MPEG video compression - The MPEG - page 147

Figure 5.18: (a) Spatial or intra-coding works on individual images. (b) Temporal or inter-coding works on
successive images. (c) In MPEG inter-coding is used to create difference images. These are then compressed
spatially.
A residual is an image of a kind, although not a viewable one, and so should contain some kind of spatial
redundancy. Figure 5.18(c) shows that MPEG takes advantage of both forms of redundancy. Residual images are
spatially compressed prior to transmission. At the decoder the spatial compression is decoded to re-create the
residual, then this is added to the previous picture/object to complete the decoding process.
Whenever objects move they will be in a different place in successive pictures. This will result in large amounts of
difference data. MPEG overcomes the problem using motion compensation. The encoder contains a motion
estimator which measures the direction and distance of motion between pictures and outputs these as vectors
which are sent to the decoder. When the decoder receives the vectors it uses them to shift data in a previous
picture to more closely resemble the current picture. Effectively the vectors are describing the optic flow axis of
some moving screen area, along which axis the image is highly redundant. Vectors are bipolar codes which
determine the amount of horizontal and vertical shift required.
In real images, moving objects do not necessarily maintain their appearance as they move. For example, objects
may move into shade or light. Consequently motion compensation can never be ideal and it is still necessary to
send a residual to make up for any shortcomings in the motion compensation.
Figure 5.19 shows how this works in MPEG-1 and MPEG-2. In addition to the motion-encoding system, the coder
also contains a motion decoder. When the encoder outputs motion vectors, it also uses them locally in the same
way that a real decoder will, and is able to produce a predicted picture based solely on the previous picture shifted
by motion vectors. This is then subtracted from the actual current picture to produce a prediction error or residual
which is an image of a kind that can be spatially compressed. The decoder takes the previous picture, shifts it with
the vectors to re-create the predicted picture and then decodes and adds the prediction error to produce the actual
picture. Picture data sent as vectors plus prediction error are said to be P coded.