Current strategy to deal with operators not supporting custom NURBS
knots is to fall back to calculated knots for curves of the custom mode
but with no `CurvesGeometry::custom_knots` allocated. Such curves are
the result of operators that copy only `Point` and `Curve` domains. This
way the problem is only postponed. It is not possible to add new custom
knot curves to such `CurvesGeometry` as custom knot offsets are
calculated all together and there is no way to distinguish between old
curves with lost knots and new ones. This is more a future problem.
The actual problem in `main` can be shown with an attached blend file
(see PR) by applying `Subdivide` to some points and then adding new
`Bezier` curve to the same object. This particular problem could be
addressed somewhere in `realize_instances.cc` but the actual problem
would persist.
This PR handles custom knots in all places where `BKE_defgroup_copy_list`
is iused, and where `bke::curves::copy_only_curve_domain` is called.
Here the assumption is made that only these places can copy custom knots
modes without copying custom knots. Depending on operator logic knots are
handled most often in one of two ways:
- `bke::curves::nurbs::copy_custom_knots`:
copies custom knots for all curves excluding `selection`. Knot modes
for excluded curves are altered from the custom mode to calculated.
This way only curves modified by the operator will loose custom knots.
- `bke::curves::nurbs::update_custom_knot_modes;`
alters all curves to calculated mode.
In some places (e.g. `reorder.cc`) it is possible to deal with knots
without side effects.
PR also adds `BLI_assert` in `load_curve_knots` function to check if
`CurvesGeometry::custom_knots` exists for custom mode curves. Thus
versioning code is needed addressing the issue in files in case such
already exists.
Pull Request: https://projects.blender.org/blender/blender/pulls/139554
46bc894570