Inbetweening or tweening is a key process in all types of animation, including computer animation. It is the process of generating intermediate frames between two images, called key frames, to give the appearance that the first image evolves smoothly into the second image. Inbetweens are the drawings which create the illusion of motion.
Traditional inbetweening involves the use of light tables to draw a set of pencil-on-paper pictures.
In the inbetweening workflow of traditional hand-drawn animation, the senior or key artist would draw the keyframes which define the movement, then, after testing and approval of the rough animation, would hand over the scene to their assistant. The assistant does the clean-up and the necessary inbetweens, or, in large studios, only some breakdowns which define the movement in more detail, before handing down the scene to their assistant, the inbetweener, who does the rest.
Typically, an animator does not draw inbetweens for all 24 frames required for one second of film. Only very fast movements require animation "on ones", as it is called. Most movements can be done with 12 drawings per second, which is called animating "on twos". When the number of inbetweens is too few, such as 4 frames per second, it will begin to lose the illusion of movement altogether. Computer-generated animation is usually animated on ones. The decision about the number of inbetweens is also an artistic one, as certain styles of animation require a not-so-smooth fashion of movement. Animation "on twos" dates to the dawn of animation – Fantasmagorie (1908), widely considered the first fully animated movie, was animated on twos.
Modern animation will use various techniques to adapt the framerate to the current shot. Slow movements may be animated on threes or fours. Different components of a shot might be animated at different framerates – for example, a character in a panning shot might be animated on twos, while everything in the shot is shifted every frame to accomplish the pan. Optical effects such as motion blur may be used to simulate the appearance of a higher framerate.
When animating in a digital context, the shortened term tweening is commonly used, and the resulting sequence of frames is called a tween. Sophisticated animation software enables the animator to specify objects in an image and define how they should move and change during the tweening process. Software may be used to manually render or adjust transitional frames by hand, or may be used to automatically render transitional frames using interpolation of graphic parameters. The free software program Synfig specializes in automated tweening.
"Ease-in" and "ease-out" in digital animation typically refer to a mechanism for defining the physics of the transition between two animation states, i.e., the linearity of a tween.
The use of computers for inbetweening was enhanced by Nestor Burtnyk and Marceli Wein at the National Research Council of Canada. They received a Technical Achievement Academy Award in 1997, for "pioneering work in the development of software techniques for computer-assisted key framing for character animation".