2. Program Description

The Distributed Graphics Laboratory (DGL) has been designed and implemented according to the requirements stated in section 1.5. This chapter describes the laboratory's functionality in detail as well as how it is used. The implemented functions are explained and demonstrated with examples. However, before we start the tour through the laboratory, a few terms should be explained:

• User: The user can access the distributed graphics laboratory from anywhere on the Internet. A user is identified by a unique user name and a password. Users are further categorized into standard users and super users. Standard users are not allowed to modify objects created by other users. Super users however, can access and modify any object. In addition, certain administrative operations are executable by super users only. An administrator must create an account before a user can access the laboratory. How this is done is explained in the Installation Guide (see Appendix A).

• Session: The distributed graphics laboratory manages a shared space, which is divided into sessions. Each session represents a conference or meeting. It is possible to create a new and empty session at any time, which can then be attended by further users. Sessions stay alive even if all session members leave the session. That makes it possible for the group of users to take a recess and to continue the session later. It is also possible for instructors to prepare material within the session that can be accessed and studied by students later. The instructor does not have to be present in the session at this time. If the session is not needed anymore, it can be terminated by a super user or by the creator of the session.

• Shared Objects: Each session consists of numerous shared objects, which are commonly accessible to every user. There are different types of shared objects, such as images, drawings, annotations and the group discussion^2.1. The shared objects follow the concept of WYSIWIS^2.2. That means that every user sees the same set of identical objects. If one user changes an object, then this object is updated and redisplayed for all other users.

• Client: The client is an application through which a user can access the distributed graphics laboratory. There are two versions of clients. The user can either use a Java client applet embedded in a web page or a stand-alone Java application. The functionality of both versions is identical. If the laboratory should be included in a web-based system, the applet version is recommended. When this is not the case, the stand-alone version should be used because it is faster than the applet version.

• Server: The server, the heart of the system, manages the distributed laboratory. Every client is connected to the server over the network in order to gain access to a shared session. The communication and interaction between clients is tunneled through the server.