Tweening

Definition

Tweening (short for “in-betweening”) is the animation process of creating intermediate frames—“tweens”—between two keyframes to produce the illusion of seamless motion. By interpolating position, rotation, scale and other attribute values over time, tweening fills the gaps that key poses define, reducing manual drawing and accelerating production.

Historical Context

Long before digital tools, traditional animators hand-drew every in-between frame between major character poses. Pioneers like Disney’s “Nine Old Men” refined timing charts to guide junior artists in drawing consistent tweens. With the advent of computer animation in the late 20th century, software such as Autodesk’s 1990s-era packages and later Adobe Flash automated much of this labor, allowing artists to specify start and end poses and let algorithms generate hundreds of intermediate frames in seconds.

Technical Process

1. Keyframe Definition: Animators set critical poses at specific frames.
2. Curve Interpolation: The software creates parameter curves (e.g., Bezier, linear) for translation, rotation and deformation channels.
3. Frame Generation: Intermediate frames are computed by evaluating curves at each time step, producing smooth motion between keys.
4. Manual Refinement: Artists adjust problematic tweens—such as unexpected arcs or volume loss—by adding breakdown poses or tweaking control tangents.

Tweening can be applied to 2D vector artwork, 3D skeletal rigs, particle systems and even camera movements. Motion-graphic designers rely on tweening principles to animate UI elements and text transitions.

Applications & Variants

• Automatic Tweening: Software-driven interpolation with minimal user input, common in 2D Flash and After Effects.
• Spline Tweening: Using spline curves for natural acceleration and deceleration (ease-in/ease-out).
• Morph Tweening: In vector animation, shapes gradually transform into new silhouettes by interpolating anchor points.

Whether generating lip-sync movements, camera pans or complex character arcs, tweening remains the backbone of modern animation workflows.

Trivia

• Early Disney studios employed full-time “in-betweeners” whose sole job was to draw tweens between master animator keys.
• Some veteran animators still prefer hand-drawing critical in-betweens to preserve artistic nuance that algorithms can’t replicate.