The macroblock is a processing unit in

image An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensio ...

and

video compression In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compressio ...

formats based on linear block transforms, typically the discrete cosine transform (DCT). A macroblock typically consists of 16×16 samples, and is further subdivided into transform blocks, and may be further subdivided into prediction blocks. Formats which are based on macroblocks include

JPEG JPEG ( ) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degree of compression can be adjusted, allowing a selectable tradeoff between storage size and imag ...

, where they are called MCU blocks, H.261, MPEG-1 Part 2, H.262/MPEG-2 Part 2,

H.263 H.263 is a video compression standard originally designed as a low-bit-rate compressed format for videotelephony. It was standardized by the ITU-T Video Coding Experts Group (VCEG) in a project ending in 1995/1996. It is a member of the H.26x fam ...

MPEG-4 Part 2 MPEG-4 Part 2, MPEG-4 Visual (formally ISO/IEC 14496-2) is a video compression format developed by the Moving Picture Experts Group (MPEG). It belongs to the MPEG-4 ISO/IEC standards. It uses block-wise motion compensation and a discrete cosi ...

, and H.264/MPEG-4 AVC. In H.265/HEVC, the macroblock as a basic processing unit has been replaced by the

coding tree unit Coding tree unit (CTU) is the basic processing unit of the High Efficiency Video Coding (HEVC) video standard and conceptually corresponds in structure to macroblock units that were used in several previous video standards. CTU is also referred to a ...

Technical details

Transform blocks

A macroblock is divided into transform blocks, which serve as input to the linear block transform, e.g. the DCT. In H.261, the first video codec to use macroblocks, transform blocks have a fixed size of 8×8 samples. In the

YCbCr YCbCr, Y′CbCr, or Y Pb/Cb Pr/Cr, also written as YCBCR or Y′CBCR, is a family of color spaces used as a part of the color image pipeline in video and digital photography systems. Y′ is the luma component and CB and CR are the blue-diff ...

color space with 4:2:0 chroma subsampling, a 16×16 macroblock consists of 16×16 luma (Y) samples and 8×8 chroma (Cb and Cr) samples. These samples are split into four Y blocks, one Cb block and one Cr block. This design is also used in

and most other macroblock-based video codecs with a fixed transform block size, such as MPEG-1 Part 2 and H.262/MPEG-2 Part 2. In other

chroma subsampling Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information, taking advantage of the human visual system's lower acuity for color differences than for luminance. It is u ...

formats, e.g. 4:0:0, 4:2:2, or 4:4:4, the number of chroma samples in a macroblock will be smaller or larger, and the grouping of chroma samples into blocks will differ accordingly. In more modern macroblock-based video coding standards such as H.263 and H.264/AVC, transform blocks can be of sizes other than 8×8 samples. For instance, in H.264/AVC main profile, the transform block size is 4×4. In H.264/AVC High profile, the transform block size can be either 4×4 or 8×8, adapted on a per-macroblock basis.

Prediction blocks

Distinct from the division into transform blocks, a macroblock can be split into prediction blocks. In early standards such as H.261, MPEG-1 Part 2, and H.262/MPEG-2 Part 2, motion compensation is performed with one motion vector per macroblock. In more modern standards such as H.264/AVC, a macroblock can be split into multiple variable-sized prediction blocks, called partitions. In an inter-predicted macroblock in H.264/AVC, a separate motion vector is specified for each partition. Correspondingly, in an intra-predicted macroblock, where samples are predicted by extrapolating from the edges of neighboring blocks, the predicted direction is specified on a per-partition basis. In H.264/AVC, prediction partition size ranges from 4×4 to 16×16 samples for both inter-prediction (motion compensation) and intra-prediction.

Bitstream representation

A possible bitstream representation of a macroblock in a video codec which uses

motion compensation Motion compensation in computing, is an algorithmic technique used to predict a frame in a video, given the previous and/or future frames by accounting for motion of the camera and/or objects in the video. It is employed in the encoding of video d ...

and

transform coding Transform coding is a type of data compression for "natural" data like audio signals or photographic images. The transformation is typically lossless (perfectly reversible) on its own but is used to enable better (more targeted) quantization, ...

is given below. It is similar to the format used in H.261.

+------+------+-------+--------+-----+----+----+--------+
,  ADDR ,  TYPE ,  QUANT ,  VECTOR ,  CBP ,  b0 ,  b1 ,  ... b5 , 
+------+------+-------+--------+-----+----+----+--------+

* ADDR — address of block in image * TYPE — identifies type of macroblock ( intra frame,

inter frame An inter frame is a frame in a video compression stream which is expressed in terms of one or more neighboring frames. The "inter" part of the term refers to the use of ''Inter frame prediction''. This kind of prediction tries to take advantage fro ...

, bi-directional inter frame) * QUANT — quantization value to vary quantization * VECTOR -

motion vector Motion estimation is the process of determining ''motion vectors'' that describe the transformation from one 2D image to another; usually from adjacent frames in a video sequence. It is an ill-posed problem as the motion is in three dimensions ...

* CBP — Coded Block Pattern, this is bit mask indicating for which blocks coefficients are present. * bN — the blocks (4 Y, 1 Cr, 1 Cb)

Macroblocking

The term ''macroblocking'' is commonly used to refer to block coding artifacts.

References