Twist angle for general trapezoids


there seems to be a discrepancy in the handling of general trapezoids between Geant4 and ROOT. In Geant4 general trapezoids with a twist are taken care of by the G4TwistedTrap class. The twist angle in this class (and all similar classes) is right-handed (clockwise when looking along/parallel to the z-axis). In ROOT this is the TGeoGtra class where the twist angle is left handed (clockwise looking anti-parallel to the z-axis). This discrepancy leads to a wrong geometry imported into ROOT from a GDML export of a Geant4 geometry using G4TwistedTrap. Unfortunately I could not find any detailed specification for the orientation of this angle in the GDML description/standard. I wonder if these are two different and incompatible, but valid, interpretations of the standard, and its the users responsibility to take care of this. Or whether I should open a bug report with either project.

Best regards,

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