DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Poynton, Charles, “Motion
portrayal, eye tracking, and
emerging display technology,” in
Proc. 30th SMPTE Advanced
Motion Imaging Conference (New
York: SMPTE, 1996), 192-202.
Figure 37.3 Static lattice
Figure 37.4 Line replication
Figure 37.5 Interfield
averaging
nonzero exposure time at the camera. (For details of
temporal characteristics of image acquisition and
display, see my SMPTE paper.)
You can think of an interlaced video signal as having its
lines in permuted order, compared to a progressive
signal. An obvious way to accomplish deinterlacing is to
write into two fields of video storage – the first field,
then the second – in video order, then read out the
assembled frame progressively (in spatial order). This
method is sometimes given the sophisticated name field
replication, or weave. This method is quite suitable for
a stationary scene, or a scene containing only slowmoving
elements. However, the image of the second
field is delayed with respect to the first, by half the
frame time (typically 1 ⁄60 s or 1 ⁄50 s). If the scene
contains an element in fairly rapid motion, such as the
disk in our test scene, the object will exhibit field
tearing: It will be reproduced with jagged edges, either
when viewed as a still frame or when viewed in motion.
The effect is sketched in Figure 37.3.
Field tearing can be avoided by intrafield processing,
using only information from a single field of video. The
simplest intrafield technique is to replicate each line
upon progressive readout. This method will reproduce
a stationary element with at most half of its potential
vertical resolution. Also, line replication introduces
a blockiness into the picture, and an apparent downward
shift of one image row. The effect is sketched in
Figure 37.4.
The blockiness of the line replication approach can be
avoided by synthesizing information that is apparently
located spatially in the opposite field, but located
temporally coincident with the same field. This can be
accomplished by averaging vertically adjacent samples
in one field, to create a synthetic intermediate line, as
depicted in Figure 37.5. (In the computer industry, this
is called “bob.”) The averaging can be done prior to
writing into the video memory, or upon reading,
depending on which is more efficient for the memory
system. Averaging alleviates the disadvantage of blockiness,
but does not compensate the loss of vertical reso-
438 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES