Processing for compression - The MPEG - page 88

Figure 3.49 (c) shows that cascading this process results in an ever smaller low-pass pixel block which eventually
converges to a single average brightness code.
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3.15 The importance of motion compensation
The optic flow axis is the locus of some point on a moving object which will be in a different place in successive
pictures. Any device which computes with respect to the optic flow axis is said to be motion compensated . Until
recently the amount of computation required in motion compensation was too expensive, but as this is no longer
the case the technology has become very important in moving image portrayal systems.
Figure 3.53 (a) shows an example of a moving object which is in a different place in each of three pictures. The
optic flow axis is shown. The object is not moving with respect to the optic flow axis and if this axis can be found
some very useful results are obtained. The process of finding the optic flow axis is called motion estimation . Motion
estimation is literally a process which analyses successive pictures and determines how objects move from one to
the next. It is an important enabling technology because of the way it parallels the action of the human eye.
Figure 3.53 (b) shows that if the object does not change its appearance as it moves, it can be portrayed in two of
the pictures by using data from one picture only, simply by shifting part of the picture to a new location. This can be
done using vectors as shown. Instead of transmitting a lot of pixel data, a few vectors are sent instead. This is the
basis of motion- compensated compression which is used extensively in MPEG as will be seen in Chapter 5 .
Figure 3.53: Motion compensation is an important technology. (a) The optic flow axis is found for a moving object.
(b) The object in picture ( n + 1) and ( n + 2) can be re-created by shifting the object of picture n using motion
vectors. MPEG uses this process for compression. (c) A standards convertor creates a picture on a new timebase