Factors affecting rate of change (speed and velocity)

The terms speed and velocity are usually used interchangeably to refer to changes in rate of acceleration. After Effects uses the terms speed and velocity in a more precise manner: velocity is the rate of change for temporal values and speed is the rate of change for spatial values.

You can fine-tune nearly all changes over time using the Speed graph or Velocity graph in the Timeline window. The Speed graph provides complete information about and control of the value and rate of change for all spatial values (such as Position) at any frame in a composition. The Velocity graph provides complete information about and control of the value and rate of change for all nonspatial values (such as Opacity) at any frame in a composition.


Velocity (nonspatial) controls A. Value at the current-time indicator B. Maximum, current, and minimum speeds C. Value graph D. Direction handle (controls speed)

The change of velocity or speed over time is affected by the following factors:

• The time difference between keyframes in the Timeline window. The shorter the time interval between keyframes, the more quickly the layer has to change before reaching the next keyframe value. If the interval is longer, the layer changes more slowly, because it must make the change over a longer period of time. Use distance to adjust the rate of change by moving keyframes forward or backward along the timeline.
• The difference between the values of adjacent keyframes. A large difference between keyframe values, such as the difference between 75% and 20% opacity, creates a faster rate of change than a smaller difference, such as the difference between 30% and 20% opacity. Use value differences to adjust the rate of change by increasing or decreasing the value of a layer property at a keyframe.
• The interpolation type applied for a keyframe. For example, it is difficult to make a value change smoothly through a keyframe when the keyframe is set to Linear interpolation; but at any time, you can switch to Bezier interpolation, which provides a smooth change through a keyframe. If you use Bezier interpolation, you can adjust the rate of change even more precisely using direction handles. (See Fine-tuning speed and velocity.)

Linear interpolation (left) causes sharp changes. Bezier interpolation (right) creates smoother change.