Graphics Reference
In-Depth Information
24 Hz
progressive
60 Hz
interlaced
30 Hz
progressive
Original
ilm frame
Video ield
(Odd)
Video ield
(Even)
Resultant
video frame
Figure 35.11: Schematic of how interlacing is exploited to adapt 24 Hz film frames for
60 Hz interlaced broadcast to NTSC televisions under the 3:2 pulldown algorithm. In the
center columns, the odd and even source film frames have been repeated three and two
times, respectively. (Created by Eric Lee.)
frames, and interpolating the remaining frames from adjacent ones would signifi-
cantly blur the images. The
telecine
or
pulldown
employed in practice is a clever
alternative that exploits interlacing.
The commonly employed algorithm is called 3:2 pulldown. We begin with a
simplified example to understand the intuition behind it. Instead of resampling
from 24 Hz progressive to 30 Hz progressive, consider the problem of resampling
from 24 Hz progressive to 60 Hz interlaced. We can approach this by replicating
each source frame “2.5” times, that is, by repeating frame
i
twice, blending frames
i
and
i
+
1, and then proceeding to process frame
i
+
1. This blends only one out
of every three frames, which is substantially better than blending five out of every
six frames. The output will be progressive, so to create an interlaced format, drop
half the rasters from every frame.
What the actual
3:2 pulldown
algorithm does is perform the blending by
choosing the source rasters more selectively. This avoids blending pixels from
separate frames and directly produces interlaced output. This is similar to stochas-
tic blending methods like dithering: The blending is spatial and is integrated by
the eye. Figure 35.11 shows the process. Given four original film frames
A
,
B
,
C
,
and
D
sampled at 24 Hz (left column), the algorithm will produce 4
·
2.5
=
10
interlaced frames at
≈
60 Hz (center columns), that correspond to five progres-
sive frames at
30 Hz (right column). Note that the interlaced frames are broad-
cast in alternating raster order, so the top frame of the left-center column will be
≈
