether they are Digital photography, digital photographs, traditional Photographic processing, photo-chemical photographs, or illustrations. Traditional analog image editing is known ...

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, 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 a pixel, the concept can be defined as bit ...

, 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. Additionally, compression in image formats such as

JPEG JPEG ( , short for Joint Photographic Experts Group and sometimes retroactively referred to as JPEG 1) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degr ...

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 is then printed 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. Modern uses of posterization also include thresholding or color segmentation that transform the image into a simplified version with fewer, highly contrasting colors.

Applications

Typically, posterization is used for tracing

contour line A contour line (also isoline, isopleth, isoquant or isarithm) of a Function of several real variables, 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 ...

s 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 that has been damaged through

compression, may consider posterizing the image as a first step to remove artifacts on the edges of the image. Studies show that using posterization can significantly improve the ability to locate landmarks or subjects in outdoor photography.

Video posterization

Temporal posterization is the visual effect of reducing the number of frames of

video Video is an Electronics, electronic medium for the recording, copying, playback, broadcasting, and display of moving picture, moving image, visual Media (communication), media. Video was first developed for mechanical television systems, whi ...

, 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 is the optical illusion that occurs when the visual perception of an object does not cease for some time after the Light ray, rays of light proceeding from it have ceased to enter the eye. The illusion has also been descr ...

that

and motion pictures normally depend on). An

animated GIF The Graphics Interchange Format (GIF; or , ) is a Raster graphics, bitmap Image file formats, image format that was developed by a team at the online services provider CompuServe led by American computer scientist Steve Wilhite and released ...

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 In signal processing and related disciplines, aliasing is a phenomenon that a reconstructed signal from samples of the original signal contains low frequency components that are not present in the original one. This is caused when, in the ori ...

. 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 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 l ...

. Compare with time stretching, which ''adds'' frames.

References

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

External links