By recording the effects of manipulations, animations can be sketched. In order to be able to edit the captured performance, a compact representation of continuous parameter evolution must be obtained. This representation must not only precisely approximate the shape of the initial parameter curves but also their timing. The data reduction algorithm must therefore treat the geometry and time components simultaneously in order to avoid the introduction of errors that would be difficult to control. We have developed an algorithm that incrementally builds, from the input sequence, a parametric B-spline preserving value and time of each input sample within a given tolerance. It is an incremental version of the Lyche and Mørken algorithm [22] that works in parallel with the interactive specification by considering only a small portion of the input curve at any time. Latency time and memory requirements for handling each portion of the curve are constant. Data reduction
may therefore be performed concurrently with interactive parameter input, and the responsiveness of the application can be ensured when handling animations defined by any number of samples. The algorithm is presented in detail in [2; 4]. This performance-based approach complements key-framing by providing the ability to create animations with straight-ahead actions. It provides complete control over the animation shape and timing, while key-framing offers control only at a limited number
of points.