Macroblock

Macroblock

Macroblock is a term used in video compression, which represents a block of 16 by 16 pixels. Each macroblock contains 4 Y block, 1 Cb block, 1 Cr block (4:2:0). (It also could be represented by 4:2:2 or 4:4:4 YCbCr format). Macroblocks can be subdivided further into smaller blocks, called partitions. H.264, for example, supports block sizes as small as 4x4.

Macroblock coding

+------+------+-------+--------+-----+----+----+--------+
| ADDR | TYPE | QUANT | VECTOR | CBP | b0 | b1 | ... B5 |
+------+------+-------+--------+-----+----+----+--------+

  • ADDR .... address of block in image
  • TYPE .... identifies type of macroblock (intra-frame, inter frame, bi-directional inter frame)
  • QUANT ... quantization value to vary quantization
  • VECTOR... motion vector
  • CBP...... Coded Block Pattern, some blocks in macroblock match well, some match poorly - this is bit mask indicating which one is present.
  • the blocks (4 Y, 1 Cr, 1 Cb)

Macroblocking

This is a non-technical term used when macroblocks are either missing and show up as video errors, or when bandwidth is not enough to encode fine detail. Square areas of the picture do not show the correct portion of the image. Instead they either show a single color block, or a low-resolution block with noticeable edges. Advanced decoders hide these types of errors with a technique called error concealment. Consumer equipment often calls this technique as MPEG Noise Reduction system.

This effect is also referred to as microblocking, tiling, mosaicking or pixelating.

See also

References

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