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VR-Link: HLA & DIS Simulation Networking

The World's Leading Simulation Networking Toolkit

The VR-Link toolkit provides an easy way to network simulators and other virtual reality applications. It uses a protocol independent API that abstracts away specific networking details and provides your software with support for industry standard High Level Architecture (HLA) and the Distributed Interactive Simulation (DIS) protocols, including HLA 1.3, HLA 1516, HLA Evolved, DIS, and DIS 7.

Protocol-Independent API

VR-Link saves development dollars by providing a single documented C++ API that abstracts away the details of the networking protocols. When you write your code to the VR-Link API, your applications become natively compliant with DIS, DIS 7, HLA 1.3, IEEE 1516-2000, and HLA Evolved. For example, most applications that already use VR-Link to support HLA 1.3 can switch to IEEE 1516-2000 or HLA Evolved by merely recompiling.


API Examples

The first example is a listen-only application that observes an exercise without simulating any entities on the network. The second is a send-only program that does not process information about remote entities. The source for both examples is included with VR-Link.

The Listen Example

This example illustrates a simple, listen-only VR-Link application. This application can be compiled for DIS or HLA, and contains no protocol-specific code.

With each iteration of the loop, the program prints an entity's updated, dead-reckoned position in topographic coordinates. In addition, if a fire PDU or interaction is detected on the network, the program prints a message showing the entity ID of the attacker.

1 // Define a callback to process fire interactions
2 void fireCb(DtFireInteraction* fire, void* usr)
3 {
4    std::cout << "Fire Interaction from " << fire->attackerId().string() << std::endl;
5 }
7 int main(int argc, char** argv)
8 {
9    // Create a connection to the exercise or federation execution
10   // based on initialization arguments passed as command-line arguments
11   // This causes the application to connect to the DIS network,
12   // HLA execution.
14   DtVrlApplicationInitializer appInit(argc, argv, "VR-Link Listen");
16   DtExerciseConn exConn(appInit);
18   // Register a callback to handle fire interactions
19   DtFireInteraction::addCallback(&exConn, fireCb, NULL);
21   // Create an object to manage remote entities.
22   DtReflectedEntityList rel(&exConn);
24   while (1)
25   {
26     // Tell VR-Link the current value of simulation time
27     exConn.clock()->setSimTime(exConn.clock()->elapsedRealTime());
28     // Process any incoming messages
29     exConn.drainInput();
31     // Find the first entity in the reflected entity list
32     DtReflectedEntity *first = rel.first();
34     if (first)
35     {
36       // Grab its state repository, where we can inspect its data
37       DtEntityStateRepository *esr = first->entityStateRep();
39       // Print the position
40       std::cout << "Position: " << esr->location().string() << std::endl;
41     }
43     // Sleep till next iteration
44     DtSleep(0.1);
45   }
46 }

Connecting to an Exercise

In lines 14-16, the program creates a DtExerciseConnection. This connection serves as the program's interface to an exercise. Depending on whether the application is compiled for DIS, HLA 1.3, HLA 1516-2000, or IEEE 1516-2010, the DtExerciseConn constructor will ask the DtVrlApplicationInitializer for the appropriate initialization parameters, such as DIS port number or the HLA federation execution name. If you do not want to use VR-Link's command-line and configuration file-based initializer class, you can call alternate, protocol-specific constructors to create a DtExerciseConn.

Managing State and Interaction Information

Applications based on VR-Link typically use callbacks to handle incoming interactions such as fire, detonations, and collisions. For example, a callback named fireCb is registered with the DtFireInteraction class at line 19. This callback (defined at line 2) prints a message containing the attacker ID. It executes whenever the exercise connection receives a Fire interaction or PDU during a call to drain- Input().

Tracking Entities

We create a reflected entity list in line 22 to keep track of entities found on the network. The entity list tracks the arrival and departure of entities, performs dead reckoning, manages time outs, and performs other entity-tracking tasks.

Listening to the Network

At the start of each iteration, the program sets VR-Link simulation time (line 27) to provide a common time value for use by time-related operations that occur within an iteration of the loop (such as the dead-reckoning of multiple entities). The drainInput() call (line 29) reads and processes any messages arriving through the exercise connection. This call triggers the execution, if needed, of any callbacks you have registered for that exercise connection. In HLA, this is where the RTI gets ticked. In line 32, the program finds the first entity in the entity list, then in line 37, retrieves the pointer to the entity's entity state repository, where VR-Link stores the values describing the remote entity's state. Line 40 obtains and prints the dead-reckoned entity location.

The Send Example

This example illustrates a simple send-only application that simulates the flight of an F18 aircraft. The program begins by sending an HLA fire interaction or a DIS fire PDU. Thereafter, the F18 flies north for 10 seconds, updating its position by sending HLA attribute updates or DIS entity state PDUs.

1 int main(int argc, char** argv)
2 {
3    // Create a connection to the exercise or federation execution
4    // based on initialization arguments passed as command-line arguments
5    // This causes the application to connect to the DIS network or
6    // HLA execution.
8    DtVrlApplicationInitializer appInit(argc, argv, "VR-Link Listen");
10   DtExerciseConn exConn(appInit);
12   DtEntityType f18Type(DtPlatform, DtPlatformDomainAir,
13   DtUnitedStates, DtFighter, DtF18, 0, 0);
15   // Create an entity publisher for the entity we are simulating
16   DtEntityPublisher f18EntityPub(f18Type, &exConn, DtDrDrmRvw, DtForceFriendly);
18   // Hold on to its state repository, where we can set data
19   DtEntityStateRepository *esr = f18EntityPub.entityStateRep();
21   // Create a topographic view on the state repository, so we
22   // can set position information in topographic coordinates
23   double refLatitude = DtDeg2Rad( 35.699760);
24   double refLongitude = DtDeg2Rad(-121.326577);
25   DtTopoView f18TopoView(esr, refLatitude, refLongitude);
27   // We can use the ESR to set state
28   esr->setMarkingText("VR-Link");
30   DtVector position(0, 0, -100);
31   DtVector velocity(20, 0, 0);
33   // Send a Fire Interaction
34   DtFireInteraction fire;
35   fire.setAttackerId(f18EntityPub.globalId());
36   exConn.sendStamped(fire);
38   // Main loop
39   DtTime timestep = 0.05;
40   DtTime simTime = 0;
41   while (simTime <= 10.0)
42   {
43      // Tell VR-Link the current value of simulation time
44      exConn.clock()->setSimTime(simTime);
46      // Process any incoming messages
47      exConn.drainInput();
49     // Set the current position information
50     f18TopoView.setLocation(position);
51     f18TopoView.setVelocity(velocity);
53     // Call tick, which insures that any data that needs to be
54     // updated is sent.
55     f18EntityPub.tick();
57     // Set up for next iteration
58     position[0] += velocity[0] * timestep;
59     simTime += timestep;
61     // Wait till real time equals simulation time of next step
62     DtSleep(simTime - exConn.clock()->elapsedRealTime());
63   }
64 }

Connecting to an Exercise

Like the listen-only example, this program creates a DtExerciseConnection to provide an interface to the RTI or DIS network (lines 8 through 10).

Managing Entities

Line 12 defines the entity type the F18 will use. To be visible to other applications in the exercise, each locally-simulated entity requires a DtEntityPublisher, created in line 16. The entity publisher manages the generation of messages for this particular entity. It provides an entity state repository where you can set state values, and a tick() function, which causes state information to be sent to the network if necessary.

Line 19 sets up a pointer to the entity state repository, then line 25 creates a topographic view on that repository. This lets us set the entity's positional data using topographic coordinates, rather than the default geocentric coordinates. (Lines 23 and 24 hard code the coordinates for this example.) Line 28 shows an example of storing a non-positional state value in the repository.

Sending interactions

An example of sending an interaction appears in lines 34-36. You can send the interaction using the exercise connection's sendStamped() function. You can use sendStamped() to send state updates as well, but it is preferable to let the entity publisher send state updates for you, using data in the entity state repository.

Sending State Messages

The main loop executes twenty times per second for ten seconds. As in the listen-only example, this program sets simulation time at the start of each iteration (line 44). While this program's main purpose is to demonstrate the sending of data to the network, it is not a true send-only application. Incoming data is also processed with the drainInput() call on line 47. This call is required for HLA, because this is where we tick the RTI. The program updates the F18's positional data in its entity state repository in lines 50 and 51, and ticks the entity publisher in line 55 to send the updated data onto the network. Thereafter, the only remaining tasks are to increment the F18's position, increment the simulation time, and sleep until it's time to begin the next iteration.

Superior Technical Support

At MAK, technical support is not just an afterthought. Our reputation for supporting our customers is one of the key reasons that people choose our products. When you call or email us with questions, you speak directly to our product developers who know the software inside and out. When you buy MAK's products, you can be sure that MAK will be in your corner as you work towards successful completion of your HLA/DIS project. We've even been known to be on the phone with customers during their HLA certification process, or during key events.

When someone reports a bug, our engineers are quick to provide a patch or workaround, meaning you will not have to wait for the next release to have your problem addressed.

If you need assistance that goes beyond the scope of technical support, our engineering services group is available to do VR-Link customization, extension or integration on demand. We have had many customers not only buy VR-Link, but also buy our engineers' time to fully manage their transition to HLA or DIS compliance.

With maintenance, you are entitled to upgrades when they are released. Typically, new releases not only add support for the latest versions of RTIs, the RPR FOM, HLA Specifications, etc, but also try to maintain compatibility with older versions as well. For example, our current release supports many versions of the MAK RTI, includes FOM Mappers for RPR FOMs 0.5, 0.7, 0.8, 1.0, and 2.0, and continues to support DIS 2.0.4, IEEE 1278.1, and IEEE1278.1a.