Using the multi-layer model for building interactive graphical applications

Most interactive graphical applications that use direct manipulation are built with low-level libraries such as Xlib because the graphic and interaction models of higher-level toolkits such as Motif are not extensible. This results in high design, development and maintenance costs and encourages the development of stereotyped applications based on buttons, menus and dialogue boxes instead of direct manipulation of the applications objects. In this article we argue that these drawbacks come from the fact that high-level toolkits rely on a visualization model to manage interaction. We introduce a model that uses several graphical layers to separate the graphic entities involved in visualization from those involved in feedback and interaction management. We describe the implementation of this MultiLayer Model and we show how it can take advantage of software and hardware graphic extensions to provide good performance. We also show how it supports multiple input devices and simplifies the description of a wide variety of interaction styles. Finally, we describe our experience in using this model to implement a set of editors for a professional animation system.