Since VR Juggler applications are objects, developers do not write the traditional main() function. Instead, developers create an application object that implements a set of pre-defined interfaces. The VR Juggler kernel controls the application's processing time by calling the object's interface implementation methods.

In traditional programs, the main() function defines the point where the thread of control enters the application. After the main() function is called, the application starts performing any necessary processing. When the operating system (OS) starts the program, it gives the main() function some unit of processing time. After the time unit (quantum) for the process expires, the OS performs what is called a “context switch” to change control to another process. VR Juggler achieves similar functionality but in a slightly different manner.

The application objects correspond to processes in a normal OS. The kernel is the scheduler, and it allocates time to an application by invoking the methods of the application object. Because the kernel has additional information about the resources needed by the applications, it maintains a very strict schedule to define when the application is granted processing time. This is the basis to maintain coherence across the system.

The first step in defining an application object is to implement the basic interfaces defined by the kernel and the Draw Managers. There is a base class for the interface that the kernel expects (vrj::App) and a base class handled by each Draw Manager interface (vrj::PfApp, vrj::GlApp, etc.). See Figure 2.1, “vrj::App hierarchy” for a visual representation of the complete application interface hierarchy. The interface defined in vrj::App specifies methods for initialization, shutdown, and execution of the application. This is the abstract type that is seen by the VR Juggler kernel. The Draw Manager interfaces specified in the vrj::*App classes define the API-specific functions necessary to render the virtual environment. For example, an interface used by a Draw Manager could have functions for drawing the scene and for initializing context-specific information.

Figure 2.1. vrj::App hierarchy

To implement an application in VR Juggler, developers simply need to “fill in the blanks” of the appropriate interfaces. To simplify this process, there are default implementations of most methods in the interfaces. Hence, the user must only provide implementations for the aspects they want to customize. If an implementation is not provided in the user application object, the default is used, but it is important to know that in most cases, the default implementation does nothing.