how to get the length of spline?

[XiangjiBU]

how to get the length of spline generated by add_spline(). for example:

fit_points=[(0,0,0),(750,500,0),(1750,500,0),(2250,1250,0)]
msp = doc.modelspace()
spline=msp.add_spline(fit_points)

[mozman]

There exist no analytical function to calculate the length of a B-spline, you have to approximate the curve and calculate the length of the polyline.

[XiangjiBU]

Could you give me some guidance to get this? do you mean that I need to get all the knots, then approximate the length? how can I get all the knots? Is there any library can do this approximate? On 3/11/2022 00:45，Manfred ***@***.***> wrote： There exist no analytical function to calculate the length of a B-spline, you have to approximate the curve and calculate the length of the polyline. — Reply to this email directly, view it on GitHub, or unsubscribe. Triage notifications on the go with GitHub Mobile for iOS or Android. You are receiving this because you authored the thread.Message ID: ***@***.***>

[mozman]

import ezdxf
from ezdxf.math import ConstructionPolyline

doc = ezdxf.new()
msp = doc.modelspace()
fit_points = [(0, 0, 0), (750, 500, 0), (1750, 500, 0), (2250, 1250, 0)]

spline = msp.add_spline(fit_points)
# Adjust the max. sagitta distance to your needs or run the calculation in a loop 
# reducing the distance until the difference to the previous run is smaller 
# than your expected precision:
polyline = ConstructionPolyline(spline.flattening(distance=0.1))
print(f"approximated length = {polyline.length:.2f}")