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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 . It can now be done photographically or with digital image processing, and may be deliberate or an unintended artifact of color quantization. Posterization is often the first step in Image tracing (tracing) of an image.
Unwanted posterization, also known as color 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. 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 bands, 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.
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.
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.
Studies show that using posterization can significantly improve the ability to locate landmarks or subjects in outdoor photography.
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.
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