Keyframe systems were developed by classical animators such as Walt Disney. An expert animator would design (choreograph) an animation by drawing certain intermediate frames, called Keyframes. Then other animators would draw the in-between frames.
The sequence of steps to produce a full animation would be as follows:
1. Develop a script or story for the animation
2. Lay out a storyboard, that is a sequence of informal drawings that shows the form, structure, and story of the animation.
3. Record a soundtrack
4. Produce a detailed layout of the action.
5. Correlate the layout with the soundtrack.
6. Create the "keyframes" of the animation. The keyframes are those where the entities to be animated are in positions such that intermediate positions can be easily inferred.
7. Fill in the intermediate frames (called "inbetweening" or "tweening").
8. Make a trial "film" called a "pencil test"
9. Transfer the pencil test frames to sheets of acetate film, called "cels". These may have multiple planes, e.g., a static background with an animated foreground.
10. The cels are then assembled into a sequence and filmed
With computers, the animator would specify the keyframes and the computer would draw the in-between frames ( "tweening"). Many different parameters can be interpolated but care must be taken in such interpolations if the motion is to look "real". For example in the rotation of a line, the angle should be interpolated rather than the 2D position of the line endpoint.
The simplest type of interpolation is linear, i.e., the computer interpolates points along a straight line. A better method is to use cubic splines for interpolation. Here, the animator can interactively construct the spline and then view the animation.
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