Slow In and Slow Out
Part of: The 12 Principles of Animation
Slow in and slow out is one of the 12 principles of animation: the practice of distributing frames within an action so the movement accelerates at the start and decelerates at the end, mimicking how real objects rarely move at a constant rate.
Almost everything in the physical world moves with a slow in and slow out. A pendulum slows at the top of its arc and speeds through the bottom. A character starting to walk does not jolt to full speed instantly. A door does not swing closed at constant velocity. Animation that ignores this principle reads as mechanical, even when every other element is right.
In a hand-drawn pipeline, slow in and slow out is achieved through the spacing of in-between drawings. Drawings packed close together read as slow motion; drawings spaced far apart read as fast. By packing drawings near the start and end of an action, and spacing them through the middle, the animator creates the slow-in-slow-out feel.
In 3D animation, the principle lives in the animation curves of the graph editor. Linear curves produce constant-rate motion; eased curves produce slow-in-slow-out. Most animator time in 3D is spent shaping these curves, not blocking the keys.
Slow in and slow out is one of the workhorse principles. It is rarely the most visible decision an animator makes, but its absence is immediately felt. On every project we work on, including Inchstones and A Modern Fairytale, this principle is part of the daily craft.
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Sources
Academic papers, recognised industry standards, and canonical industry texts that back up claims in this entry.
- The Illusion of Life: Disney Animation. Thomas, Johnston, Walt Disney Productions, 1981Supports: 12 principles historical origin
- The Animator's Survival Kit. Williams, Faber & Faber, 2001Supports: hand-drawn in-between spacing
- Computer Animation: Algorithms and Techniques. Parent, Addison-Wesley, 2002Supports: technical easing deceleration
Frequently asked questions
Is slow in and slow out always desirable?
Almost always. The exceptions are deliberate stylistic choices, like the snap-cut feel of certain motion graphics or the constant-rate motion of some retro animation styles. Even then, the absence is a deliberate choice rather than an oversight. The default for character animation is to apply it.
How does this apply to 3D animation?
Through the shape of the animation curves in the graph editor. Most 3D animator time is spent refining these curves rather than placing keys, and slow-in-slow-out shaping is the heart of that work.
What about motion design?
Equally important. Easing curves in motion design (often expressed as cubic bezier curves) are exactly slow-in-slow-out applied to graphic elements rather than characters. The principle is universal across animation disciplines.