This Processing applet called Lineplay implements a little bit better the ideas discussed in the previous post on real-time animation of freehand strokes.
The program takes as input freehand line strokes, which are converted into Bezier curves. The control points in the curves are connected to each other using spring and damping forces, in order to preserve the overall shape of the stroke. Then the anchor points can be used to animate the curves: by moving the anchors around, their motions are recorded into small loops which are repeated continuously. The curves are connected to the anchor points when the strokes go through them, again using spring and damping forces. In this way, translations in the anchors are transmitted to the whole curve.
The results of this little demo can bear some resemblance to the animations produced by Sodaplay, but instead of explicitly connecting points, Lineplay automatically converts freehand strokes into “springy” Bezier curves.