Adobe After Effects CS4 - Manual (Page 99)

93 USING AFTER EFFECTS CS4
Importing and managing footage items
Last updated 12/21/2009
Separate video fields and determine field order
If you want to use interlaced or field-rendered footage (such as NTSC video) in an After Effects project, you 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 image. 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 consistently and at
the highest quality.
After Effects creates field-separated footage from a single formerly interlaced frame by splitting it 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 output. 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.
Always separate fields for interlaced footage. Never separate fields for noninterlaced footage items.
You can only remove pull-down after you have separated fields.
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.
To quickly give video footage a more film-like appearance, import the footage twice, and interpret each footage item
with a different field order. Then add them both to the same composition and blend them together. The misinterpreted
layer adds some film-like blur.
After Effects automatically separates fields for D1 and DV video footage items. You can manually separate fields for
all other types of video footage in the Interpret Footage dialog box.
1 Select the footage item in the Project panel.
2 Choose File > Interpret Footage > Main.
3 Choose an option from the Separate Fields menu.
4 Click Preserve Edges (Best Quality Only) to increase image quality in nonmoving areas when the image is rendered
at Best quality. Then click OK.
Note: If the field settings in the Interpret Footage dialog box are correct for the input footage and the field settings in the
Render Settings dialog box are correct for the output device, you can mix footage items of different field orders in a
composition. If either of these settings is incorrect, however, the frames will be in the correct order, but the field order may
be reversed, resulting in jerky, unacceptable images.
FieldsKit from RE:Vision Effects creates full noninterlaced frames from fields and vice versa. For information, see the
RE:Vision Effects website .
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.