1 1. motion tracking - polhemus liberty latus references: 1. liberty latus manual, polhemus, 2005 2....

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1. Motion Tracking - Polhemus Liberty Latus

References:

1. Liberty Latus Manual, Polhemus, 2005

2. G. Burdea and P. Coiffet, Virtual Reality Technology 2nd Ed., Wiley, New Jersey, 2003, Ch.2

3. J.M. Hart, Windows System Programming, 3rd Ed., Addison-Wesley, 2005, Ch.12

Department of ELECTRONIC AND INFORMATION ENGINEERING

1. Motion Tracking – Polhemus Liberty Latus by Dr Daniel Lun

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Motion Tracking Systems A motion tracking

system is often the most important I/O Devices of a Virtual Reality system

It detects the position and orientation (P&O) of multiple objects in the 3D space and feeds into the VR Engine



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3-D System of Coordinates of Objects

3D objects have 6 degrees of freedom (D.O.Fs) of movement: three translations –

(x, y, z); three rotations –

(Azimuth (az), Elevation (el) and Roll (ro)).



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Tracking Technologies Trackers measure the motion of “objects” such as user’s

wrist or his head vs. a fixed system of coordinates.

Technologies to perform this task: Magnetic (prevalent, less expensive)

E.g. Polhemus’s Liberty Latus, Ascension’s FOB Ultrasonic (less used, low resolution) Mechanical (special cases, only when size and weight are

not matter) E.g. Fakespace’s Push1280

Vision-based (expensive, high accuracy) E.g. A.R.T. GmbH’s ARTrack

Inertial/Vision-based (cheap but limited functionality) E.g. WII of Nintendo



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Characteristics of Trackers Different tracking systems have different characteristics

Measurement rate – Readings/sec; Sensing latency; Sensor noise and drift; Measurement accuracy (errors); Measurement repeatability; Tethered or wireless; Sensing degradation; Restriction on the working environment; Cost

To build a VR system using a particular tracking system, good understanding of its technology and limitations is required



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Magnetic Tracking Systems

Definition: A magnetic tracker is a non-contact position measurement device that uses a magnetic field produced by a stationary TRANSMITTER to determine the real-time position of a moving RECEIVER element



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Working principle Magnetic field picked up by an antenna can be

converted into electrical signal The electric power induced depends on the

magnetic field strength The longer is the distance between the transmitter and

antenna, the lower is the power

The strength of magnetic field follows inverse square law Magnitude decreases as the square of the distance

Magnetic field is directional E.g. vertical magnetic field can only be picked up by

antenna placed vertically



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Working Principle

Receiver antenna(stationary)

Transmitter antenna(moving)

Tx G

Tx BTx RRx BRx RRx G

Time

When the object is moving or rotating, voltage of different magnitude will be generated in its three antennas based on the distance moved and the rotation angle

Such information will be sent to the computer for analyzing the P&O information of the moving object

Magnetic wave

Send back to the computer for further analysis

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Limitations

Performance will be significantly degraded if there is ferromagnetic material nearby the transmitter or receiver



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Case study – Polhemus’s Liberty Latus The LIBERTY LATUS (Large Area Tracking Untethered System)

is a magnetic based motion tracking system Design to track the position and orientation (P&O) of multiple

objects in the 3D space Comprise 3 major components:

SEU – System Electronic Unit Receptors Wireless Markers



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Working Principle

Markers/Transmitters(moving)

Receptor / Receiver(stationary)

Magnetic wave of different frequencies

Signals of multiple frequencies are sent back to SEU for analysis

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Characteristics Each transmitter “marker” position is measured by a receiver

“receptor” within 8 ft. One receiver can track 4 markers. In practice, inaccurate data will be received if the distance

between marker and receptor is larger than 1m The system can have up to 12 markers and up to 16 receptors

In DE505a, 8 markers and 4 receptors are installed Sampling rate is 188 Hz up to 8 markers and drops to 94 Hz from

9 to 12 markers Markers are battery powered up to 2.5 hours and weigh 2 ounces

each Battery life can be shorter when using for long time

For one marker and one receptor accuracy is 0.04 mm and 0.0012 degree at 1 ft range and drops afterwards



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Precautions When Using the System

1. Never put metallic objects near the receptors or markers. Wrong data will be generated

2. Never put receptors and markers on the ground in DE505a since there are many metallic conduits laid under the ground

3. Whenever a marker is switched on, there is a possibility that the phase of the data is wrong (from +ve to –ve or vice versa). It will also happen when the marker is moving too far away from the receptors. Switch the marker off and on again to obtain a correct phase

4. When a marker is moved outside 1.5m from the receptors, the data received will have much noise. Denoising technique should be implemented in order to use the data in your game

5. A marker should be moved too close (e.g. < 10cm) to the receptor. Wrong data will be generated



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More Precautions

The floor of DE505a is not leveled. Some parts may be slightly higher than the other parts

Try not to use the absolute values of the received data to determine the exact 3D coordinates of a marker in the room, as the values can drift in time

Use the relative difference between measures as much as possible

The battery of the marker can be exhausted much earlier than expected. Replace the battery once noticing that there is abnormality in the received data



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Software Architecture

For easy access of the data generated by Liberty Latus, a simple network server is installed that accepts clients’ connection thru Winsock

Motion Tracking

Server

receptor

Marker

Marker

Network backbone

SEU

Windows SocketServer IP: 158.132.148.212Port: 8888

receptor

receptor

receptor



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Motion Tracking Server

The server accepts string commands and replies different string for different requests

Some commands are particularly important:1. “hello” – When the user program sends a string

“hello” to the server, the server will return a string “Hello client, have a nice day!”

2. “Q” – For closing a Winsock connection.3. “P” – Return a string that contains the P&O data

of all markers and the status of the markers



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Retrieve the P&O Data

When the string of the P&O data is received, the function memcpy() has to be used to convert the string into an array of the structure MarkerData:

#define NMarker 8 //There are 8 markers in DE505a

struct MarkerData //Structure of marker data{

float x, y, z, az, el, ro; //6DOF of a markerbool status; //Status of a marker

};

MarkerData marker[NMarker]; //Define an array of 8 markers

memcpy(marker, stringFromServer, sizeof(marker));//stringFromServer is the string received from server//Must use this command to convert between data type

#define NMarker 8 //There are 8 markers in DE505a

struct MarkerData //Structure of marker data{

float x, y, z, az, el, ro; //6DOF of a markerbool status; //Status of a marker

};

MarkerData marker[NMarker]; //Define an array of 8 markers

memcpy(marker, stringFromServer, sizeof(marker));//stringFromServer is the string received from server//Must use this command to convert between data type

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Socket API and Socket The socket API is an Interprocess Communication (IPC) programming

interface originally provided as part of the Berkeley UNIX OS Ported to all modern operating systems, including Sun Solaris and

Windows systems It is a de facto standard for programming IPC, and is the basis of more

sophisticated IPC interface such as remote procedure call and remote method invocation

a socket

Process A Process B

• A socket API provides a programming construct termed a socket

• A process wishing to communicate with another process must instantiate such a construct

• The two processes then issue operations provided by the API to send and receive data.



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Connection-oriented stream socket A socket programming construct can make use of either

the UDP or TCP protocol Sockets that use UDP for transport are known as

datagram sockets, while sockets that use TCP are termed stream sockets

Socket APIs can support both connectionless and connection-oriented communication at the application layer

For the Motion Tracking Server, it uses stream socket and works in connection-oriented mode, i.e. a connection has to be made before data can be sent or received

Process AsocketAPI runtime

supportProcess B

socketAPI runtime

support

transport layer software transport layer software

connection-oriented datagram socket

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Windows Sockets API

An extension of the Berkeley Sockets API used in the Windows environment

Porting of code already written for Berkeley Sockets Windows stations easily integrated into TCP/IP networks

Winsock API is supported by a DLL (ws2_32.dll) that can be accessed by linking ws2_32.lib with your project

Include <winsock.h> as well for the definition of some names



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General Procedure for Connecting to a Server

WSAStartup( ... ); //1. Initialize

socket ( ... ); //2. Create a socket

connect ( ... ); //3. Connect to the server

send ( ... ); / recv ( ... );

//4. Send or receive data

:

closesocket ( ... );//5. After using the socket,

// close it.

WSACleanup ( ... ); //6. Allow the system to

// free resource allocated

WSAStartup( ... ); //1. Initialize

socket ( ... ); //2. Create a socket

connect ( ... ); //3. Connect to the server

send ( ... ); / recv ( ... );

//4. Send or receive data

:

closesocket ( ... );//5. After using the socket,

// close it.

WSACleanup ( ... ); //6. Allow the system to

// free resource allocated

Just follow the procedure below and fill in the required information:

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Winsock Initialization

To initialize, a nonstandard Winsock-specific function WSAStartup must be the first function a program calls

For example,

WORD wVersionRequired; //Store the socket version

WSADATA wsaData; //Store socket info

wVersionRequired = ... ; //Input the version number

int error = WSAStartup( // Return non-zero if error

wVersionRequired,

(LPWSADATA)&wsaData);

WORD wVersionRequired; //Store the socket version

WSADATA wsaData; //Store socket info

wVersionRequired = ... ; //Input the version number

int error = WSAStartup( // Return non-zero if error

wVersionRequired,

(LPWSADATA)&wsaData);

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WSAStartup() Parameters wVersionRequired

Indicates the highest version of the WinSock DLL you need Returns a non-zero value if the DLL cannot support the

version you want Can use a macro MAKEWORD to generate the number wVersionRequired = MAKEWORD(2,2);

//version 2.2

lpWSAData points to a WSADATA structure that returns information on the configuration of the DLL

To obtain the error number, the function WSAGetLastError()



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Create a Socket

Call socket() to create (or open) a socket For example,

typedef unsigned int SOCKET;

//Should have been included in <winsock.h>

int af = ...;

int type = ...;

int protocol = ...;

SOCKET theSocket = socket(af, type, protocol);

typedef unsigned int SOCKET;

//Should have been included in <winsock.h>

int af = ...;

int type = ...;

int protocol = ...;

SOCKET theSocket = socket(af, type, protocol);



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socket() parameter

Input parameters include af denotes the address family. PF_INET

designates the Internet protocol (IP) type specifies connection-oriented

(SOCK_STREAM) or datagram communications (SOCK_DGRAM)

protocol – unnecessary if using TCP/IP Use IPPROTO_TCP

socket returns SOCKET_ERROR upon failure



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Connect to a Server

To connect to the Motion Tracking Server, a client program can call the function connect() if it has a socket

For example,

SOCKET s = ...;

SOCKADDR_IN serverInfo = ...;

int nNameLen = ...;

int connectResult = connect(s,(LPSOCKADDR)&serverInfo,

nNameLen);

SOCKET s = ...;

SOCKADDR_IN serverInfo = ...;

int nNameLen = ...;

int connectResult = connect(s,(LPSOCKADDR)&serverInfo,

nNameLen);

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connect Parameters

Input parameters: s – a socket created by socket() function &serverInfo – a LPSOCKADDR pointer that points to a

SOCKADDR_IN structure designating the server machine name and port address

nNameLen – should give the size of SOCKADDR_IN Use sizeof()

Return 0 indicates a successful connection, otherwise SOCKET_ERROR indicate failure Possibly due to the server socket is not listening to clients’

request Department of ELECTRONIC AND INFORMATION ENGINEERING


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SOCKADDR_IN Structure

The structure SOCKADDR_IN is protocol dependent. For TCP/IP, it is defined as

struct sockaddr_in{

short sin_family; // PF_INETu_short sin_port;struct sin_addr; //4-byte IP addrchar sin_zero [8];

};typedef struct sockaddr_in SOCKADDR_IN

//Should have been defined in //<winsock.h>

struct sockaddr_in{

short sin_family; // PF_INETu_short sin_port;struct sin_addr; //4-byte IP addrchar sin_zero [8];

};typedef struct sockaddr_in SOCKADDR_IN

//Should have been defined in //<winsock.h>

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Parameters of SOCKADDR_IN sin_family

Address family, should be the same as the parameter af when calling socket(), i.e. PR_INET, e.g.

sin_port Specify port number of the server Should call the function htons() to enter this

value. E.g.

int portNo = …; //Specify the port numberserverInfo.sin_port = htons(portNo);

int portNo = …; //Specify the port numberserverInfo.sin_port = htons(portNo);

serverInfo.sin_family = PF_INET;serverInfo.sin_family = PF_INET;

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Parameters of SOCKADDR_IN sin_addr

sin_addr is a structure that has a sub-member s_addr. Should be filled with server’s IP address

The function inet_addr() can be used as follows:

std::string ipAddress; // Assume ipAddress contains // the IP address of the serverSOCKADDR_IN sa;

//Define an instance of the structuresa.sin_addr.s_addr = inet_addr(ipAddress.c_str());

//Set the IP address of server

std::string ipAddress; // Assume ipAddress contains // the IP address of the serverSOCKADDR_IN sa;

//Define an instance of the structuresa.sin_addr.s_addr = inet_addr(ipAddress.c_str());

//Set the IP address of server

sin_zeros Just to pad enough zeros to make the structure the

same size as SOCKADDR. No need to initialize it

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Send or Receive Data

After the connection is made, the client can send or receive data to or from the server

Partner stations exchange data using send() and recv()

send() and recv() have identical arguments:int send ( int recv (

SOCKET s, SOCKET s,

LPSTR lpBuffer, LPSTR lpBuffer,

int nBufferLen, int nBufferLen,

int nFlags); int nFlags);



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send / recv Parameters lpBuffer

send: the string to be sent recv: keep the received string

nBufferLen send: the length of the string recv: the size of the buffer used to keep the string

nFlags Can be used to indicate urgency Can also be used to allow reading the data but not removing it In general, use 0

Return the actual number of bytes transmitted or received. An error is indicated by the value SOCKET_ERROR



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When Finish …

When finish transmitting or receiving data, remember to destroy the socket and release the resource

Use closesocket() and WSACleanup() as follows:

SOCKET s;: //After finish using the socket ...:

closesocket(s);//The name of the socket is s

WSACleanup(); //Release the resource acquired

SOCKET s;: //After finish using the socket ...:

closesocket(s);//The name of the socket is s

WSACleanup(); //Release the resource acquired

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System Software Architecture

Motion Tracking

Server

Network backbone


Motion Tracking

Server Simulator


Latus

mMarker[8]

WindowSocket

ClientWindows Socket

Your Client Program

Ogre Graphics Engine

Should be used to keeping the current markers’ data

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Responses of the Servers

String sent from the client

Response of the server

“hello” “Hello client, have a nice day!”

“Q” The Winsock connection will be terminated

“P” Return a string that contains the P&O data of all markers and the status of the markers



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Structure of the Client Program

Latus-mMarker[NMARKER] : MarkerData

+Initialize(...)+UpdatePO(...)+GetX(...)+GetY(...)+GetZ(...)+GetAZ(...)+GetEL(...)+GetRO(...)+GetStatus(...)

WindowSocket-mSocket : SOCKET-mConnect : bool

+ConnectToHost(...)+CloseConnection(...)+SendMsg(...)+ReceiveMsg(...)+IsConnect(...)

WindowSocket is the base class for realizing Windows Sockets

Latus is derived from WindowSocket that extends its function to obtain markers’ data on the network

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Things to be done in Lab1

Construct a basic Ogre game Implement the class WindowSocket Implement the class Latus Apply the markers’ data obtained using the

Latus class to the movement of Ogre objects



1 1. motion tracking - polhemus liberty latus references: 1. liberty latus manual, polhemus, 2005 2....

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