Information Technology Reference
In-Depth Information
Figure 5.47: The alpha plane is used in MPEG-4 to describe the shape of an object. Within a bounding rectangle
are three different types of macroblock: those within the object, those outside and those which are intersected by
the boundary.
Within the bounding rectangle are three kinds of macroblock. Trans- parent macroblocks are entirely outside the
object and contain no texture data. The shape data are identical over the entire block and result in a key signal
which deselects this block in the compositing process. Such shape data are trivial to compress. Opaque
macroblocks are entirely within the object and are full of texture data. The shape data are again identical over the
entire block and result in the block being keyed at the compositor. Boundary macroblocks are blocks through which
the edge of the object passes. They contain essentially all the shape information and somewhat fewer texture data
than an opaque macroblock.
5.18 An MPEG-2 decoder
The decoder is only defined by implication from the definitions of syntax and any decoder which can correctly
interpret all combinations of syntax at a particular profile will be deemed compliant however it works. Compliant
MPEG-2 decoders must be able to decode an MPEG-1 elementary stream. This is not particularly difficult as
MPEG-2 uses all the tools of MPEG-1 and simply adds more of its own. Consequently an MPEG-1 bitstream can
be thought of as a subset of an MPEG-2 bitstream.
The first problem a decoder has is that the input is an endless bitstream which contains a huge range of
parameters many of which have variable length. Unique synchronizing patterns must be placed periodically
throughout the bitstream so that the decoder can identify known starting points. The pictures which can be sent
under MPEG are so flexible that the decoder must first find a sequence header so that it can establish the size of
the picture, the frame rate, the colour coding used, etc.
The decoder must also be supplied with a 27 MHz system clock. In a DVD player, this would come from a crystal,
but in a transmission system this would be provided by a numerically locked loop running from a clock reference
parameter in the bitstream (see Chapter 6 ). Until this loop has achieved lock the decoder cannot function properly.
Figure 5.42 shows a bidirectional decoder. The decoder can only begin decoding with an I picture and as this only
uses intra-coding there will be no vectors. An I picture is transmitted as a series of slices which begin with
subsidiary synchronizing patterns. The first macroblock in the slice contains an absolute DC coefficient, but the
remaining macroblocks code the DC coefficient differentially so the decoder must subtract the differential values
from the previous value to obtain the absolute value.
 
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