A Visual Programming Model for User Interface Animation

havior; its smoothness corresponds to our perception of motion and changes in the real world. In a UI, animation may illustrate the dynamic behavior of both interacting entities: the user and the underlying application. When changes are illustrated by a sudden switch from one visual configuration to another, the user has to deduce the causal link between the two states. Continuous animation provides object constancy and replaces this cognitive effort by a simple perceptual activity [10]. Moreover, animation is a very efficient way of conveying information in UIs. Animation is a natural way to represent dynamic behavior, but the lack of models and the difficulty of programming animation have limited its introduction into the user interface (UI). Visual programming techniques may greatly simplify animation creation, but most of the current approaches are limited to a specific application class. This paper describes HandMove, a general and powerful model for UI animation, which allows visual programming. HandMove is able to express and to homogeneously combine various UI animation types: spontaneous, interactive and application-controlled animation. Its visual model extends path-based specification to describe continuous and sudden changes, dynamic trajectory modification, communication between objects, etc. The article also proposes a formal definition of the HandMove model as a visual programming language. With few exceptions, current UI animation models are restricted to a specific application. The bipolarity of human-computer interaction may suggest to classify such applications into two categories the first one is centred on program behavior and the second one on user actions. The first class includes well-known animation applications: simulation of dynamic systems [8], algorithm animation [2] [11], data visualization [10]. The second category implements animated feedback to user actions in order to provide a more realistic interaction [4] [14]. A third category may be added to this classification : predefined UI animation, not controlled either by the user or by the underlying application. This case is typically found in animated presentations or animated help [1] [13].