Quantization (image processing): Difference between revisions

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{{Short description|Lossy compression technique}}

{{RefimproveMore citations needed|date=November 2012}}

'''Quantization''', involved in [[image processing]], is a [[lossy compression]] technique achieved by compressing a range of values to a single quantum (discrete) 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]].

== Color quantization ==

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

== Grayscale quantization ==

Grayscale quantization, also known as gray level quantization, is a process in digital image processing that involves reducing the number of unique intensity levels (shades of gray) in an image while preserving its essential visual information. This technique is commonly used for simplifying images, reducing storage requirements, and facilitating processing operations. In grayscale quantization, an image with ''N'' intensity levels is converted into an image with a reduced number of levels, typically ''L'' levels, where ''L''<''N''. The process involves mapping each pixel's original intensity value to one of the new intensity levels. One of the simplest methods of grayscale quantization is uniform quantization, where the intensity range is divided into equal intervals, and each interval is represented by a single intensity value. Let's say we have an image with intensity levels ranging from 0 to 255 (8-bit grayscale). If we want to quantize it to 4 levels, the intervals would be [0-63], [64-127], [128-191], and [192-255]. Each interval would be represented by the midpoint intensity value, resulting in intensity levels of 31, 95, 159, and 223 respectively.

The formula for uniform quantization is:

<math>Q(x) = \left \lfloor \frac{x}{\Delta} \right \rfloor \times \Delta + \frac{\Delta}{2} </math>

Where:

* ''Q''(''x'') is the quantized intensity value.

* ''x'' is the original intensity value.

* Δ is the size of each quantization interval.

Let's quantize an original intensity value of 147 to 3 intensity levels.

Original intensity value: ''x''=147

Desired intensity levels: ''L''=3

We first need to calculate the size of each quantization interval:

<math>\Delta = \frac{255}{L-1} = \frac{255}{3-1} = 127.5</math>

Using the uniform quantization formula:

<math>Q(x) = \left \lfloor \frac{147}{127.5} \right \rfloor \times 127.5 + \frac{127.5}{2}</math>

<math>Q(x) = \left \lfloor 1.15294118 \right \rfloor \times 127.5 + \frac{127.5}{2}</math>

<math>Q(x) = 1 \times 127.5 + 63.75 = 191.25</math>

Rounding 191.25 to the nearest integer, we get <math>Q(x) = 191</math>

So, the quantized intensity value of 147 to 3 levels is 191.

== Frequency quantization for image compression ==

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'', https://web.archive.org/web/20111115004238/http://www.john-wiseman.com/technical/MPEG_tutorial.htm</ref>

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=== Quantization matrices ===

A typical video codec works by breaking the picture into discrete blocks (8×8 pixels in the case of MPEG<ref name="wiseman"/>). These blocks can then be subjected to [[discrete cosine transform]] (DCT) to calculate the frequency components, both horizontally and vertically.<ref name="wiseman"/> The resulting block (the same size as the original block) is then pre-multiplied by the quantisationquantization scale code and divided element-wise by the quantization matrix, and rounding each resultant element. The quantization matrix is designed to provide more resolution to more perceivable frequency components over less perceivable components (usually lower frequencies over high frequencies) in addition to transforming as many components to 0, which can be encoded with greatest efficiency. Many video encoders (such as [[DivX]], [[Xvid]], and [[3ivx]]) and compression standards (such as [[MPEG-2]] and [[H.264/AVC]]) allow custom matrices to be used. The extent of the reduction may be varied by changing the quantizer scale code, taking up much less bandwidth than a full quantizer matrix.<ref name="wiseman"/>

This is an example of DCT coefficient matrix: <!--NOTE: this matrix was generated using random numbers and the other two matricies. It may not actually work well with an iDCT. -->

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* [[Image segmentation]]

* [[Image-based meshing]]

* [[Range segmentation]]

==References==

{{reflist}}<ref>{{Cite book |last=Smith |first=Steven W. |title=Digital signal processing: a practical guide for engineers and scientists |date=2003 |publisher=Newnes |isbn=978-0-7506-7444-7 |series=Demystifying technology series |___location=Amsterdam Boston}}</ref>{{Compression Methods}}

{{reflist}}

{{Compression Methods}}

{{DEFAULTSORT:Quantization (Image Processing)}}

[[Category:Lossy compression algorithms]]

[[Category:Image compression]]

[[Category:Data compression]]