In some cases the positive direction of the interface normal along some parts of an interface is opposite to the positive direction along other parts of the same interface. This may occur if there are some very thin, long, needle-like triangles in the interface, and especially if such triangles stretch all across the interface. To fix this, make the triangles more regular. There are different tools to fix this:
- ‘Interfaces > QC’ with the options ‘Flat triangles’ and ‘Try to repair’ enabled.
- ‘Interfaces > Smooth > Smooth and resample’, usually with the ‘Border’ option ‘Use to stamp out’ enabled.
- ‘Interfaces > Subdivide’
- The tools in ‘Primitives > Move > Single Vertex’ to move triangle vertices one by one.