79 How does MPEG achieve compression? Picture layer:
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
79 How does MPEG achieve compression? Picture layer:
In MPEG-2, a frame may be coded progressively or interlaced, signaled
by the progressive_frame variable. In interlaced frames
(progressive_frame==0), frames may then be coded as either a frame
picture (picture_structure==frame) or as two separately coded field
pictures (picture_structure==top_field or
picture_structure==bottom_field). Progressive frames are a logic
choice for video material which originated from film, where all pixels
are integrated or captured at the same time instant. Most electronic
cameras today capture pictures in two separate stages: a top field
consisting of all odd lines of the picture are nearly captured in the
time instant, followed by a bottom field of all even lines. Frame
pictures provide the option of coding each macroblock locally as either
field or frame. An encoder may choose field pictures to save memory
storage or reduce the end-to-end encoder-decoder delay by one field
period.
There is no longer such a thing called D pictures in MPEG-2 syntax.
However, Main Profile @ Main Level MPEG-2 decoders, for example, are
still required to decode D pictures at Main Level (e.g. 720x480x30
Hz). The usefulness of D pictures, a concept from the year 1990, had
evaporated by the time MPEG-2 solidified in 1993.
repeat_first_field was introduced in MPEG-2 to signal that a field or
frame from the current frame is to be repeated for purposes of frame
rate conversion (as in the 30 Hz display vs. 24 Hz coded example
above). On average in a 24 frame/sec coded sequence, every other coded
frame would signal the repeat_first_field flag. Thus the 24 frame/sec
(or 48 field/sec) coded sequence would become a 30 frame/sec (60
field/sec) display sequence. This processes has been known for decades
as 3:2 Pulldown. Most movies seen on NTSC displays since the advent of
television have been displayed this way. Only within the past decade
has it become possible to interpolate motion to create 30 truly unique
frames from the original 24. Since the repeat_first_field flag is
independently determined in every frame structured picture, the actual
pattern can be irregular (it doesnt have to be every other frame
literally). An irregularity would occur during a scene cut, for
example.
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