[geom] Keep reflections through a GDML round trip - #25
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sawenzel merged 1 commit intoSep 2, 2026
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This fixes root-project#23237, a bug in the GDML writer and reader that could lose reflections when a geometry was exported to GDML and read back. The writer now preserves reflections combined with rotations by factoring the reflection into a z-mirror and writing the remaining proper rotation. Diagonal reflection matrices keep their existing representation. The reader now also handles the scale of placements inside assemblies, so reflections are preserved there as well. A new test exports and re-imports a reflected box with an off-centre daughter, both in the world and in an assembly. It checks the global position, frame handedness and TGeoHMatrix::IsReflection() before and after the round trip. The ALICE geometry is now correctly exported to GDML, with reflections preserved through the round trip.
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This fixes a bug in the GDML writer and reader that could lose reflections when a geometry was exported to GDML and read back.
The writer now preserves reflections combined with rotations by factoring the reflection into a z-mirror and writing the remaining proper rotation. Diagonal reflection matrices keep their existing representation.
The reader now also handles the scale of placements inside assemblies, so reflections are preserved there as well.
A new test exports and re-imports a reflected box with an off-centre daughter, both in the world and in an assembly. It checks the global position, frame handedness and TGeoHMatrix::IsReflection() before and after the round trip.
The ALICE geometry is now correctly exported to GDML, with reflections preserved through the round trip.