Base Application Object Interface
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Base Application Object Interface

Within this chapter, we provide a brief overview of the member functions from the base VR Juggler application interface. This interface is defined by vjApp, and the member functions are shown in Figure 2.3. Refer to Figure 2.2 for a visual presentation of the order in which the methods are invoked.

The base interface of the application object defines the following functions:

As previously described, the VR Juggler kernel calls these functions from its control loop to allocate processing time to them. These functions handle initialization and computation. Other member functions that can be used for reconfiguration, focus control, resetting, and exiting will be covered later in this book.

Figure 2.3. Application object interface

Initialization

The following is a description of the application objects related to the initialization of a VR Juggler application. The order of presentation is the same as the order of execution when the application is executed by the kernel.

vjApp::init()

The init() method is called by the kernel to initialize any application data. When the kernel prepares to start a new application, it first calls init() to signal the application that it is about to be executed.

Timing

This member function is called immediately after the kernel is told to start running the application and before any graphics API handling has been started by VR Juggler.

Uses

Typical applications will utilize this method to load data files, create lookup tables, or perform some steps that should be done only once per execution. In other words, this method is the place to perform any pre-processing steps needed by the application to set up its data structures.

vjApp::apiInit()

This member function is for any graphics API-specific initialization required by the application. Data members that cannot be initialized until after the graphics API is started should be initialized here.

Note

In OpenGL, there is no concept of initializing the API, so this method is not normally used in such applications.

Timing

This member function is called after the graphics API has been started but before the kernel frame is started.

Uses

In most cases, scene graph loading and other API-specific initialization should be done in this method.

Frame Functions

Once the application object has been initialized by the VR Juggler kernel, the kernel frame loop begins. Each frame, there are specific application object methods that are invoked, and understanding the timing and potential uses of these methods can improve the functionality of the immersive application. In some cases, it is possible to use these member functions to optimize the application to improve the frame rate and the level of interactivity.

vjApp::preFrame()

The preFrame() method is called when the system is about to trigger drawing. This is the time that the application object should do any last-minute updates of data based on input device status. It is best to avoid doing any time-consuming computation in this method. The time used in this method contributes to the overall device latency in the system. The devices will not be re-sampled before rendering begins.

Timing

This method is called immediately before triggering rendering of the current frame.

Uses

In general, this method should be reserved for “last-millisecond” data updates in response to device input (latency-critical code).

vjApp::intraFrame()

The code in this method executes in parallel with the rendering method. That is, it executes while the current frame is being drawn. This is the place to put any processing that can be done in advance for the next frame. By doing parallel processing in this method, the application can increase its frame rate because drawing and computation can be parallelized. Special care must be taken to ensure that any data being used for rendering does not change while rendering is happening. One method for doing this is buffering. Use of synchronization primitives is not recommended because that technique could lower the frame rate.

Timing

This method is invoked after rendering has been triggered but before the rendering has finished.

Uses

The primary use of this method is performing time-consuming computations, the results of which can be used in the next frame.

vjApp::postFrame()

Finally, the postFrame() method is available for final processing at the end of the kernel frame loop. This is a good place to do any data updates that are not dependent upon input data and cannot be overlapped with the rendering process (see the discussion on vjApp::intraFrame() above).

Timing

This method is invoked after rendering has completed but before VR Juggler updates devices and other internal data.

Uses

Some possible uses of this method include “cleaning up” after the frame has been rendered or synchronizing with external networking or computational processes.

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