If you ask most people what a tween is, they may tell you that it is a child between the ages of 10-12; an age before their teen age. However, there is another use for the word which means, loosely, in-between frames of animation.
Animation is arduous work. Anyone familiar with stop motion animation knows that to make an appearance of motion, you take a picture onto film of your performer (be it clay, or any other ordinarily inanimate object), move it a bit, and then take another picture. If the movements of each frame is minute enough, when played back at 20-30 of those pictures per second, the illusion of motion is made. Much sweat and hard work is made though per each second! Further, imagine you have to move a toy car four feet in a particular direction in one second and you had 24 pictures to get from the first picture to the end of that first second. You’d have to move the toy 24 times to enable that car to smoothly go four feet!
The advent of the computer has made this easier with some help from Nestor Burtnyk and Marceli Wein who came up with the idea of key frames, an Academy Award winning idea, according to Wikipedia’s article, Inbetweening. If we were to take our toy car example and instead substitute it with a virtual toy car created in the computer we can start a process of animation far easier. The first virtual picture of the toy car we could start at the beginning of the four feet (in this case a virtual four feet). In our animation key frame capable software we would declare this first frame the key frame. We could then use the software to change the frame to the last of our animation, in our original example, the 24th frame.
We can then move the car to its final destination four feet away. We then assign this last frame as a key frame as well. The computer now will automatically figure out all the movements that the car needed from the start to the finish. Frame two has the car move just a tiny bit, three a bit more all calculated precisely be the computer to get it to the finish. Does the movement seem too simplistic? Would you like the car to veer to the right or left on its way to the finish line? It’s easy to add that complexity. Mid-point, in this example at frame 12, create another key frame. At this point, we already have the car at about two feet, halfway to our destination. Keep the car at mid point but move it to the left or right and create another key frame. Now if you played the animation, the computer would calculate the car smoothly going from start to finish veering slightly to the left or right then back in place to the destination.
Tweening with this concept of key frames is extremely popular and is used in such software packages as Synfig (free at this writing!), Hash’s Animation Master, Adobe’s Flash, SmithMicro’s Anime Studio series and many more. You can basically tell your friends that you’ve been slaving over thousands of frames of animation, when you’ve actually been modifying a much smaller amount of key frames and you’ve let the computer do most of the difficult work for you.
Source: Author(s): Contributors to the Wikipedia Project. Title: Inbetween Website: Wikipedia Web address: http://en.wikipedia.org/wiki/Inbetweening