VRED - Virtual Reality Editor - PI-VR GmbH

3.19. TRACKING 3. USER INTERFACE 

3.18.3 Gap Measurement 

The most complex measurement is the gap measurement 

which is a useful substitute of the line-objectmeasurement 

in the case no appropriate line geometry 

is available. The gap measurement is calculated along 

the line segment between two picked points on two distinct 

objects. The viewer will try to find multiple minimum 

distances between the two objects, where each distance 

calculation is restricted to be on a plane perpendicular to 

the line segment. The result will be displayed in the render 

window using a special visualization of all distances. 

Figure 3.123: Gap Measurement 

3.19 Tracking 

The tracking module is responsible for connecting an 

external tracking system to VRED by communicating to 

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one or more VRPN (Virtual Reality Peripheral Network) 

servers. VRPN is an open source standard library for accessing 

many different tracking systems with a unified approach. 

Each VRPN tracking server can contain any number 

of sensors, which in turn are classified as follows: Buttons, 

Analogs, Dials and Bodies. Each server is configured 

by a VRPN configuration file, which must be edited 

by hand. 

3.19.1 Tracking Modes 

The tracking system supports the following four different 

modes, which can be selected independent for each 

tracked body. 

• Powerwall Mode. The powerwall mode is used for 

headtracking a spectator in front of a powerwall. In 

order for this mode to work correctly you either have 

to enable the Powerwall Mode in the Stereo menu 

in the main menu bar or you have to use a cluster for 

viewing. The powerwall tracking mode is enabled by 

setting the bodies target name to powerwall in the 

tracking module. 

• HMD Mode. The HMD mode is intended for headtracking 

a head mounted display. The HMD tracking 

mode is enabled by setting the bodies target name 

to powerwall in the tracking module. You do not 

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need to change anything in contrast to the Powerwall 

Mode as described above. 

• Relative Object Mode. For tracking objects other 

than the spectator there is the Relative Object 

Mode. This mode tracks a physical body and moves 

a node in the virtual scene relativ to the camera position. 

That means if you navigate through the virtual 

scene, the tracked virtual object remains at the 

same location relative to the camera (and seems 

to move through the scene as you navigate). This 

mode is useful for items that the spectator can carry 

around (a flashlight or simply his hands). 

• Absolute Object Mode. As opposed to the relative 

object mode the Absolute Object Mode tracks 

a physical body and moves a node in the virtual 

scene in absolute mode. That means that when you 

navigate away from the virtual representation of the 

tracked physical body, this representation departs 

from the viewer in the virtual world. This mode is 

useful if the virtual scene has a static physical representation 

in reality and you need to track objects 

relative to this representation. For example you put 

a tracked item onto a table that coexists in the virtual 

scene and in the real world. 

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3.19.2 Coordinate Systems 

The main difficulty with tracking lays within the three different 

coordinate systems that have to be considered: 

• Scene coordinate system. This is the coordinate 

system of your virtual world, that is the scene loaded 

into VRED. Note that all virtual representation of 

bodies must be transform nodes. These use a local 

coordinate system that is determined by the product 

of all transform nodes above itself. 

• Tracking coordinate system. The tracking coordinate 

system belongs to the tracking hardware. This 

coordinate system may be the same as the physical 

coordinate system of your application, but it may 

also be completely different, concerning both the 

origin and the orientation. 

• Physical coordinate system. The physical coordinate 

system is that of the real world and is normally 

defined by your needs (e.g. where the origin is and 

the orientation). 

The difficult task is to get these three coordinate systems 

working together. Fortunately VRED has some built-in 

capabilities to calibrate the coordinate systems to easily 

change the origin or orientation. But still, you - as the 

user - have to be aware of what you are doing and how 

the coordinate systems should be mapped on each other. 

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Figure 3.124: An example of a tracking setup, where both 

the tracking coordinate system and the bodies coincide 

with their virtual representations. 

Hint 

Of course it is the easiest, when all three coordinate systems 

are the same – but in most cases this is not possible. 

But a careful design of the scene and the setup in the 

physical world can guarantee that these two coordinate 

systems are the same. Plus it is much easier if the orientation 

of the tracking coordinate system and of the virtual 

scene is the same. This might require a recalibration of 

the tracking system with the original tracking software. 

3.19.3 Calibration 

In order to help the user at bringing together the three different 

coordinate systems, VRED offers the ability to cali- 

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brate a complete tracking space and individual sensors. 

Calibrating a Sensor 

Calibrating a sensor effectively does the following thing: 

The tracking system resets the orientation of the sensor 

and moves its origin to its current location. This sort of 

calibration is useful for initially resetting the position of a 

sensor. 

In VRED calibrating a body simply resets its virtual 

representation to the origin of its local coordinate system. 

Figure 3.125: A body that needs calibration, because its 

virtual position and orientation differ from its real position 

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Figure 3.126: After calibrating a body, it will be reset, such 

that its virtual position and orientation coincide with its virtual 

local coordinate system. 

Calibrating a Coordinate System 

The calibration of a tracking space is a bit more complicated. 

In order to calibrate the space you need to specify 

one sensor within this space. Then the calibration will 

move and rotate the space in such a manner that the origin 

and orientation of the complete tracking space coincides 

with the origin and orientation of the chosen sensor. 

This is especially useful to change the orientation of the 

tracking space in order to meet the needs of the given 

virtual and real scene. 

In VRED the tracking space is calibrated with the help 

of a body. You can use any body of the tracking system 

and use its implicit coordinate system as a reference for 

the tracking coordinate system. After calibrating the coor- 

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dinate system, that body will be at the origin of the calibrated 

tracking space. 

Figure 3.127: The tracking coordinate system and the virtual 

coordinate system do not overlap. 

Figure 3.128: Calibration of the tracking coordinate system 

by using a body as a reference coordinate system. 

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3.19.4 Tracking Devices 

The first and most important tab of the tracking module 

within VRED is dedicated to the tracking devices and their 

sensors. Each tracking device represents a complete 

tracking system consisting of a tracking space, a coordinate 

system and one or more sensors. 

Figure 3.129: List of all Tracking Devices 

You can add a new tracking system by using the context 

menu in the device list. You have to enter its name (in 

the form trackername@computername) and then a connection 

to that VRPN server will be established. Then you 

can start or stop the complete tracking process by clicking 

on the checkbox Enable Tracking left of the device list. 

By using the last two columns in the device list you can 

rotate the coordinate system (Z-Up) and enable or disable 

the complete device (On). 

You can also calibrate the coordinate system as de- 

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scribed above. A calibration needs one sensor that describes 

the new coordinate systems, both its new origin 

and its new orientation. You can also edit the calibration 

matrix directly, but it is discouraged to use this method. 

When a device is created and selected, sensors can 

be added to the device. Unfortunetaly VRPN does not 

tell which sensors are connected, so you have to enter 

them manually in the sensor list. You can also name each 

sensor for accessing these via Python-scripts – the name 

has no other function. The following sensor types are supported: 

• Buttons. 

• Analogs. 

• Dials. 

• Bodies. 

Bodies probably are the most interesting sensor, as these 

are the only sensors that have a position and orientation 

in space. Thus these are also the only sensors that can 

be calibrated or connected to a node. 

Connecting a body to a node is an important task that 

will update a transform nodes matrix with the values coming 

from the corresponding sensor. In order to connect a 

sensor with a node (only transform nodes are supported), 

simply enter the name of the node in the field Target of 

the sensor. There are three special targets that connect 

a sensor to the camera instead of a node. These are as 

follows: 

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• powerwall. This special target enables headtracking 

as needed for powerwall projections. In order to 

make this mode work, you either have to use a render 

cluster or enable the powerwall viewing mode 

from the main menu bar in VRED. 

• hmd. The HMD mode is for head mounted displays. 

• camera. The camera mode is a very special mode 

that simply tracks the position of a sensor but does 

not track its orientation. 

In addition to these special targets you can also enable or 

disable the absolute tracking mode as described above 

by clicking the checkbox in the last column of the sensor 

list. Unchecking the absolute tracking mode will enable 

a tracking mode that transforms the target relativ to the 

camera position. 

Using the context menu you can also calibrate any 

sensor of type body, as described above. The calibration 

will reset the position and orientation of the node that 

is connected to the body. 

3.19.5 Tracking Servers 

The second tab of the tracking module contains a list of 

VRPN servers which are controlled by the cluster service. 

You can add new servers by using the context menu and 

entering their network adress. All servers in this list can 

be started and stopped using the context menu. Note 

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Figure 3.130: List of all Tracking Servers 

that the cluster service must be running on the remote 

machine to be useable with this list. 

3.19.6 Connecting the Camera to a Node 

One important tracking scenario is attaching the camera 

to a tracked node. This is especially useful for driving simulators, 

where a spectator sits within a car that is driving 

through the scene, but the camera should move with the 

car instead of remaining at a fixed point. 

Such effects can be achieved by attaching the camera 

to any node in the scene graph. To connect the camera 

to a nodem, first select the corresponding node and then 

press on the button Connect to Beacon within the camera 

module. This establishes a connection. To activate 

the connection, you need to check Enable Camera Bea- 

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Figure 3.131: Attaching the Camera to a Node 

con. This will set the camera to the nodes location. As 

long as Enable Mouse Navigation still is checked, you 

can use your mouse for navigation (relative to the node). 

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