2D offset fails if multiple faces would be created

[CarpeNecopinum]

There are two cases in which offsetting a single face could create multiple resulting faces:

1. A face with a "choke point" breaks apart due to a negative offset larger than the choke point:

cs = Circle(2) + Pos(3.9, 0) * Circle(2)
cso = offset(cs, -0.5)
# -> RuntimeError: Multiple Wires generated

2. A face with a hole is cut apart by the whole growing through some thin sections:

f = Rectangle(15, 10) - Circle(4.9)
fo = offset(f, -0.15)
# -> AttributeError: 'Compound' object has no attribute 'normal_at'

both of these currently fail (with different errors)

[CarpeNecopinum]

My current workaround is

def wireOffset(wire: Wire, dist: float):
    builder = BRepOffsetAPI_MakeOffset()
    builder.Init(GeomAbs_JoinType.GeomAbs_Arc)
    builder.AddWire(wire.wrapped)
    builder.Perform(dist)
    obj = downcast(builder.Shape())
    if isinstance(obj, TopoDS_Compound):
        return Compound(obj).wires()
    elif isinstance(obj, TopoDS_Wire):
        return ShapeList[Wire]([Wire(obj)])
    else:
        raise RuntimeError("Unexpected result type")

def safeOffset(face: Face, dist: float):
    outer = wireOffset(face.wire(), dist)
    inner = [x for i in face.inner_wires() for x in wireOffset(i, -dist) ]
    main = Sketch(Face(wire) for wire in outer)
    if len(inner) > 0:
        return main.cut(*(Face(i) for i in inner))
    return main

which works for both cases (but is otherwise a lot less flexible than the offset part of build123d)

[MarcWeber]

from OCP.GeomAbs import GeomAbs_JoinType
from OCP.BRepOffsetAPI import BRepOffsetAPI_MakeOffset
from OCP.TopoDS import ( TopoDS_Compound, TopoDS_Wire )

from build123d import ( Wire, ShapeList, Face, Sketch)
from build123d.topology import downcast, Compound

missing imports :-)

Alternative libraries for Python which can do wires and offsets
which build123 could import/export to / from:

• Inkex (Librarie of Inkscape)
• svgpathtools
• shapely

My current case doesn't turn one into 2. But I create code for cadquery once (positive and negative side for paper embossing and eventually hitting a similar case. And using any line art from inkscape gallery likely causes issues as mentioned here.