Digital Signal Processing Reference
In-Depth Information
FIELD 0
ODD NUMBERED ROWS
FIELD 1
EVEN NUMBERED ROWS
ROW1
ROW2
=
VIDEO FRAME
30 frames/s
Figure 6.2. Weave deinterlacing: combining two
elds to create one frame.
VIDEO fields
60 fields/s
when the two fields are generated from a single progressive
video frame.
In practice the two fields are normally offset in time
typically
e
=
60
th
of a second. So while field zero is captured at time t
0
s,
field one is captured at time t
0
þ
by 1
s. Thus the weave technique is
fine if the image does not change in that 1
=
60
th
of the second. But,
if the image changes, then the pixels in field zero will not line up
with pixels in field one (especially for the portion of the image
that changed)
e
and you will see a jagged edge as shown in
Figure 6.3
.
Figure 6.3
shows this combing effect
also called mouse
e
teeth
using a simple Excel model. You can immediately and
intuitively understand how jagged edges appear where the image
has changed and how the resultant image is fine where the image
does not change.
Figure 6.4
shows how an image looks when these artifacts
appear as a result of weave deinterlacing.
Another form of deinterlacing is called “bob” deinterlacing.
where each field becomes its own frame of video. This doubles
the resultant frame rate. So an interlaced NTSC clip at 29.97 fps
(fields per second) stream becomes a 59.94 fps (fps) progressive.
The lines in each field are also doubled as the field becomes
a frame
e
which is why this technique is also sometimes
described as spatial line doubling.
Since each field has only half the scan lines of a full frame,
interpolation must be used to form the missing scan lines.
Interpolation is a fancy term for guessing the value of the line of
pixels. In the video scaling chapter we saw how this technique
was used to create new pixel values in order to make an image
bigger or smaller.
e
