GammaLib

to implement a correct reflected-region background method, GSkyRegions must have a rotation angle implemented, so that arbitrary regions end up in the correct orientation on the sky.

The suggested way to implement this is to add:
@
GSkyDir m_rotation_point;
double m_rotation_angle;

GSkyDir  transform_to_region_system( GSkyDir &dir );
 GSkyDir  transform_from_region_system( GSkyDir &dir);


to the base GSkyRegion class. The transformation functions apply the rotation to any point for the current region, and should be applied in @contains() etc., before checking if a coordinate is inside a region. This way the mathematical definition of the regions does not need to take into account rotation. E.g. a rectangle region can be assumed to be aligned to the X-Y axis, and rotation is taken into account by the coordinate transformation.