Standalone conversion from LCIO to EDM4hep

The lcio2edm4hep executable reads LCIO (.slcio) files and converts its contents into EDM4hep. Each LCEvent of the input file will be put into a podio::Frame in the output file (under the events category). The most basic usage is simply

lcio2edm4hep <input.slcio> <output.edm4hep.root>

Patching missing collections on the fly

A major difference between LCIO and EDM4hep is that in LCIO an LCEvent can effectively have arbitrary contents, whereas in EDM4hep the assumption is that each event consists of the same collections (even if some of them are empty). Hence, it is necessary to either ensure that all events in the LCIO file have the same contents or or to give lcio2edm4hep some additional information such that it can patch in potentially missing collections on the fly. This additional information comes in the form of a third argument to lcio2edm4hep and is effectively a list of collection names and their types that comprise the superset of all collections appearing in at least one event in the input LCIO file. The format looks like this, where each collection is a single line containing the name first and than its type, e.g.

SETSpacePoints             TrackerHit
RecoMCTruthLink            LCRelation[ReconstructedParticle,MCParticle]

The easiest way to obtain such a file is to use the check_missing_cols executable that comes with LCIO using the --minimal flag. The output of this can be directly consumed by lcio2edm4hep

Example:

Get the patch file

check_missing_cols --minimal \
  /pnfs/desy.de/ilc/prod/ilc/mc-2020/ild/rec/250-SetA/higgs/ILD_l5_o2_v02/v02-02-01/00015671/000/rv02-02-01.sv02-02-01.mILD_l5_o2_v02.E250-SetA.I402005.Pe3e3h.eL.pR.n000_002.d_rec_00015671_493.slcio \
  > patch.txt

Pass it to lcio2edm4hep

lcio2edm4hep \
  /pnfs/desy.de/ilc/prod/ilc/mc-2020/ild/rec/250-SetA/higgs/ILD_l5_o2_v02/v02-02-01/00015671/000/rv02-02-01.sv02-02-01.mILD_l5_o2_v02.E250-SetA.I402005.Pe3e3h.eL.pR.n000_002.d_rec_00015671_493.slcio \
  Output.root \
  patch.txt

Converting `LCRelation` collections

For collections of LCRelation type it is necessary to define the FromType and ToType as well, as otherwise the converter will not be able to create the correct edm4hep file. The check_missing_cols executable will try to determine these types from the collection parameters and will warn if it cannot do it for certain collections. In this case it is the users responsibility to provide the missing types as otherwise the conversion will simply skip these collections, or potentially even crash.

Converting only a subset of collections

Using the same mechanism as for patching collections it is also possible to only convert a subset of all available collections. lcio2edm4hep uses the contents of the colltypefile to determine the contents of the output. If that contains only a subset of all collections, only that subset will be converted. Missing collections will still be patched in, in this case.

Library usage of the conversion functions

The conversion functions are designed to also be usable as a library. The overall design is to make the conversion a two step process. Step one is converting the data and step two being the resolving of the relations and filling of subset collection.

Converting collection (data)

The main entry point is convertCollection which will automatically dispatch to the correct conversion function depending on the type information that is stored in the input LCCollection. It is also possible to access the individual conversion functions for each type. All of the conversion functions take a map of LCIO to EDM4hep objects of their specific type that will be filled during the conversion. for convenience all necessary maps are bundled in the LcioEdmTypeMapping struct.

Handling relations

Once all necessary collections have been converted, it is necessary to resolve the relations between the objects. This is done using the resolveRelations function. This will again dispatch to the correct relation resolving function for the corresponding types, which can obviously also be invoked directly.

Handling of subset collections

Subset collections are handled similar to relations using the function fillSubset. Internally this simply forwards to handleSubsetColl which handles all the type details and can obviously also be used directly.

Handling of `LCRelation`s

LCRelation only exist in LCIO and their conversion is limited to what is available in EDM4hep. They use the "FromType" and "ToType" collection parameters to get the necessary type information.

The AssociationCollections in EDM4hep are then created using createAssociations.

Converting entire events

Converting an entire event can be done calling the convertEvent. This can also be used as an example to guide the implementation of custom conversions using the available functionality.

Converting Event parameters

This can be done by calling convertObjectParameters that will put all the event parameters into the passed podio::Frame.

Subtle differences between LCIO and EDM4hep

There are a few small differences between LCIO and EDM4hep that shine through in the conversion, these are:

CaloHitContributions are part of the SimCalorimeterHits in LCIO while being their own data type in EDM4hep. They are created by createCaloHitContributions.
The event information like an event number is part of the LCEvent in LCIO. In EDM4hep there is a separate EventHeader Collection. It can be created using EventHeaderCollection which is stored under the name "EventHeader".
Particle IDs are converted during the conversion of the the reconstructed Particle collection.

Example for a ReconstructedParticle Collection

#include "k4EDM4hep2LcioConv/k4Lcio2EDM4hepConv.h"

// the struct defined in the header file is used for the maps linking Lcio particles
// to their EDM counterparts.

auto typeMapping = LcioEdmTypeMapping{};

// We assume that this is a collection of ReconstructedParticles!
LCEVENT::LCCollection* lcCollection;

// Convert the data
auto edmCollections = convertReconstructedParticle("name",
                                                   lcCollection,
                                                   typeMapping.recoParticles,
                                                   typeMapping.particleIDs);

// Resolve relations (only converted objects will be available)
// This has to be called at the very end, after all collection data has been
// converted
resolveRelations(typeMapping);