Inbetweening

• 1. Introduction

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  Inbetweening is the quiet craft that makes cartoons feel alive. Without it, animation would be a series of frozen poses, each image snapping abruptly to the next. The whole illusion of a character running, jumping, or even blinking depends on smooth transitional frames called inbetweens, slotted precisely between two keyframes.

  What makes inbetweening remarkable is how old and yet how technical it is. Dick Huemer developed the traditional system in the 1920s, and its basic logic still runs through every animated film and digital interface today. The questions worth exploring: who actually does this work, how much of it is handcraft versus computation, and what happens when the number of drawings per second drops too low?

• 2. The Studio Assembly Line

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  Dick Huemer built the inbetweening workflow in the 1920s as a way to divide labor without losing coherence. Art Davis is credited as the first inbetweener, meaning the first person to hold that role as a dedicated studio position. The division of responsibility that Huemer created proved efficient enough to survive for a century.

  In large studios, a primary artist draws the key frames that define the essential poses of a movement. After rough animation is tested and approved, the scene moves to assistants, who clean up the drawings and add breakdowns. Breakdowns are intermediate drawings that define movement in greater detail, sitting between the keyframes. The final pass belongs to the inbetweener, whose job is to complete the remaining gaps. Small animation teams collapse this hierarchy entirely, with a single animator carrying out every stage.

• 3. Ones, Twos, and the Limits of Illusion

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  One second of film requires 24 frames, but most animated movement does not need all 24 drawings. Animating "on twos" means producing one drawing for every two frames, yielding 12 drawings per second. That rate has been the standard for over 100 years. Fantasmagorie, the 1908 film widely considered the first fully animated movie, was animated on twos.

  Only the fastest movements demand 24 drawings per second, known as animating "on ones". Computer-generated animation defaults to ones as a matter of course. At the other extreme, dropping to roughly four drawings per second risks breaking the illusion of movement altogether. Modern productions handle this range fluidly. A character in a panning shot might be animated on twos while the entire background shifts every frame on ones, creating the sensation of camera movement without doubling the drawing workload. Optical effects like motion blur can also simulate the appearance of a higher frame rate without adding drawings.

• 4. Digital Tweening and the Software Era

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  Nestor Burtnyk and Marceli Wein at the National Research Council of Canada expanded what computers could do for inbetweening. Their work was recognized with a Technical Achievement Academy Award in 1997, given for pioneering work in the development of software techniques for computer-assisted keyframing for character animation.

  In a digital context, the process is called tweening and the resulting sequence is called a tween. Software allows animators to define how objects should move and change, either by manually adjusting transitional frames or by letting the program interpolate graphic parameters automatically. Among the earliest tools to use automatic interpolation were Adobe Flash and Animo, developed by Cambridge Animation Systems, both appearing in the late 1990s. Tweenmaker followed around 2006. The free program Synfig specializes specifically in automated tweening.

  Ease-in and ease-out are the digital controls that govern the physics of a transition. An ease-in animation begins slowly and accelerates; ease-out does the reverse. Both terms describe the linearity of the tween, allowing animators to shape timing without drawing every intermediate frame by hand.