In-Betweening (Tweening)

Definition - What does In-Betweening (Tweening) mean?

Inbetweening is the process of creating transitional frames between two separate objects in order to show the appearance of movement and evolution of the first object into the second object. It is a common technique used in many types of animation. The frames between the key frames (the first and last frames of the animation) are called “inbetweens” and they help make the illusion of fluid motion.

Sophisticated animation software have complex algorithms that identify the key frames in an image and define how the transition goes, creating the inbetweens for the tweening process and completing the animation. This is done by interpolating graphics parameters or data. On the other hand, all of this can be done manually, which is what animators often do as they need more control of the process than what an automatic algorithm can afford. Some animation software that automate the inbetweening process still allow manual means of editing each inbetween frame so that the animator can be sure that the movement is fluid and lifelike or whatever demand is.

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Techopedia explains In-Betweening (Tweening)

Inbetweening is a technique used in animation. Two images are used as the key frames which serve as the beginning and the ending of the animation sequence. The process is about taking those two key frames and filling in the animation or frames in between. These inbetweens are what makes the animation look fluid.

If you are making an animation about a man walking, you would probably have the following frames:

Man standing straight as initial key frame
The man is now shown slightly lifting his right leg
The leg is now half way to a full bend
The leg is now fully bent
The leg is starting to unbend forward
and so on
Until you get to the final key frame of the step which is a full stride

Without the inbetweens the animation would look so jugged and jittery. It helps just to have the key frames of the beginning and end of the motion as this gives the animator a very good idea of how the frames in between should look like.