3D Arc dxf entity flattening

[ValerioMussi]

Hi @mozman!
There is something I cannot properly understand about using the flattening method of the arc dxf entity. Can you please help me figuring out what I'm doing wrong?

When I try to use the flattening method of a 3D arc dxf entity I get vertexes that actually do not lie on the arc.
Here is an example modified from the arc.py example.

import ezdxf
from ezdxf.math import Vec3, ConstructionArc, UCS
# create a 3D arc from 3 points in WCS
start_point_wcs = Vec3(3, 0, 0)
end_point_wcs = Vec3(0, 3, 0)
def_point_wcs = Vec3(0, 0, 3)

# create UCS
ucs = UCS.from_x_axis_and_point_in_xy(
    origin=def_point_wcs,
    axis=start_point_wcs - def_point_wcs,
    point=end_point_wcs,
)
start_point_ucs = ucs.from_wcs(start_point_wcs)
end_point_ucs = ucs.from_wcs(end_point_wcs)
def_point_ucs = Vec3(0, 0)  # origin of UCS

# create arc in the xy-plane of the UCS
arc = ConstructionArc.from_3p(start_point_ucs, end_point_ucs, def_point_ucs)
arc.add_to_layout(modelspace, ucs, dxfattribs={"color": 1})  # red arc

arc2=modelspace.query("ARC")[0]
points=list(arc2.flattening(1))
print(f"start: {arc2.start_point} end: {arc2.end_point}")
print(points)
for p in points:
    modelspace.add_point(p)

# saving DXF file
filename = "arcs.dxf"
doc.saveas(filename)
print("drawing '%s' created.\n" % filename)`

The output I get is:

start: (3.000000000000001, 5.551115123125783e-16, -4.440892098500626e-16) end: (6.661338147750939e-16, 3.0, 2.220446049250313e-16)
[Vec3(2.0, -0.999999999999999, -1.000000000000001), Vec3(1.0000000000000007, 0.9999999999999993, -2.0), Vec3(-0.9999999999999986, 1.9999999999999996, -1.000000000000001)]

And here is a screenshot from Draftsight:

Thanks,
Valerio.

[mozman]

This is an error: issue #655

[ValerioMussi]

Thanks for the quick fix!
Just one more question related to that.
While waiting for you answer I was looking for a quick workaround and I found that,
for an arc entity (ezdxf.entities.Arc) I was able to use arc.vertices(arc.angles(5)) getting a generator for Vec3 in WCS. And this perfectly feets my needs.
However these methods are not in the documentation for the ezdxf.entities.Arc
Is it fine using them or they are not documented for a particular reason and should not be used?

Thanks,
Valerio.

[mozman]

Arc.vertices() is Inherited from Circle(), and therefore documented only for Circle().

[ValerioMussi]

I got it.
Thanks!
Valerio.