[geom] Keep reflections through a GDML round trip - #23238
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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.
agheata
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Sep 3, 2026
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Looks good, both the fix and the regression test. Thanks! LGTM
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Thanks @agheata |
Test Results 20 files 20 suites 2d 22h 6m 36s ⏱️ Results for commit 81043cf. |
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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.
This PR fixes #23237