Adobe After Effects CS3 - Manual (Page 77)

AFTER EFFECTS CS3
User Guide
72
Determine the original field order
The field order for an interlaced video footage item determines the order in which the two video fields (upper and
lower) are displayed. A system that draws the upper lines before the lower lines is called upper-field first; one that
draws the lower lines before the upper lines is called lower-field first. Many standard-definition formats (such as DV
NTSC) are lower-field first, whereas many high-definition formats (such as 1080i DVCPRO HD) are upper-field
first.
Note: Upper-field first corresponds to Even Field First in an ElectricImage file.
The order in which the fields are displayed is important, especially when the fields contain motion. If you separate
video fields using the wrong field order, motion will not appear smooth.
Some programs, including After Effects, label the field order when rendering interlaced video files. When you
import a labeled video file, After Effects honors the field order label automatically. You can override this field order
by applying different footage interpretation settings.
If a file does not contain a field order label, you can match the original field order of your footage. If you are not sure
which field order was used to interlace a footage item, use the procedure below to find out.
1 Select the item in the Project panel.
2 Choose File > Interpret Footage > Main.
3 In the Interpret Footage dialog box, select Upper Field First from the Separate Fields menu, and then click OK.
4 In the Project panel, press Alt (Windows) or Option (Mac OS) as you double-click the footage to open it in the
Footage panel.
5 If the Time Controls panel is not visible, choose Window > Time Controls.
6 In the Footage panel, find a segment that contains one or more moving areas.
7 Using the Next Frame button in the Time Controls panel, step forward at least five frames in the Footage panel.
Moving areas should move consistently in one direction. If the moving areas move backward every other frame, the
wrong field-separation option has been applied to the footage.
Remove 3:2 or 24Pa pulldown from video
When you transfer 24-fps film to 29.97-fps video, you use a process called 3:2 pulldown, in which the film frames are
distributed across video fields in a repeating 3:2 pattern. The first frame of film is copied to fields 1 and 2 of the first
frame of video, and also to field 1 of the second video frame. The second frame of film is then spread across the next
two fields of video—field 2 of the second video frame and field 1 of the third frame of video. This 3:2 pattern is
repeated until four frames of film are spread over five frames of video, and then the pattern is repeated.
The 3:2 pulldown process results in whole frames (represented by a W) and split-field frames (represented by an S).
The three whole video frames contain two fields from the same film frame. The remaining two split-field frames
contain a video frame from two different film frames. The two split-field frames are always adjacent to each other.
The phase of 3:2 pulldown refers to the point at which the two split-field frames fall within the first five frames of the
footage.
Phase occurs as a result of two conversions that happen during 3:2 pulldown: 24-fps film is redistributed through
30-fps video, so each of four frames of 24-fps film is spread out over five frames of 30(29.97)-fps video. First, the film
is slowed down 0.1% to match the speed difference between 29.97 fps and 30 fps. Next, each film frame is repeated
in a special pattern and mated to fields of video.