The macroblock is a processing unit in

image An image or picture is a visual representation. An image can be Two-dimensional space, two-dimensional, such as a drawing, painting, or photograph, or Three-dimensional space, three-dimensional, such as a carving or sculpture. Images may be di ...

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 compression ...

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 ( , short for Joint Photographic Experts Group and sometimes retroactively referred to as JPEG 1) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degr ...

, where they are called MCU blocks, H.261, MPEG-1 Part 2, H.262/MPEG-2 Part 2, H.263, MPEG-4 Part 2, 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 In H.265/HEVC, the macroblock as a basic processing unit has been replaced by the coding tree unit.

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 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 Chrominance, chroma information than for luma (video), luma information, taking advantage of the human visual system's lower acuity for color differences t ...

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 and transform coding 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 fr ...

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