Tweening, short for in-betweening, is the process of generating the intermediate frames between two defined keyframes in an animation, creating smooth motion by filling in the transitional positions between a starting state and an ending state. In traditional hand-drawn animation, in-betweening was the task of junior animators who drew the frames connecting the key poses established by lead animators. In digital animation, tweening is performed automatically by software based on interpolation algorithms.

Digital tweening calculates the position, size, rotation, opacity, and other properties of an animated element at every frame between two keyframes, using different interpolation methods to control the character of the motion. Linear tweening moves at constant speed from start to finish. Ease-in starts slowly and accelerates. Ease-out starts fast and decelerates. Ease-in-out combines both, creating motion that accelerates from the first keyframe and decelerates into the second - the most natural-feeling motion for most animated objects. The principles behind tweening are central to all animation software, from Adobe After Effects and Animate to 3D tools like Maya and Blender, and the quality of tweening - how naturally the interpolated motion reads - is a significant factor in whether animation feels mechanical or organic.

In AI video generation, tweening concepts apply when thinking about how the model transitions between states within a generated clip. When generating a transformation or any shot with clear start and end states, the way the model interpolates between those states is essentially performing an automated form of tweening - and describing the desired quality of that motion (smooth, gradual, easing naturally) in the prompt helps shape how fluidly the model moves between the defined visual states.