A deblocking filter is a video filter

Filter (video)

A video filter is a software component that is used to decode audio and video. Multiple filters can be used in a filter chain, in which each filter receives input from its previous-in-line filter upstream, processes the input and outputs the processed video to its next-in-line filter downstream...

 applied to blocks in decoded video to improve visual quality and prediction performance by smoothing the sharp edges which can form between macroblock

Macroblock

Macroblock is an image compression component and technique based on discrete cosine transform used on still images and video frames. Macroblocks are usually composed of two or more blocks of pixels. In the JPEG standard macroblocks are called MCU blocks....

s when block coding

Discrete cosine transform

A discrete cosine transform expresses a sequence of finitely many data points in terms of a sum of cosine functions oscillating at different frequencies. DCTs are important to numerous applications in science and engineering, from lossy compression of audio and images A discrete cosine transform...

 techniques are used. The filter aims to improve the appearance of decoded pictures.

H.264 deblocking filter

In contrast with older MPEG-1/2

H.262/MPEG-2 Part 2

H.262 or MPEG-2 Part 2 is a digital video compression and encoding standard developed and maintained jointly by ITU-T Video Coding Experts Group and ISO/IEC Moving Picture Experts Group . It is the second part of the ISO/IEC MPEG-2 standard...

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MPEG-4 Part 2

MPEG-4 Part 2, MPEG-4 Visual is a video compression technology developed by MPEG. It belongs to the MPEG-4 ISO/IEC standards. It is a discrete cosine transform compression standard, similar to previous standards such as MPEG-1 and MPEG-2...

 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 in-loop effects of the filter are taken into account in reference macroblock

Macroblock

Macroblock is an image compression component and technique based on discrete cosine transform used on still images and video frames. Macroblocks are usually composed of two or more blocks of pixels. In the JPEG standard macroblocks are called MCU blocks....

s 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.

The filter operates on the edges of each 4×4 or 8×8 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

Reference frame (video)

Reference frames are frames of a compressed video that are used to define future frames. As such, they are only used in inter-frame compression techniques. In older video encoding standards, such as MPEG-2, only one reference frame – the previous frame – was used for P-frames...

 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 encoder, the result of an edge detection test, and other factors).H.263

H.263 Annex J 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

H.263

H.263 is a video compression standard originally designed as a low-bitrate compressed format for videoconferencing. It was developed by the ITU-T Video Coding Experts Group in a project ending in 1995/1996 as one member of the H.26x family of video coding standards in the domain of the ITU-T.H.263...

Annex J.

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 tended to produce significant blocking artifacts that could be substantially reduced by such a filter.