Using the Calo Detector Element interface

The Calorimeter (includes SPD, PS, ECAL, HCAL) is described in terms of DetectorElement. This short note summarize the usage of this object.

1- Get the Detector Element object:

One need to get a 'Smart Pointer' to the wanted calorimeter, starting from the LHCb object. The code looks like:

SmartDataPtr<DetectorElement> cave(detDataService(),"/dd/Structure/LHCb" ); 
if ( !cave ) { ...report problem... } 
SmartDataPtr<DeCalorimeter> ecal( cave, "Ecal" ); 
if( !ecal ) { ...report problem...} 
ecal->buildCells(); 

2- Use this object

Then on can use the cell interface to the Calorimeter, with the following functions:

CaloCellID ID ; // This is not correct, a CaloCellID has to be either initialized 
                // with a value (unfrequent), or is just returned by a method, 
                // like data access method. 

// Center position of a specific cell 
  double x = ecal->cellX( ID ) ; 
  double y = ecal->cellY( ID ) ; 
  double z = ecal->cellZ( ID ) ; 
  HepPoint3D center = ecal->cellCenter( ID ) ; 

// Size of a cell (x or y). Note : The size of a non existent cell is zero. 
  double size = ecal->cellSize( ID ) ; 

// Neighbors of a specified cell
   CaloNeighbors neigh = ecal->neighborCells( ID ) ; 

// Cell ID at a given position
   HepPoint3D globalPoint ; 
   CaloCellID anID = ecal->Cell( globalPoint ) ; 

3- Some examples 

The most delicate is to use the neighbors, at least for first time use of C++ features. The following code fragment loops on all neiughbors of a given cell, and gets the x value of all these cells.

   CaloCellID anID ; // Cell ID obtained by some way 
   for ( CaloNeighbors::const_iterator it = ecal->neighborCells(anID).begin(); 
         it != ecal->neighborCells(anID).end(); it++ ) { 
      CaloCellID neighbor = *it; 
      double x = ecal->cellX( neighbor ); 
   } 

This means that one shoud use an iterator variable, and that the CaloCellID is pointed at by this iterator variable. Note that one can use directly *it at argument to the ecal functions. One can also use a 'while' loop, but with some care for the incrementation of the iterator...

    CaloNeighbors::const_iterator it = ecal->neighborCells(anID).begin(); 
    while ( it != ecal->neighborCells(anID).end() ) {
      CaloCellID neighbor = *it++; 
      ... 
    }