The After Effects expression:

valueAtTime(linear(effect("Slider Control")("Slider"), 0, 100, key(1).time, key(numKeys).time))

is used to control the timing of an animation based on the value of a slider control. Let’s break down what this expression does and provide some examples.

Explanation

effect("Slider Control")("Slider"):

This part references a slider control effect applied to the layer. The slider’s value will drive the timing of the animation.

linear(effect("Slider Control")("Slider"), 0, 100, key(1).time, key(numKeys).time):

• The linear() function maps the value of the slider to a time range between the first keyframe (key(1).time) and the last keyframe (key(numKeys).time).
• As the slider moves from 0 to 100, the expression linearly maps it to a time range defined by the keyframes. This allows you to “scrub” through an animation using the slider control.

valueAtTime(...):

This expression tells After Effects to display the property value at a specific time, determined by the result of the linear() function. Essentially, the slider acts as a controller to play back the animation by mapping slider values to different points in time.

Example Scenarios

Imagine you have a position animation where an object moves from point A to point B over two keyframes.

1. Position Animation:
   • The position property has two keyframes:
     • The first keyframe is at 0 seconds, and the object is at x=100.
     • The second keyframe is at 5 seconds, and the object is at x=500.
   • By applying this expression to the position property with a slider range of 0 to 100, moving the slider will “scrub” through the animation:
     • Slider at 0: Displays the position at 0 seconds (object at x=100).
     • Slider at 50: Displays the position halfway through the animation (approximately at x=300).
     • Slider at 100: Displays the position at 5 seconds (object at x=500).
2. Opacity Animation:
   • Suppose an object’s opacity animates from 0% to 100% over 3 seconds.
   • With this expression and a slider ranging from 0 to 100, you could control the opacity’s animation progress:
     • Slider at 25: Displays opacity at 0.75 seconds.
     • Slider at 75: Displays opacity at 2.25 seconds.
   • Moving the slider will allow you to scrub through the opacity transition smoothly.

Practical Uses

This expression is beneficial for:

• Creating custom controllers for time-based animations.
• Allowing users to scrub through animations via a slider.
• Giving users fine-tuned control over animation timing without changing the actual keyframes.

This setup makes animation control more intuitive, especially for complex projects, by providing a single slider interface to scrub through keyframe timing.