Adobe AFTER EFFECTS CS3 PROFESSIONAL User Manual page 85

AFTER EFFECTS CS3 79
User Guide
A
B
C
Interlaced scanning of interlaced video fields compared with progressive scanning of noninterlaced video frame.
A. For interlaced video, entire upper field is drawn to screen first, from top to bottom, in one pass. B. Next, entire lower field is drawn to screen,
from top to bottom, in one pass. C. For noninterlaced video, entire frame is drawn to screen, from top to bottom, in one pass.
Noninterlaced video frames aren't separated into fields. A progressive-scan monitor displays a noninterlaced video
frame by drawing all the horizontal lines, from top to bottom, in one pass. Computer monitors are almost all
progressive-scan monitors, and most video displayed on computer monitors is noninterlaced.
The terms progressive and noninterlaced are thus closely related and are often used interchangeably, but progressive
refers to the recording or drawing of the scan lines by a camera or monitor, whereas noninterlaced refers to the fact
that the video data itself isn't separated into fields. For example, it's possible with some modern cameras to use
progressive scanning to record two simultaneous fields per frame of interlaced video.
Separate video fields
If you want to use interlaced or field-rendered footage (such as NTSC video) in an After Effects project, you will get
the best results if you separate the video fields when you import the footage. After Effects separates video fields by
creating a full frame from each field, preserving all of the image data from the original footage.
Separating fields is critical if you plan to make significant changes to the footage. When you scale, rotate, or apply
effects to interlaced video, unwanted artifacts, such as crossed fields, are often introduced. By separating fields, After
Effects accurately converts the two interlaced frames in the video to noninterlaced frames, while preserving the
maximum amount of image quality. Using noninterlaced frames allows After Effects to apply edits and effects consis-
tently and at the highest quality.
After Effects creates field-separated footage from a single formerly interlaced field by splitting fields into two
independent frames. Each new frame has only half the information of the original frame, so some frames may appear
to have a lower resolution than others when viewed at Draft quality. When you render the final composition, After
Effects reproduces high-quality interlaced frames for videotape. When you render a movie at Best quality, After
Effects interpolates between the scan lines of a field to produce maximum image quality.
If your output will not be interlaced, it's best to use noninterlaced source footage, to avoid the need to separate fields.
However, if a noninterlaced version of your source footage is not available, interlaced footage will work fine.
When you render a composition containing field-separated footage, set the Field Rendering option to the same field
order as your video equipment. If you don't field-render the composition, or if you field-render with the incorrect
settings, the final movie may appear too soft, jerky, or distorted.