Apple Shake 4 - Manual (Page 194)

194 Chapter 6 Importing Video and Anamorphic Film
Because each interlaced frame of video consists of two fields that contain half the
information for that frame, each field can be thought of as a half-height version of the
originating frame. Because, during playback, the television displays these images
quickly, one after the other, the human eye is fooled into perceiving the image as
having a higher resolution than each individual field actually possesses. To sum up, each
field sacrifices quality in terms of vertical resolution (perceived as image sharpness) for
the benefit of improved temporal quality (perceived as smoothness of motion).
Common Issues When Compositing Interlaced Images
In order to avoid pitfalls when compositing interlaced media, it’s important to
understand the peculiarities of the format. This section describes the compositing
operations in Shake that are affected by improperly setting a script’s interlacing
parameters.
Parameter Animation Across Fields
The first problem occurs when you animate any parameter. The animation must be
understood and applied to every field of video—at half-frame intervals. If you read in
an interlaced clip and apply a static Gamma, no problems occur because both fields
receive the same correction. If, however, you animate the gamma correction, you must
enable field rendering in order to apply the correct gamma value to each of the two
fields, in the correct order.
Transforms Applied to Fields
The second, and trickier, issue arises when you apply spatial effects with a node such as
the Blur or a Move2D node. For example, panning an image up by 1 pixel in the Y axis
has the inadvertent effect of reversing the two fields within that frame, because the
even lines are moved to the odd field, and the odd lines are moved to the even field.
The resulting motion artifacts will appear as a jittering effect because the fields are
playing backwards even though the frames are still playing forwards. This would be the
same as if you inverted a standard film frame sequence 1, 2, 3, 4, 5, 6 to play as 2, 1, 4,
3, 6, 5.