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OpenIGTLink protocol description


Every message transfered from a sender to a receiver through OpenIGTLink communication consists of the header byte array followed by the body.

The header contains type string and size of the body, which helps receiver to interpret the body.

The type string may be one of the names defined in OpenIGTLink protocol specification (this page) or a name defined by a user.

The sender does not need to care whether the receiver can interpret the data type, since the receiver can skip reading the body as long as the body size information in the header is correct.

The header also contains device name. This would be helpful when the sender sends same type of data acquired from multiple sources. For example, if the sender has multi-channel tracking device, sender can assign different name to each channel, then the receiver can recognize from which channel data has been acquired.

Details of the header and body structures are defined in following sections.

Protocol extensions

Client - Server

A device (scanner, tracker) exposing functionality and executing commands is usually the Server. Once started will stand by listening for connection requests.

A Client (for ex. a PC) usually connects to multiple devices and coordinates the whole process.

Header Structure

 0   2                       14                                      34             42               50              58
 | V |          TYPE         |              DEVICE_NAME              |   TIME_STAMP  |   BODY_SIZE   |     CRC64     |   BODY  		  

Byte Order

Big endian should be used for all numerical values (version, body size, crc). Unused spaces are padded with 0 (binary).

Header Fields

Data Type Description
V Unsigned short (16bit) Version number (1)
TYPE char[12] Type name of data
DEVICE_NAME char[20] Unique device name
TIME_STAMP 64 bit unsigned int Timestamp or 0 if unused
BODY_SIZE 64 bit unsigned int Size of body in bytes
CRC 64 bit unsigned int 64 bit CRC for body data

Data types (Common across implementations)

Data type:

  • Command (get position, get status, go to target, stop, start continuous push, stop push, ready)
  • Data (image, position, status)


58  60  62  64          70                      82                      94                      106                     118
| V |T|S|E|O|RI |RJ |RK |  TX   |  TY   |  TZ   |  SX   |  SY   |  SZ   |  NX   |  NY   |  NZ   |  PX   |  PY   |  PZ   |
118         124         130


Data Type Description
V unsigned short version number
T 8bit unsigned int Image type (1: Scalar, 3: Vector)
S 8bit unsigned int Scalar type (2:int8 3:uint8 4:int16 5:uint16 6:int32 7:uint32 10:float32 11:float64)
E 8bit unsigned int Endian for image data (1:BIG 2:LITTLE) (NOTE: values in image header is fixed to BIG endian)
O 8bit unsigned int image coordinate (1:RAS 2:LPS)
RI, RJ, RK 16 bit unsigned int Number of pixels in each direction
TX, TY, TZ 32 bit float Transverse vector (direction for 'i' index) / The length represents pixel size in 'i' direction
SX, SY, SZ 32 bit float Transverse vector (direction for 'j' index) / The length represents pixel size in 'j' direction
NX, NY, NZ 32 bit float Normal vector of image plane(direction for 'k' index) / The length represents pixel size in 'z' direction or slice thickness
PX, PY, PZ 32 bit float center position of the image *
DI, DJ, DK 16 bit unsigned int Starting index of subvolume
DRI, DRJ, DRK 16 bit unsigned int number of pixels of subvolume
IMAGE_DATA Binary image data () Image data (endian is determined by "E" field)

* NOTE 1/10/11: There was a discrepancy between the protocol document and the library. Since the library has been used by most of OpenIGTLink software, the protocol document was modified to be consistent with the library.


The transform data is upper three rows of 4x4 transformation matrix, where element (3,3) is assumed to be 1.

Each element is a 4 byte (32 bit) float, 12x4 = 48 byte in total.

 Bytes (Body)
 58      62      66      70      74      78      82      86      90      94      98     102     106
 | (0,0) | (1,0) | (2,0) | (0,1) | (1,1) | (2,1) | (0,2) | (1,2) | (2,2) | (0,3) | (1,3) | (2,3) | 


Quaternion representation of position / orientation.

Parameters (12 or 24 or 28 byte total):

  • Vector of three 32 bit (3x4 byte) floats: position X, Y, Z
  • Vector of 3 or 4, 32 bit each (3x3 or 3x4 byte) floats: orientation quaternion, normalized

The orientation is optional, if not used the values are (0,0,0,1)
The W element of quaternion is also optional, defaults to 1.

 Bytes (Body)
 58      62      66      70      74      78       82
 |   x   |   y   |   z   |   ox  |   oy  |   oz  |    W  |


Query for device capabilities.

Parameters: none (BODY_SIZE=0)
Response: one CAPABILITY packet for each implemented device


List of available TYPEs.

 Bytes (Body)
 58                     70
 |    TYPE (12bytes)     |    TYPE (12bytes)     |       |    TYPE (12bytes)     |


Parameters: none (BODY_SIZE=0)
Response: one STATUS packet for each implemented device


Any STATUS that's not 1 (OK) should be logged by the receiver.

The uniform error reporting allows easy logging and can be the base of exception class in libraries.

 Bytes (Body)
 58   60            68                           88
 | C |  Sub Code   |       Status name           |   Status message (optional)
Data Type Description
C Unsigned short (16bit) Status code groups: 1-Ok, 2-Generic Error, ... (see below)
Sub Code 64 bit integer Sub code for the error (ex. 0x200 - file not found)
Error name char[20] "Error", "OK", "Warning" - can be anything, don't relay on this
Status Message (optional) char[ BodySize - 30 ] Optional (English) description (ex. "File C:\test.ini not found")

Status codes:
0 Invalid packet - 0 is not used
1 OK (Default status)
2 Unknown error
3 Panic mode (emergency)
4 Not found (file, configuration, device etc)
5 Access denied
6 Busy
7 Time out / Connection lost
8 Overflow / Can't be reached
9 Checksum error
10 Configuration error
11 Not enough resource (memory, storage etc)
12 Illegal/Unknown instruction (or feature not implemented / Unknown command received)
13 Device not ready (starting up)
14 Manual mode (device does not accept commands)
15 Device disabled
16 Device not present
17 Device version not known
18 Hardware failure
19 Exiting / shut down in progress