 Move start point to end point of curve

Hi;
How can I move the start point of curve to the end point of curve and let curve aways open ?

Do you mean how can you make a closed curve read as open?

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You can’t (in Rhino). Curves with concurrent start/end points are automatically closed. Why do you need this?

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Well, there seems to be reasons…
for example if you try to make a shape like this: this is a loft with History and I closed the upper curve afterwards 3 Likes

Hi@ Michael_Pryor:
Yes, do you have any idea?

Hi@ Helvetosaur:
How abaut create a new curve with the curve’s control point?

Sorry if I missed the point with my previous approach.
The beast you are talking about may be an un-clamped curve. In Rhino all curves are clamped at start and end by default and as far I know there is no way with the standard interface to model it manually. But you can use the SDK and create an un-clamped curve with a custom knot spacing. non-clamped curve.3dm (26.1 KB)

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An open curve in Rhino can have end points an arbitrarily small but non-zero distance apart. I don’t believe there is anything in the math which precludes an open curve from having end points coinciding with exactly the same coordinates. However if the end points coincide then Rhino automatically changes the curve type to closed. This appears to be due to a decision made when the code was written, not the underlying math.

In the attached example the ends the gap at the end of the open curve is less than 0.0000000 and the open and closed curves differ by less than 10e-08. Both curves are clamped at both ends.
Open Vs Closed DC01.3dm (56.5 KB)

Geometry:
Valid curve.
Closed NURBS curve
start = (64.0944100,40.4634158,0.0000000)
end = (64.0944100,40.4634158,0.0000000)
degree = 3
control points: non-rational, count=7 (1 duplicate)
knots: uniform (delta=30.7558793), domain = 0.0000000 to 123.0235174
clamped at start and end

curve

ID: 3e257faf-7269-44bf-87cc-06d8da89b243 (57)
Object name: (not named)
Layer name: Open
Render Material:
source = from layer
index = -1

Geometry:
Valid curve.
Open NURBS curve
start = (16.7885861,40.4634158,0.0000000)
end = (16.7885861,40.4634158,0.0000000)
degree = 3
control points: non-rational, count=7
knots: uniform (delta=30.7558793), domain = 0.0000000 to 123.0235174
clamped at start and end

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Hi @suc_kiet
Could you solve your problem? What was it actually?

Hi@ Jess;
I cant not make the curve open if the start point are at the same position, so I move the end point back a tolerance distance, this way meet my request.
Thanks for all friend’s reply.

Yes, unfortunately it is not possible anymore to open a non-periodic closed curve by dragging the last/first point at the seam location. I would agree that this should be possible! Personally I don’t like to introduce tiny inaccuracies to hack around such shortcomings. Maybe this can be added to the bug tracker? @pascal Thanks.

Hi Jess, all - there is testSeamOpener but it ‘does what it does’, you don’t get to tell it how much…

-Pascal

Half of the control point distance. That is fine for me - Thanks Pascal!