Possible bug in ezdxf open_uniform_knot_vector function

[subsurfaceiodev]

Hi @mozman !
ezdxf_spline_issue.zip contains the following files:

1. to_import.dxf created in AutoCAD containing 3 splines: 2 individual splines on the left and the product of joining those splines as a single spline on the right
   

2. spline_mwe.py containing the following code:

import ezdxf
from ezdxf import zoom
from ezdxf.entities import Spline

doc = ezdxf.readfile('to_import.dxf')
msp = doc.modelspace()

doc_new = ezdxf.new()
msp_new = doc_new.modelspace()

for e in msp:
    if isinstance(e, Spline):
        spline = msp_new.add_open_spline(
            control_points=e.control_points,
            # knots=e.knots,
            degree=2,
        )
        # actual knots
        knots = e.knots

        # open_uniform_knot_vector
        knots_ezdxf = spline.knots
        print(f'{knots=}, {knots_ezdxf=}')
zoom.extents(msp_new)
doc_new.saveas('exported.dxf')

3. exported.dxf which is the product of the above code:
   

As you may see, the right spline is not exported correctly when commenting line 15 of the above code (knots=e.knots), we noticed when knots are not provided add_open_spline function calls open_uniform_knot_vector to calculate them. Said calculation works fine for the left splines, but for the right spline you may see AutoCAD knots are: [0.0, 0.0, 0.0, 1.0, 1.0, 2.0, 2.0, 2.0] while calculated knots by ezdxf are [0.0, 0.0, 0.0, 1.0, 2.0, 3.0, 3.0, 3.0] which seems to be a bug causing exported right spline to not look correct.

[mozman]

This is a special spline configuration and not an open B-spline with uniform knot distribution because the interior knots (1.0) are repeated and the curve passes the third control vertex. See definition for open uniform knots.

This means that this spline configuration cannot be created using theadd_open_spline() factory method. To get the same spline you have to use the original knots as done by commenting out the line knots=e.knots.

[subsurfaceiodev]

It makes sense, is there any function in ezdxf to get said special spline configuration knots given control points?