Quantization (image processing): Difference between revisions

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'''Quantization''', involved in [[image processing]], is a [[lossy compression]] technique achieved by compressing a range of values to a single quantum value. When the number of discrete symbols in a given stream is reduced, the stream becomes more compressible. For example, reducing the number of colors required to represent a digital [[image]] makes it possible to reduce its file size. Specific applications include [[Discrete cosine transform|DCT]] data quantization in [[JPEG]] and [[Discrete wavelet transform|DWT]] data quantization in [[JPEG 2000]].

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== Color quantization ==

{{mainarticlemain|Color quantization}}

Color quantization reduces the number of colors used in an image; this is important for displaying images on devices that support a limited number of colors and for efficiently compressing certain kinds of images. Most bitmap editors and many operating systems have built-in support for color quantization. Popular modern color quantization algorithms include the nearest color algorithm (for fixed palettes), the [[Median cut|median cut algorithm]], and an algorithm based on [[octree]]s.

It is common to combine color quantization with [[dither|dithering]]ing to create an impression of a larger number of colors and eliminate [[colour banding|banding]] artifacts.

== Frequency quantization for image compression ==

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The human eye is fairly good at seeing small differences in [[brightness]] over a relatively large area, but not so good at distinguishing the exact strength of a high frequency (rapidly varying) brightness variation. This fact allows one to reduce the amount of information required by ignoring the high frequency components. This is done by simply dividing each component in the frequency ___domain by a constant for that component, and then rounding to the nearest integer. This is the main lossy operation in the whole process. As a result of this, it is typically the case that many of the higher frequency components are rounded to zero, and many of the rest become small positive or negative numbers.

As human vision is also more sensitive to [[luminance]] than [[chrominance]], further compression can be obtained by working in a non-RGB color space which separates the two (e.g. [[YCbCr]]), and quantizing the channels separately.<ref name="wiseman">John Wiseman, ''An Introduction to MPEG Video Compression'', http://www.john-wiseman.com/technical/MPEG_tutorial.htm, retrieved 6 Nov 2010.</ref>

=== Quantization matrices ===

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{{Compression Methods}}

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[[Category:Lossy compression algorithms]]