Information Technology Reference
In-Depth Information
Figure 5.38:
The zig-zag scan for an interlaced image has to favour vertical frequencies twice as much as
horizontal.
Using the standard 45° zig-zag scan with this different coefficient distribution would not have the required effect of
putting all the significant coefficients at the beginning of the scan. To achieve this requires a different zig-zag scan,
which is shown in Figure 5.38. This scan, sometimes known as the Yeltsin walk, attempts to match the elliptical
probability of interlaced coefficients with a scan slanted at 67.5° to the vertical.
Motion estimation is more difficult in an interlaced system. Vertical detail can result in differences between fields
and this reduces the quality of the match. Fields are vertically subsampled without filtering and so contain alias
products. This aliasing will mean that the vertical waveform representing a moving object will not be the same in
successive pictures and this will also reduce the quality of the match.
Even when the correct vector has been found, the match may be poor so the estimator fails to recognize it. If it is
recognized, a poor match means that the quality of the prediction in
P
and
B
pictures will be poor and so a large
prediction error or residual has to be transmitted. In an attempt to reduce the residual, MPEG-2 allows field-pictures
to use motion-compensated prediction from either the adjacent field or from the same field type in another frame. In
this case the encoder will use the better match. This technique can also be used in areas of frame-pictures which
use field DCT.
The motion compensation of MPEG-2 has half-pixel resolution and this is inherently compatible with an interlace
because an interpolator must be present to handle the half-pixel shifts. Figure 5.39(a) shows that in an interlaced
system, each field contains half of the frame lines and so interpolating half-way between lines of one field type will
actually create values lying on the sampling structure of the other field type. Thus it is equally possible for a
predictive system to decode a given field type based on pixel data from the other field type or of the same type.
