65 MPEG Myths p2
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
65 MPEG Myths p2
4. Display picture size is the same as the coded picture size
In MPEG, the display picture size and frame rate may differ from the
size (resolution) and frame rate encoded into the bitstream. For
example, a regular pattern of pictures in a source image sequence may
be dropped (decimated), and then each picture may itself be filtered
and subsampled prior to encoding. Upon reconstruction, the picture may
be interpolated and upsampled back to the source size and frame rate.
In fact, the three fundamental phases (Source Rate, Coded Rate, and
Display Rate) may differ by several parameters. The MPEG syntax can
separately describe Coded and Display Rates through sequence_headers,
but the Source Rate is known only by the encoder.
5. Picture coding types (I, P, B) all consist of the same macroblocks types.
All macroblocks within an I picture must be coded Intra (like a
baseline JPEG picture). However, macroblocks within a P picture may
either be coded as Intra or Non-intra (temporally predicted from a
previously reconstructed picture). Finally, macroblocks within the B
picture can be independently selected as either Intra, Forward
predicted, Backward predicted, or both forward and backward
(Interpolated) predicted. The macroblock header contains an element,
called macroblock_type, which can flip these modes on and off like
switches. macroblock_type is possibly the single most powerful element
in the whole of video syntax. Picture types (I, P, and B) merely enable
macroblock modes by widening the scope of the semantics. The component
switches are:
1. Intra or Non-intra
2. Forward temporally predicted (motion_forward)
3. Backward temporally predicted (motion_backward)
(2+3 in combination represent ¿Interpolated¿)
4. conditional replenishment (macroblock_pattern).
5. adaptation in quantization (macroblock_quantizer).
6. temporally predicted without motion compensation
The first 5 switches are mostly orthogonal (the 6th is derived from the
1st and 2nd in P pictures, and does not exist in B pictures). Some
switches are non-applicable in the presence of others. For example, in
an Intra macroblock, all 6 blocks by definition contain DCT data,
therefore there is no need to signal either the macroblock_pattern or
any of the temporal prediction switches. Likewise, when there is no
coded prediction error information in a Non-intra macroblock, the
macroblock_quantizer signal would have no meaning.
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