#181: AEILocalInterp should not off-centre the interpolation stencil by default ---------------------+------------------------------------------------------ Reporter: hinder | Owner: Type: defect | Status: confirmed Priority: major | Milestone: Component: Cactus | Version: Resolution: | Keywords: AEILocalInterp ---------------------+------------------------------------------------------
Comment (by hinder):
We just ran into this issue again. However, I don't understand in our case why there is a problem. We are using order=4 Lagrange interpolation, which has a 5-point interpolation stencil, and we use 3 ghost and symmetry points. Hence, the interpolator should never have to off-centre the stencil, because the interpolation point will always be in the 'default- centering-region', as defined in the documentation (https://einsteintoolkit.org/documentation/ThornDoc/Numerical/AEILocalInterp/). In fact, 2 ghost points should be sufficient. We are using periodic boundary conditions, and the interpolation point is exactly on the boundary in one (maybe two) dimensions. If CarpetInterp always gives the point to the component that "owns" it, and the symmetry interpolator the same, then setting these options (boundary_off_centering_tolerance= boundary_extrapolation_tolerance={0...}) should have no effect. Instead, we see that without these parameters set, there is a symmetry violation (a geodesic which should evolve along the cell edge by symmetry diverges from it), and with them set, everything is fine. Further, my understanding was that if AEILocalInterp previously decided to off-centre, and you set these options to 0, then it would return an error code, which we do not see, rather than giving "better" results. If it could give "good" results without off-centering, then why is it deciding to off-center?