Information Technology Reference
In-Depth Information
Figure 5.37:
(a) In Frame-DCT, a picture is effectively de-interlaced. (b) In Field-DCT, each DCT block only
contains lines from one field, but over twice the screen area. (c) The same DCT content results when field-pictures
are assembled into blocks.
Even small amounts of motion in any direction can destroy the correlation between odd and even lines and a frame
DCT will result in an excessive number of coefficients. Figure 5.37(b) shows that instead the luminance component
of a frame can also be divided into
field DCT
blocks. In this case one DCT block contains odd lines and the other
contains even lines. In this mode the chroma still produces one DCT block from both fields as in Figure 5.37(a).
When an input frame is designated as two
field-pictures
, the macro- blocks come from a screen area which is 32
lines high. Figure 5.37(c) shows that the DCT blocks contain the same data as if the input frame had been
designated a
frame-picture
but with
field DCT
. Consequently it is only frame-pictures which have the option of field
or frame DCT. These may be selected by the encoder on a macroblock-by-macroblock basis and, of course, the
resultant bitstream must specify what has been done.
In a frame which contains a small moving area, it may be advantageous to encode as a frame-picture with frame
DCT except in the moving area where field DCT is used. This approach may result in fewer bits than coding as two
field-pictures.
In a field-picture and in a frame-picture using field DCT, a DCT block contains lines from one field type only and this
must have come from a screen area 16 scan lines high, whereas in progressive scan and frame DCT the area is
only 8 scan lines high. A given vertical spatial frequency in the image is sampled at points twice as far apart which
is interpreted by the field DCT as a doubled spatial frequency, whereas there is no change in the horizontal
spectrum.
Following the DCT calculation, the coefficient distribution will be different in field-pictures and field DCT frame-
pictures. In these cases, the probability of coefficients is not a constant function of radius from the DC coefficient as
it is in progressive scan, but is elliptical where the ellipse is twice as high as it is wide.
