A deblocking filter is applied to blocks in decoded video to improve visual quality and prediction performance by smoothing the sharp edges which can form between blocks when block coding techniques are used. The filter aims to improve the appearance of decoded pictures. 2-D DCT compared to the DFT The discrete cosine transform (DCT) is a Fourier-related transform similar to the discrete Fourier transform (DFT), but using only real numbers. ...

H.264 deblocking filter

In contrast with older MPEG-1/2/4 standards, the H.264 deblocking filter is not an optional additional feature in the decoder. It is a feature on both the decoding path and on the encoding path, so that the effects of the filter are taken into account in reference macroblocks used for prediction. When a stream is encoded, the filter strength can be selected, or the filter can be switched off entirely. Otherwise, the filter strength is determined by coding modes of adjacent blocks, quantization step size, and the steepness of the luminance gradient between blocks. H.264 is a high compression digital video codec standard written by the ITU-T Video Coding Experts Group (VCEG) together with the ISO/IEC Moving Picture Experts Group (MPEG) as the product of a collective partnership effort known as the Joint Video Team (JVT). ...

The filter operates on the edges of each or transform block in the luma and chroma planes of each picture. Each small block's edge is assigned a boundary strength based on whether it is also a macroblock boundary, the coding (intra/inter) of the blocks, whether references (in motion prediction and reference frame choice) differ, and whether it is a luma or chroma edge. Stronger levels of filtering are assigned by this scheme where there is likely to be more distortion. The filter can modify as many as three samples on either side of a given block edge (in the case where an edge is a luma edge that lies between different macroblocks and at least one of them is intra coded). In most cases it can modify one or two samples on either side of the edge (depending on the quantization step size, the tuning of the filter strength by the endcoder, the result of an edge detection test, and other factors).

H.263 Annex I deblocking filter

Although the concept of an "in loop" deblocking filter is best known in relation to H.264, it was actually also standardized earlier (in a somewhat different form) in the H.263v2 Annex J. H.263. ...

Deblocking filters as post-processors

Prior to H.263 Annex J and H.264, the use of a deblocking filter as a post-processing technique to improve the visual quality of decoded pictures was already a well-known technology. This was particularly true in the video conferencing industry, where the low bit rates used there tended to produce significant blocking artifacts that could be substantially reduced by such a filter.

External links

• H.264 : Advanced video coding for generic audiovisual services - subclause 8.7 "Deblocking filter process"