TraCI/Object Context Subscription
Context subscriptions are allowing the obtaining of specific values from surrounding objects of a certain so called "EGO" object. With these datas one can determine the traffic status around that EGO object. Such an EGO Object can be any possible Vehicle, inductive loop, points-of-interest, and such like. A vehicle driving through a city, for example, is surrounded by a lot of different and changing vehicles, lanes, junctions, or points-of-interest along his ride. Context subscriptions can provide selected variables of those objects that surround the EGO object within a certain range.
The subscription for a structure's context is initiated using a "Subscribe ... Context" command (0x80-0x8e). The command is evaluated immediately on request, verifying it this way. It returns a "Subscribe ... Context" response (0x90-0x9e). If EGO is a vehicle, the subscription is descheduled as soon as the vehicle leaves the simulation.
As soon as the subscription was accepted, it is executed after each call of Simulation Step(2) command.
The command ID describes the type of the EGO-object which is the center of the context; currently, the following EGO-objects are supported:
For a given EGO-structure, each subscription allows to address multiple variables from structures of a certain kind. This means, if you have an EGO-vehicle, you may ask for speeds and positions of vehicles in range of EGO. If you also need information about surrounding points-of-interest, you have to add a further context subscription for PoIs. Vehicles, and PoIs belong to different "context domains". The following domains are available:
|Type||Context Domain ID||Notes|
|inductive loops||0xa0||retrievable variables|
For each domain, the same variables can be retrieved as using the Get ... Variable commands (see above).
Command 0x8X: Subscribe ... Context
|begin Time||end Time||Object ID||Context Domain||Context Range||Variable Number||The list of variables to return|
- begin Time: the subscription is executed only in time steps >= this value; in ms
- end Time: the subscription is executed in time steps <= this value; the subscription is removed if the simulation has reached a higher time step; in ms
- Context Domain: the type of objects in the addressed object's surrounding to ask values from
- Context Range: the radius of the surrounding
- The size of the variables list must be equal to the field "Variable Number".
Response 0x9X: ... Subscription Response
|Object ID||Context Domain||Variable Count||Objects Count||Object #1 ID||Object #1 / Variable #1 Id||Object #1 / Variable #1 status||Object #1 / Return type of the variable #1||Object #1 / <VALUE#1>||...||Object #1 / Variable #n Id||Object #1 / Variable #n status||Object #1 / Return type of the variable #n||Object #1 / <VALUE#n>||...||Object #m ID||Object #m / Variable #1 Id||Object #m / Variable #1 status||Object #m / Return type of the variable #1||Object #m / <VALUE#1>||...||Object #m / Variable #n Id||Object #m / Variable #n status||Object #m / Return type of the variable #n||Object #m / <VALUE#n>|
The respond to a "Subscribe ... Variable".
The status is 0x00 (RTYPE_OK) if the variable could be retrieved successfully. If not, the status is 0xff (RTYPE_ERR). In the second case, the variable type is set to string and the variable value contains the error message. Please note, that the status should be ok in all cases.
- Variable Count is the number of returned variables per object
- Objects Count is the number of objects in range
This implies that n*m values are returned (Variable Count * Objects Count).
Client library methods
- In the python library, all domains support the methods subscribeContext and unsubscribeContext
- In the C++ library, the method simulation.subscribeContext takes an additional argument that encodes the source domain
- When asking for domains "point-of-interest" and "polygon", the current version does not support asking for these structures when their position was changed using TraCI