Clarify situation around subtractive sweeps

[hannobraun]

Subtractive Sweeps

Fornjot supports sweeping a face of a shell, extending that shell (SweepFaceOfShell). I know for a fact that this works as an additive sweep. Nothing in the API or the documentation suggests that it shouldn't also work as a subtractive sweep.

I don't know if this works. I'm guessing no, and I'm guessing the reason for that is, that the orientation of the new side faces need to be different in either scenario. Supporting evidence for that is, that I've chosen to implement blind holes based on lower-level primitives (but that might also haven been just a case of idiocy).

Actions Items

1. Confirm that subtractive sweeps actually don't work.
2. Update the documentation of SweepFaceOfShell to explicitly mention that. Refer to AddHole as a (limited) alternative.
3. Update documentation of AddHole to clarify the context it lives in. (If I'm right and subtractive sweeps really don't work, they serve as stopgap to make them work in limited ways, and could later become just a higher-level wrapper around subtractive sweeps.
4. Open issue for implementing subtractive sweeps.

Thoughts on Implementation

If I'm correct, and the orientation of the side faces is the only issue, then this isn't a hard problem in principle. All the operation would need to do, is figure out whether the sweep direction goes towards the outside or inside, which can be determined from the normal of the swept face. SweepSketch already does this, but that code is currently limited due to our primitive geometry representation.

This is probably another one of those cases that should wait for a better geometry representation.