Language Integration for ActiveVRML
December 12, 1995
The Active Virtual Reality Modeling Language (ActiveVRML) is designed specifically for multimedia interactive animation. Existing popular programming languages, such as Microsoft® Visual Basic®, Java, and C++, are designed for coding general (nonmultimedia) computations, such as file system management, network protocols, intensive numerical computation, and so on. An important aspect of ActiveVRML is its ability to communicate with programs written in these existing general programming languages. This strategy results in a symbiosis, allowing each language to be used for its strengths. This document provides an overview of how ActiveVRML components interoperate with components written in other applications to form a single, multilingual application.
Application Connections
Although conventional programming languages do not support behaviors as first-class values, most languages do support some form of event modeling. For this reason, ActiveVRML's language integration mechanism is based on events with data.
There are two communication paths between ActiveVRML and imperative programming languages:
- ActiveVRML-originated events. ActiveVRML can notify an imperative program that a particular event has occurred and provide data (parameters) along with this notification. In this case, the imperative program imports an event defined in ActiveVRML and defines its own response code to go with it. The response code is executed when the ActiveVRML event occurs, using the data provided by the event.
- Program-originated events. An imperative program can notify ActiveVRML that an event has occurred and provide data along with this notification. In this case, the imperative program creates an object with a fire method that includes parameterized event data. This object can then be passed to ActiveVRML to be incorporated into a model. The imperative program's object acts as a kind of remote control that triggers event occurrences that affect the model.
The next section develops a hypothetical application that will be used to illustrate these techniques.
Example Connections: The Personal Weather Channel
To further explain how language integration works between ActiveVRML and other programming languages, consider the following example of a weather server. When connected to the weather server, a high-quality image of the user's geographic area is displayed at roughly state or province resolution, annotated with place names. The user clicks a town name, causing an animated diagram to appear, superimposed on the map. The animation is one of the following:
- Sunny: An animated smiling sun.
- Partly cloudy: Wispy, light clouds dance over the town.
- Overcast: Thick clouds billow and roil, mostly obscuring the town.
- Rain: Cartoon rain droplets fall.
- Snow: Richly detailed ice crystals appear and blanket the town.
The obvious language division for this application is to perform the graphics, animation, and user input functions from ActiveVRML and obtain current forecast information over the Internet from an imperative program (a weather information service). There are many possibilities in designing the communication protocol. A very simple protocol consisting of just two event messages could be as follows:
- A select-town event is linked from ActiveVRML to the program. Each time the user selects a town, the name of the town is passed as event data. The program responds by locating a forecast for that town and relaying the information with the update-forecast event. The select-town event also instructs the program to continue to track the forecast for this town, requesting an update-forecast message each time it changes.
- An update-forecast event is linked from the program to ActiveVRML. This event is sent in response to a select-town event and each time the program determines that the forecast has changed. The update-forecast event communicates one of the five forecasts: sunny, partly-cloudy, overcast, rainy, or snowy. In response to this event, ActiveVRML updates the weather animation to reflect the forecast.