Adobe AFTER EFFECTS CS3 PROFESSIONAL User Manual page 69

Set the default alpha channel preferences
1 Choose Edit > Preferences > Import (Windows) or After Effects > Preferences > Import (Mac OS).
Choose options from the Interpret Unlabeled Alpha As menu. The options in this menu are similar to the options
2
in the Interpret Footage dialog box. Ask User specifies that the Interpret Footage dialog box opens each time a
footage item with an unlabeled alpha channel is imported.
Change frame rate
The composition frame rate determines the number of frames displayed per second, and how time is divided into
frames in the time ruler and time display. Composition frame rate is usually determined by the type of output that
you are targeting. NTSC video has a frame rate of 29.97 frames per second (fps), PAL video has a frame rate of 25
fps, and motion picture film typically has a frame rate of 24 fps. Depending on the broadcast system, DVD video can
have the same frame rate as NTSC video or PAL video, or a frame rate of 23.976. Cartoons and video intended for
CD-ROM or the web are often 10 to 15 fps.
Setting the composition frame rate to twice the rate of the output format causes After Effects to display each field of
interlaced source footage as its own, separate frame in the Composition panel. This process lets you set keyframes on
individual fields and gain precision when animating masks.
When you render a movie for final output, you can choose to use the composition frame rate or another frame rate.
This is useful when you are using the same composition to create output for multiple media.
Each motion-footage item in a composition can also have its own frame rate. The relationship between the footage-
item frame rate and the composition frame rate determines how smoothly the layer plays. For example, if the
footage-item frame rate is 30 fps and the composition frame rate is 30 fps, then whenever the composition advances
one frame, the next frame from the footage item is displayed. If the footage-item frame rate is 15 fps and the compo-
sition frame rate is 30 fps, then each frame of the footage item appears in two successive frames of the composition.
(This assumes, of course, the simple case in which no time stretching or frame blending has been applied to the
layer.)
Ideally, use source footage that matches the final output frame rate. This way, After Effects renders each frame, and
the final output does not omit or duplicate frames. If, however, the source footage has a frame rate slightly different
from what you want to output to (for example, 30-fps footage and 29.97-fps final output), you can make the footage
frame rate match the composition frame rate by conforming it.
Conforming the frame rate of a footage item does not alter the original file, only the reference that After Effects uses.
When conforming, After Effects changes the internal duration of frames but not the frame content. Afterward, the
footage plays back at a different speed. For example, if you conform the frame rate from 15 fps to 30 fps, the footage
plays back twice as fast. In most cases, conform the frame rate only when the difference between the footage frame
rate and the output frame rate is small.
Note: Conforming can change the synchronization of visual footage that has an audio track, because changing the frame
rate changes the duration of the video but leaves the audio unchanged. If you want to stretch both audio and video, use
the Time Stretch command. (See "Time-stretch a layer" on page 219.) Keyframes applied to the source footage remain at
their original locations (which retains their synchronization within the composition but not the visual content of the
layer). You may need to adjust keyframe locations after conforming a footage item.
You can change the frame rate for any movie or sequence of still images. For example, you can import a sequence of
ten still images and specify a frame rate for that footage item of 5 frames per second (fps); this sequence would then
have a duration of two seconds when used in a composition.
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User Guide