Apple Shake 4 - Manual (Page 192)

192 Chapter 6 Importing Video and Anamorphic Film
Understanding Video Interlacing
Dividing each frame of video into two fields is a technique originally developed for
television broadcasting to solve a number of technical difficulties with early TV
equipment. In essence, interlacing reduces the perceived strobing of 30 images playing
every second on a television screen. Interlacing divides each frame into two fields, each
of which contains half of the image information. Consequently, the television screen
displays 60 fields each second, resulting in smoother motion.
The following example depicts an animation sequence showing frames 1 and 3 of a
moving image. These are non-interlaced, full frames—each frame contains the entire
image at that instant in time.
The images below depict the same frames up to (but not including) frame 3 as they
appear when they’re interlaced. The image on the left shows some information from
frame 1, field 1 and from frame 1, field 2 (unconventionally labeled here as frame 1.5).
The image on the right shows frame 2, field 1 and frame 2, field 2 (unconventionally
labeled frame 2.5 for illustration purposes).
As you can see, each field contains only half of the horizontal lines of the image. If
you’ve ever seen a still photograph of an interlaced source clip, you’ve probably already
seen this type of image: the moving subject appears to be in two places at once,
blurred because each image contains only half the scan lines.