Apple MA434Z/A User Manual - Page 194

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. 194 Chapter 6 Importing Video and Anamorphic Film