What Is Linear Keyframe Interpolation in After Effects

This article provides a comprehensive overview of linear keyframe interpolation in Adobe After Effects, covering its core definition, visual characteristics, and practical application. You will learn how linear interpolation affects the speed and movement of animation properties, how it compares to other interpolation types, and the specific production scenarios where this default keyframe style is best utilized.

Understanding Linear Interpolation

Keyframe interpolation determines how Adobe After Effects calculates the values between two set points in time. When you create a property change using standard keyframes, After Effects connects them using linear interpolation by default.

Linear keyframe interpolation creates a uniform, constant rate of change between two keyframes. This means the transition from point A to point B happens at a mathematically steady pace without any acceleration or deceleration. In the After Effects timeline, linear keyframes are represented by the standard diamond icon.

Characteristics of Linear Motion

Because linear interpolation maintains an identical rate of change across the entire duration of the animation, it produces distinct behavioral traits:

  • Instant Velocity: The animated property reaches its designated travel speed instantly upon passing the first keyframe, with zero ramp-up time.
  • Abrupt Stops: The motion halts abruptly the precise frame it hits the ending keyframe, without trailing off or slowing down.
  • Mechanical Feel: Because physical objects in the real world require time to accelerate and decelerate due to mass, inertia, and friction, linear motion often looks robotic, unnatural, or purely digital.

When viewed in the Graph Editor, linear interpolation displays as a straight, angled line in the Value Graph and a completely flat horizontal line in the Speed Graph.

Linear vs. Bezier and Hold Keyframes

To understand linear interpolation clearly, it helps to contrast it with the other primary interpolation types in After Effects:

  • Bezier Keyframes (Hourglass Icon): Use mathematical curves to smoothly accelerate into and decelerate out of keyframes (commonly referred to as "Easy Ease"). Bezier motion mimics real-world physics.
  • Hold Keyframes (Square Icon): Prevent any intermediate calculation entirely. The property retains its initial value until the playback head reaches the second keyframe, resulting in an instantaneous jump.
  • Linear Keyframes (Diamond Icon): Calculate intermediate values in a strictly direct line without changes in velocity.

Spatial vs. Temporal Linear Interpolation

Interpolation applies to both time and space in After Effects:

  • Temporal Linear Interpolation: Controls the timing and speed of the value change. The rate of progress along the timeline remains entirely constant.
  • Spatial Linear Interpolation: Controls the physical path of 2D or 3D positional movement. When spatial interpolation is linear, an object moves along a rigid, straight line between coordinates without any rounded curves.

Best Use Cases for Linear Keyframes

While animators frequently smooth out motion using Bezier curves, linear keyframe interpolation remains essential in specific situations:

  • Continuous Loops: Objects that cycle indefinitely, such as rotating gears, conveyor belts, clock hands, or continuously scrolling backgrounds, require a constant speed to loop seamlessly without a visible hiccup or stutter.
  • Mechanical and UI Animations: Elements designed to look computer-generated, such as robotic assemblies, progress bars, cursors, and certain technical HUD graphics, benefit from the rigid precision of linear motion.
  • Constant Camera Pans: Fixed-speed tracking shots across a large landscape or text document often require steady linear movement to ensure readability and consistent pacing.

How to Set Keyframes to Linear

If you have converted keyframes to Bezier or Hold types and need to revert them to linear interpolation:

  1. Select the keyframes on your timeline layer.
  2. Hold Ctrl (Windows) or Command (macOS) and click directly on the selected keyframe until the icon returns to a diamond shape.
  3. Alternatively, right-click any selected keyframe, choose Keyframe Interpolation, and change both Temporal Interpolation and Spatial Interpolation back to Linear.