Architecture Tutorial: Retrieve laser scans

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Following the Robotic Architecture proposed here, laser scans from either a simulated or a real robot should be accessible from a single module, named BS_RangeSensors.

The interface of this module is described in this section and this tutorial explains how to write a client module to query the latest laser scans from BS_RangeSensors.

Notice that this tutorial only covers the specific case of writing the codification of the client module in the C/C++ language.


Calling getObservations

At the point in your module where you need to request the laser scans, the service "getObservations" from "BS_RangeSensors" must be invoked. This is the definition of that service, just to know how it looks like:

Tut-scans service.jpg

To call that service, you must use the aracne-atom "request-synchronous&blocking". It can be inserted in your code from the BABEL_MD menu. A dialog will appear asking you which module and service you want to call. Write this information by hand or by using the button "From ICE..." as in the image:

Tut-scans wizard1.jpg

Accept and a wizard will appear asking you for the arguments of the service. Fill it out as in the figure:

Tut-scans wizard2.jpg

Now, this aracne atom must have some preparatory C++ code before it, and some C++ code after it to process the results.

You can use the following fragments of code, self-explained in its comments. Pay attention to:

  • The label "YOUR CODE HERE" for the place where to process the laser scan.
  • The point "ARACNE_ATOM_HERE" at which the aracne atom "request-synchronous&blocking" must be inserted.

<cpp> using namespace mrpt; using namespace mrpt::utils; using namespace mrpt::slam;

// Retrieve the latest laser scan // ----------------------------------------------------- BABEL::BS_RangeSensors::SeqOfBytes *SF; // The sequence of bytes sent by the server module BABEL::Boolean sensorError; // Sensor error? bool err_var; // BABEL communication error?

/**** ARACNE_ATOM_HERE ****/

// Communications error? Propagate it: if (err_var) { // communication error } else { // service requested ok

CObservation2DRangeScanPtr laserScan; // A smart-pointer to the laser scan (read below)

if (!sensorError) { try { // De-serialize the object: mrpt::utils::CSerializablePtr obj = SeqOfBytes2MRPTObject(SF);

// Assure it's a valid observation (if not, the "catch" block will take control): ASSERT_(obj) ASSERT_( IS_CLASS(obj,CSensoryFrame) )

CSensoryFramePtr theSF = CSensoryFramePtr(obj);

laserScan = theSF->getObservationByClass<CObservation2DRangeScan>(); // Get the first scan, if any. } catch(std::exception &e) { // Process ERROR, message in e.what() } }

// Free memory of the Sequence delete SF; SF = NULL;

// Now, we have the latest scan laser in the variable "laserScan", which // is a smart pointer to a mrpt::slam::CObservation2DRangeScan // ------------------------------------------------------------------------- if (laserScan) { // ******* YOUR CODE HERE ************** // Access to the object data members to get the scan data. // Refer to: // Examples: // Scan size: laserScan->scan.size() // Scan point[0]: laserScan->scan[0] // Scan point[0] is valid: laserScan->validRange[0]!=0 } else { // For some reason (comms error, hardware error, etc...) there is no laser scan. } } </cpp>

Auxiliary code

Add the following auxiliary function to your module "Auxiliary logic" section, since this function is used in the code above:

<cpp> // SeqOfBytes2MRPTObject: Convert a BABEL sequence of bytes into a MRPT object. // IMPORTANT: "T of seqofbytes" must be a POINTER to a BABEL sequence, not a reference to it. template <typename T> mrpt::utils::CSerializablePtr SeqOfBytes2MRPTObject(const T &seqofbytes) { if (seqofbytes->length()==0) return mrpt::utils::CSerializablePtr(); mrpt::utils::CMemoryStream memBlock;

	memBlock.assignMemoryNotOwn(&(*seqofbytes)[0], seqofbytes->length() );

return memBlock.ReadObject(); } </cpp>

Non-deportabilities of the module

If your module didn't use MRPT before adding these code fragments, it must be inserted by:

  • Selecting the tab "Non-deportabilities" in BABEL_MD.
  • Select "Execution".
  • Add "mrpt-core" from the list below.
  • Save the module.

You'll need MRPT installed as explained here.