Fillet Fails on Simple 2D Geometry, Succeeds in 3D

[ZacharyKF]

First here's the minimal example:

from build123d import *

# Simplest form of the face
box = Spline((-10, 1), (0, 0), (10, 1)).offset_2d(1, side=Side.LEFT, closed=False)
box = (
    Line(box @ 0, (box @ 0) - Vector(0, 5))
    + box
    + Line(box @ 1, (box @ 1) - Vector(0, 5))
)
box = make_face(box + Line(box @ 0, box @ 1)) + Location((0, 2.5)) * Rectangle(3, 5)

# Showing the vertex selection works
verts = box.vertices().sort_by_distance((0, 0))[0:2]

# Trying to fillet, these won't work
# edges = box.edges().sort_by_distance(verts[0])[0:2]
# box.max_fillet(edges)
# box = fillet(edges, 0.1)

from ocp_vscode import show, set_defaults, Camera, set_port

set_port(3939)
set_defaults(reset_camera=Camera.KEEP, ortho=True, black_edges=True)
show(box, verts)

Jern pointed out that it works in 3D:

threed = extrude(box,amount=1)
edges2 = threed.edges().filter_by(Axis.Z).sort_by_distance(verts[0])[0:2]
threed = fillet(edges2,1)

[gumyr]

The problem appears to be related to the spine as shown by these examples:

with BuildSketch() as fail:
    with BuildLine() as outline:
        l1 = Spline((-10, 2), (0, 1), (10, 2))
        l2 = Polyline(l1 @ 0, (-10, 0), (10, 0), l1 @ 1)
    make_face()
    Rectangle(3, 5, align=(Align.CENTER, Align.MIN))
    verts = fail.vertices().sort_by_distance((0, 0))[0:2]
    # fillet(verts, 1)

with BuildSketch() as pass1:
    Rectangle(10, 2, align=(Align.CENTER, Align.MIN))
    with Locations((0, 20)):
        Circle(19, mode=Mode.SUBTRACT)
    Rectangle(3, 5, align=(Align.CENTER, Align.MIN))
    # Trapezoid(3, 5, 95, align=(Align.CENTER, Align.MIN))
    verts = pass1.vertices().sort_by_distance((0, 0))[0:2]
    fillet(verts, 1)

with BuildSketch() as pass2:
    Rectangle(10, 2, align=(Align.CENTER, Align.MIN))
    Trapezoid(3, 5, 95, align=(Align.CENTER, Align.MIN))
    verts = pass2.vertices().sort_by_distance((0, 0))[0:2]
    fillet(verts, 1)

OCCT is refusing to fillet the spline which admittedly is a much more difficult shape to work with. Determining where the circle of the fillet meets the spline requires the search algorithm that is present in the DoubleTangentArc object.

To fix this build123d would have to identify that one or more of the edges is a complex curve (splines, bezier curves, etc.) and implement the fillet itself possibly using the DoubleTangentArc.

[gumyr]

An alternative fix may be to attempt a 3D fillet and extract the appropriate 2D shape.