DIVE TCL Interface

DRAFT
Principles and Guidelines

Emmanuel Frécon
The Swedish Institute of Computer Science
Stockholm March 28, 1995

This text describes the guidelines of a recent effort which tends to replace the current behaviours attached to objects by scripts written in a "real" programming language. Even if the recent developments of the behaviours add testing guards in state transitions, and thus making loops possible, it appeared to us that further enhancements would lead to the development of our own programming language. Instead, we currently try to attach scripts written in a programming shell language (currently TCL) to objects.

TCL has been choosen for a number of reasons, among which:

• ease of use. TCL is a mixture between C and shell programming. It is spreading rapidly among the computing community, which means that more and more know how to use it.
  
• TCL is open. It is easy to add new commands to TCL.
• TCL is easy to connect to C.
• TCL is composed of a very small and fast kernel, which makes it possible to add to DIVE applications without much overhead.
• If the DIVE X interface is extended with Tk, TCL is the companion of TK.
  
• Having a TK version of DIVE and TCL commands to access and interact with DIVE would open DIVE to other applications, using the TK send command.

Programming behaviours in TCL is not anymore based on a state machine. Instead, it is done in a way similar to programming a DIVE application. Registered TCL procedures are called each time a DIVE event occurs. There are no limit to the number of procedures an object can register for a given event. As in C, and opposite to current behaviours, all the arguments carried by the incoming event are given to the procedure as arguments. For example, a procedure listening to incoming interaction signals can know which person has sent the signal, and which object (hand) it is coming from.

Scripts can use the usual looping and testing statements of TCL. They can also use local variables (global variables should use DIVE properties). In addition, specific DIVE commands are offered to them, most of them correspond to the distr_ functions, some others can access DIVE database. Follows an open-ended list:

• All translations and rotation functions, in whatever coordinate system, both in absolute or relative coordinates.
• All object attribute modification functions.
• All object hierarchy modification functions (i.e. adding and removing subs).
  
• Reading a vr file.
• Playing a sound.
• Getting a random number.
• Some DIVE event generation functions, namely sending behaviour signals.
  
• A set of functions to access the DIVE database: iterating among all the objects, among all the children, all the hierarchy under an object, finding parents and ancestors, finding object by their name and getting as much information as possible about a person or an object.
  
• Operating with properties.
• Sending TCL commands to other objects.

Given such a set of commands, vishnu (the successor of the visualizer) can offer, at no cost, a TCL prompt in its starting window: it is then possible to interactively enter DIVE and TCL commands and watch the result.

PS: Note about properties: Properties are variables of any type and name that DIVE application can leave in objects and distribute. Standard types such as int, strings, ... and also point, object identifiers, ... are supported; new types, only supported by some applications running in worlds are also possible.