80 How does MPEG achieve compression? Slice:
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
80 How does MPEG achieve compression? Slice:
To aid implementations which break the decoding process into parallel
operations along horizontal strips within the same picture, MPEG-2
introduced a general semantic mandatory requirement that all
macroblock rows must start and end with at least one slice. Since a
slice commences with a start code, it can be identified by
inexpensively parsing through the bitstream along byte boundaries.
Before, an implementation might have had to parse all the variable
length tokens between each slice (thereby completing a significant
stage of decoding process in advance) to know the exact position of
each macroblock within the bitstream. In MPEG-1, it was possible to
code a picture with only a single slice. Naturally, the mandatory
slice per macroblock row restriction also facilitates error recovery.
MPEG-2 also added the concept of the slice_id. This optional 6-bit
element signals which picture a particular slice belongs to. In badly
mangled bitstreams, the location of the picture headers could become
garbled. slice_id allows a decoder to place a slice in the proper
location within a sequence. Other elements in the slice header, such
as slice_vertical_position, and the macroblock_address_increment of the
first macroblock in the slice uniquely identify the exact macroblock
position of the slice within the picture. Thus within a window of 64
pictures, a lost slice can find its way.
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