Processing for compression - The MPEG - page 93

Figure 3.60: (a) A two-dimensional correlation surface has a central peak when there is no motion. (b) In the case
of a pan, the peak moves laterally. (c) A camera tilt moves the peak at right angles to the pan.
Where more complex motions are involved, perhaps with several objects moving in different directions and/or at
different speeds, one peak will appear in the correlation surface for each object.
It is a fundamental strength of phase correlation that it actually measures the direction and speed of moving objects
rather than estimating, extrapolating or searching for them. The motion can be measured to sub-pixel accuracy
without excessive complexity.
However, the consequences of uncertainty are that accuracy in the transform domain is incompatible with accuracy
in the spatial domain. Although phase correlation accurately measures motion speeds and directions, it cannot
specify where in the picture these motions are taking place. It is necessary to look for them in a further matching
process. The efficiency of this process is dramatically improved by the inputs from the phase-correlation stage.
[ 21 ] Limb, J.O. and Murphy, J.A., Measuring the speed of moving objects from television signals. IEEE Trans.
Commun. , 474-478 (1975)
[ 22 ] Thomas, G.A., Television motion measurement for DATV and other applications. BBC Res. Dept. Rept, RD
1987/11 (1987)#
[ 23 ] Pearson, J.J. et al., Video rate image correlation processor. SPIE, Vol. 119, Application of digital image
processing, IOCC (1977)
3.17 Motion-compensated displays
MPEG coding is frequently used with film-originated material in DVD and in DVB. When this is done the pictures
are coded at their native rate of 24 (or sometimes 25) Hertz. Figure 3.61 (a) shows the time axis of film, where
entire frames are simultaneously exposed, or sampled, at typically 24 Hz. The result is that the image is effectively
at right angles to the time axis. During filming, some of the frame period is required to transport the film, and the
shutter is closed whilst this takes place. The temporal aperture or exposure is thus somewhat shorter than the
frame period.