Temporal Preprocessing - A Practical Guide to Video and Audio Compression - page 540

Frame 1 Frames 1 & 2a Frames 2b & 3 Frame 3

Figure 33-3 Pulldown fields.

33.3 Using Averaging to Remove Interlacing

Averaging the fields together might seem like a good idea, but this just moves the prob-
lem of the horizontal interlacing artifact. Instead of having odd and even fields, there is
a single field per frame and this converts readily to progressive scanning. However, this
only solves the mechanical problem. The image has not actually been de-interlaced at all.
It is going to compress very badly and waste bandwidth. So we see that a simple aver-
aging technique yields unsatisfactory results and is therefore not recommended (see
Figure 33-4).
If you look closely, you can see that there is some grain reduction and the lines in the
interlaced images are blended out by the averaging, so the compression may spend less
time on the horizontal detail but the superimposed effect is still evident. In this case the
averaging involved simply taking pairs of lines, one from each field.
This is obviously not a good solution.

33.4 Dropping a Field to De-Interlace

The simplest technique is to just drop the odd or even field. This preserves the horizontal
resolution but throws away half of the vertical detail. The eye is amazingly forgiving of
this, but there are implications.