96 What's the deal with slice_vertical_position and macroblock_address_increment? (MPEG-2)
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
96 What's the deal with slice_vertical_position and macroblock_address_increment? (MPEG-2)
The absolute position of the first macroblock within a slice is known
by the combination of slice_vertical_position and the
macroblock_address_increment. Therefore, the proper place of a lost
slice found in a highly corrupt bitstream can be located exactly within
the picture. These two syntax elements are also the only known means
of detecting slice gaps----areas of the picture which are not
represented with any information (including skipped macroblocks). A
slice gap occurs when the current macroblock address of the first
macroblock in a slice is greater than the previous macroblock address
by more than 1 macroblock unit. A slice overlap occurs when the current
macroblock address is less than or equal to the previous macroblocks
address. The previous macroblock in both instances is the last known
macroblock within the previous slice. Because of the semantic
interpretation of slice gaps and overlaps, and because of the syntactic
restrictions for slice_vertical_position and
macroblock_address_increment, it is not syntactically possible for a
skipped macroblock to be represented in the first and last positions of
a slice. In the past, some (bad) encoders would attempt to signal a
run of skipped macroblocks to the end of the slice. These evil skipped
macroblocks should be interpreted by a compliant decoder as a gap, not
as a string of skipped macroblocks.
Continue to:
My Books
