Image Processing Reference
In-Depth Information
33
Temporal Preprocessing
33.1
Introduction
Temporal preprocessing is an important part of the preparation for compressing your
video. Getting this wrong will significantly compromise your compression ratios and
waste bit rate by preserving unwanted artifacts in the video in order to describe the con-
tent you do want to deliver.
The main area we are concerned with here is removing interlacing effects when con-
verting the images to a progressively scanned format. This operation will also remove any
redundant frames that contain duplicated fields. This is also good for compression pur-
poses. We can eliminate the pulldown that is introduced when playing 24-fps movie film
on a 60-Hz TV service such as that used in the United States. However, in territories like
Europe where the display runs at 50 Hz there is no pulldown and therefore no reduction
in frame count. Nevertheless, converting from an interlaced to a progressive display
regime is still valuable and helps the compression process.
33.2
Interlace Removal
Broadcast SDTV has two fields per frame, with the lines interlaced. This is not the same as
computer video, which is progressively scanned in one pass. If you are deploying the
video to the web, you must remove the interlacing. If you leave the interlacing intact, then
the coded video treats moving objects very poorly and you get double images of things
appearing. Because of the interlacing effect, you get lots of alternating single horizontal
lines on the image, and these are very hard to compress. Check whether you have to
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