Apple Final Cut Express 4 - Manual (Page 776)

776 Part IX Effects and Color Correction
Controlling Speed Along a Motion Path
The speed at which a clip travels along a motion path is determined by two factors:
 The spatial, or physical, distance between two keyframes in the Canvas.
The farther a clip has to travel in a given duration, the faster its apparent movement. The
less distance a clip moves, given the same amount of time, the slower it appears to go.
 The duration, or distance in time, between two keyframes in the Center parameter of
a clip’s Motion tab.
Two keyframes 1 second apart result in faster motion than two keyframes
4 seconds apart.
You can modify a clip’s velocity, changing the quality of its movement. With no velocity
adjustments, clips move at full speed and then come to a full stop. This can result in
abrupt, artificial-looking motion. Final Cut Express gives you the ability to change the
velocity of a clip’s motion over time, using velocity handles to modify the keyframes of
a clip’s motion path in the Canvas.
By adjusting a keyframe’s velocity in the Canvas, inertia can be added to a clip’s motion.
You can adjust the first keyframe of a motion path so that, instead of taking off at full
speed from a complete stop, the clip starts off slowly and then speeds up over time.
These speed changes are indicated by velocity tick marks along that clip’s motion path.
Note: In the two examples below, the motion is at a single constant speed.
To vary the acceleration of this clip along its motion path, you must first add Bezier
handles to the keyframes you want to adjust (see “
Creating Curved Motion Paths Using
Bezier Handles ” on page 774). A velocity handle appears as a small purple dot between
the keyframe and the end of the Bezier handle. You can then modify the velocity
handle attached to each Bezier handle for these keyframes.
The velocity of a clip’s motion from one keyframe to the next can be modified by
dragging the velocity handle in and out along the Bezier handle.