Guidelines on using the geometry (v10+)

Detailed information is available on the geometry system page and at important concepts.

• Always use complete ID classes to identify a part of the detector. For example, identify a plane with a geo::PlaneID object instead of just one index. The geo::PlaneID object also includes information about the cryostat and TPC to which the plane belongs. Complete ID types are defined here.
  • Use geometry methods that take geometry IDs as arguments: prefer geo::WireReadoutGeom::Nplanes(geo::TPCID const&) over geo::WireReadoutGeom::Nplanes(), as the latter will use a default TPC, which is beyond the control of the user.
  • There is seldom a need for a geometry ID to “decay” to an index: e.g., a vector keyed by plane index. The ID classes have comparison operators thus supporting associative lists (e.g. std::map<geo::PlaneID, DataClass>).
  • When a vector is the only reasonable choice, make sure the corresponding parent ID (geo::TPCID in the above example) is available so that the original ID can be easily reconstituted.
• Use world coordinates whenever possible.
  • Avoid computing these yourself (use e.g. geo::TPCGeo::MinX(), geo::TPCGeo::MaxX(), etc.)
• Instead of assuming the ID of the next channel is just one more than the channel ID of the current wire, use the channel ID of the next wire.
• Beware of assumptions:
  • There can be more than one cryostat and more than one TPC within each cryostat.
  • There are not always 3 wire planes; use geo::WireReadoutGeom::Nplanes(tpcid) to find out how many.
  • Do not rely on knowing where “0” is on an axis–use the GeometryCore interface instead.
  • Don’t assume that mappings between readout channels and wires are one-to-one.
  • Don’t assume that the wire pitch is the same on all planes (currently all wires on the same wire plane should have the same pitch, barring bugs)

When reviewing your (or others’) code:

• Keep track of how the geometry object IDs move around.
• Investigate any hard-coded number and where it comes from.
• Be aware under what circumstances a return ID’s validity should be checked.
• Check any try/catch blocks to make sure exceptions are correctly handled; if some code ignores an exception, it must explain why.
• Be suspicious of any code whose behavior relies on geo::GeometryCore::DetectorName() as it implies detector dependency.