Image Processing Reference
In-Depth Information
Figure 33-8 Pulldown removed
Frame 1
Frame 2
Frame 3
Recovering the correct frames is theoretically possible by dropping those fields that
duplicate one another and combining the correct pairs of the remaining frames.
Now let's revisit our original four-frame segment and see if we can recover three per-
fectly intact frames by taking the odd and even fields from different frames.
You can see in Figure 33-8 that we have successfully reconstructed the original Frame
2 that was divided by the pulldown. There are some residual artifacts, but these would be
eliminated if a better-quality master-input format were used instead of the MPEG-2 orig-
inal that was used for this test.
When It Gets Nasty
Some significant problems present themselves if more advanced telecine approaches have
been used.
One problem occurs because the technology companies that develop telecine machines
have introduced some cleverness into the scanning process. The film is scanned into a frame
store at 24 fps, and when that frame store is dual ported the output is clocked at a different
frequency. The output may combine the top half of one frame and the bottom half of the
next. The pulldown is applied not at the frame boundaries but on a line-by-line basis. This
is much harder to de-interlace, which is apparent in the middle frame of Figure 33-9.
Figure 33-9 Line-by-line pulldown.
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