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Setting curves to another spline type makes them shorter
Confirmed, NormalPublicKNOWN ISSUE


System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 460.79

Blender Version
Broken: version: 2.92.0, branch: master, commit date: 2021-02-24 16:25, hash: rB02948a2cab44
Worked: (newest version of Blender that worked as expected)

Short description of error
I discussed this with Campbell during a meeting 2021-02-23. He suggested that I report it as a bug.

When setting curve spline type from nurbs to bezier, the curve becomes shorter. It also becomes shorter when I convert it from bezier to nurbs.

It is worth to note that when converting from bezier to nurbs, the curve can get back the length by changing the following settings in
object data properties > active spline

Screen grab of issue

Exact steps for others to reproduce the error

  1. Add curve > path
  2. Go into curve edit mode
  3. Subdivide the curve 2 times in order to get enough vertices for changing spline type to bezier
  4. Change spline type from Nurbs to Bezier - observe that the curve becomes shorter.
  5. Change spline type from Bezier to Nurbs - observe that the curve becomes shorter

Event Timeline

Falk David (filedescriptor) changed the task status from Needs Triage to Confirmed.Mar 1 2021, 9:43 AM

Can confirm this on the 2.92 release and the latest 2.93.0 Alpha, branch: master, commit date: 2021-03-01 06:12, hash: rBbfc70a6a958b.

Bastien Montagne (mont29) changed the subtype of this task from "Report" to "Known Issue".Mar 1 2021, 3:42 PM
Bastien Montagne (mont29) moved this task from Backlog to Known Issues on the Modeling board.

Would consider that a known issue rather than a bug, this is just the conversion 'algorithm' being dumb and super-basic, instead of trying to be smart...

Note that you will always lose some data during such conversion anyway, since interpolation methods are quiet different.