Posterization
Encyclopedia
Poster
A poster is any piece of printed paper designed to be attached to a wall or vertical surface. Typically posters include both textual and graphic elements, although a poster may be either wholly graphical or wholly text. Posters are designed to be both eye-catching and informative. Posters may be...
s. It can now be done photographically or with digital image processing, and may be deliberate or may be an unintended artifact of color quantization
Color quantization
In computer graphics, color quantization or color image quantization 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 visually similar as possible to the original image. Computer algorithms to perform color...
.
Cause
The effect may be created deliberately, or happen accidentally.As an artistic effect, posterization may be created deliberately using most photo-editing programs or using photographic processes.
Unwanted posterization, also known as banding, may occur when the color depth
Color depth
In computer graphics, color depth or bit depth is the number of bits used to represent the color of a single pixel in a bitmapped image or video frame buffer. This concept is also known as bits per pixel , particularly when specified along with the number of bits used...
, 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 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...
s, such as LCD and plasma televisions, this effect is referred to as false contouring. The result may be compounded further by an optical illusion
Optical illusion
An optical illusion is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source...
, called the Mach band illusion
Mach bands
Mach bands is an optical illusion named after the physicist Ernst Mach. The illusion consists of light or dark stripes that are perceived next to the boundary between two regions of an image that have different lightness gradients .-Explanation:The Mach bands effect is due to the spatial...
, 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 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 and vectorizing photo-realistic imagesRaster to vector
In computer graphics, vectorization refers to the process of converting raster graphics into vector graphics.- Popular applications :*In computer-aided design drawings are scanned, vectorized and written as CAD files in a process called paper-to-CAD conversion or drawing conversion.*In geographic...
. 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
Line art
Line art is any image that consists of distinct straight and curved lines placed against a background, without gradations in shade or hue to represent two-dimensional or three-dimensional objects...
that has been damaged through JPEG
JPEG
In computing, JPEG . The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and image quality. JPEG typically achieves 10:1 compression with little perceptible loss in image quality....
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 of reducing the number of framesFrame rate
Frame rate is the frequency at which an imaging device produces unique consecutive images called frames. The term applies equally well to computer graphics, video cameras, film cameras, and motion capture systems...
of video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
, 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
Telecine
Telecine is transferring motion picture film into video and is performed in a color suite. The term is also used to refer to the equipment used in the post-production process....
, the unused frames are simply discarded, and it is intended to be apparent (longer than the persistence of vision
Persistence of vision
Persistence of vision is the phenomenon of the eye by which an afterimage is thought to persist for approximately one twenty-fifth of a second on the retina....
that video
Video
Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
and motion pictures normally depend on). An animated GIF often looks posterized because of its normally-low frame rate.
More formally, this is downsampling
Downsampling
In signal processing, downsampling is the process of reducing the sampling rate of a signal. This is usually done to reduce the data rate or the size of the data....
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
"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 and/or missing or poor anti-aliasing filtering prior to sampling....
; formally, a form of aliasing
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
. 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
Temporal anti-aliasing seeks to reduce or remove the effects of temporal aliasing. Temporal aliasing is caused by the sampling rate of a scene being too low compared to the transformation speed of objects inside of the scene; this causes objects to appear to jump or appear at a location instead of...
, which yields motion blur
Motion blur
Motion blur is the apparent streaking of rapidly moving objects in a still image or a sequence of images such as a movie or animation. It results when the image being recorded changes during the recording of a single frame, either due to rapid movement or long exposure.- Photography :When a camera...
.
Compare with time stretching, which adds frames.
See also
- DownsamplingDownsamplingIn signal processing, downsampling is the process of reducing the sampling rate of a signal. This is usually done to reduce the data rate or the size of the data....
- Quantization errorQuantization errorIn analog-to-digital conversion, the difference between the actual analog value and quantized digital value is called quantization error or quantization distortion. This error is either due to rounding or truncation...
- Discretization errorDiscretization errorIn numerical analysis, computational physics, and simulation, discretization error is error resulting from the fact that a function of a continuous variable is represented in the computer by a finite number of evaluations, for example, on a lattice...
- Color quantizationColor quantizationIn computer graphics, color quantization or color image quantization 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 visually similar as possible to the original image. Computer algorithms to perform color...
External links and sources
- http://desktoppub.about.com/cs/graphicstips/f/posterization.htm
- http://www.sphoto.com/techinfo/wdtech.html
- Source: Langford, Michael. The Darkroom Handbook. New York: Dorling Kindersley Limited, 1981. 245-249.
- Source: Jasc Software. Paint Shop Pro Help, 1998.