Quantization (image processing): Difference between revisions

Revision as of 07:08, 12 November 2022

Revision as of 18:46, 12 January 2023

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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. -->