NURBS subdivision wrong. #44332

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humphries boyce · 2015-04-10T00:18:23+02:00

humphries boyce commented

2015-04-10 00:18:23 +02:00

Curve>>segments>>Subdivide

  for NURBS circle changes the shape to a rounded rectangle.

This seems to be wrong for all NURBS.

The Bohm algorithm for dividing a single interval is (this is for a cubic NURBS)

Q_i = P_i for i <= j-3
Q_i = (1-a_i) P_i-1 + a_i P_i for j-2 <= i <= j
Q_i+1 = P_i for j+1 <= i

a_i=(t' - t_i)/(t_i+3 - t_i)
The interval being divided is between t_j and t_j+1, and is split at t'.
Q_i are the new control points.
P_i are the old control points.

This is taken from Computer Graphics: Principles and Practice
by Foley, van Dam, Feiner and Hughes, page 509.

The Oslo algorithm for multiple divisions can be found in
"Discrete B-Splines and Subdivision Techniques in Computer-Aided Geometric Design and Computer Graphics" by Cohen, E.T. Lyche and R. Riesenfeld
CGIP 14(2), October 1980 87-111.

Blender Version
Broken: (example: 2.69.7 4b206af, see splash screen)
Worked: (optional)

Short description of error

Exact steps for others to reproduce the error
Based on a (as simple as possible) attached .blend file with minimum amount of steps

Curve>>segments>>Subdivide ``` for NURBS circle changes the shape to a rounded rectangle. ``` This seems to be wrong for all NURBS. The Bohm algorithm for dividing a single interval is (this is for a cubic NURBS) Q_i = P_i for i <= j-3 Q_i = (1-a_i) P_i-1 + a_i P_i for j-2 <= i <= j Q_i+1 = P_i for j+1 <= i a_i=(t' - t_i)/(t_i+3 - t_i) The interval being divided is between t_j and t_j+1, and is split at t'. Q_i are the new control points. P_i are the old control points. This is taken from Computer Graphics: Principles and Practice by Foley, van Dam, Feiner and Hughes, page 509. The Oslo algorithm for multiple divisions can be found in "Discrete B-Splines and Subdivision Techniques in Computer-Aided Geometric Design and Computer Graphics" by Cohen, E.T. Lyche and R. Riesenfeld CGIP 14(2), October 1980 87-111. **Blender Version** Broken: (example: 2.69.7 4b206af, see splash screen) Worked: (optional) **Short description of error** **Exact steps for others to reproduce the error** Based on a (as simple as possible) attached .blend file with minimum amount of steps

humphries boyce commented

2015-04-10 00:18:23 +02:00

Changed status to: 'Open'

humphries boyce self-assigned this 2015-04-10 00:18:23 +02:00

humphries boyce commented

2015-04-10 00:18:23 +02:00

Added subscriber: @humphries

Bastien Montagne commented

2015-04-10 16:40:37 +02:00

Added subscriber: @mont29

Bastien Montagne commented

2015-04-10 16:40:37 +02:00

Not quite sure what you are referring to? Do you have an actual example of this failure (like a .blend file showing it)? If you are referring to our code, adding a precise reference to it (file and lines) would help too…

Just throwing another algo like that does not help much without context…

Not quite sure what you are referring to? Do you have an actual example of this failure (like a .blend file showing it)? If you are referring to our code, adding a precise reference to it (file and lines) would help too… Just throwing another algo like that does not help much without context…

humphries boyce commented

2015-04-10 22:51:23 +02:00

HOW to test

Run blender
Create a NURBS circle
Select it
Change mode to edit
Subdivide it with Curve>>Segments>>Subdivide /* This should only increase the number of control points */

The NURBS object is no longer a circle!

All NURBS objects have there shape change by Curve>>Segments>>Subdivide.

HOW to test 1. Run blender 2. Create a NURBS circle 3. Select it 4. Change mode to edit 5. Subdivide it with Curve>>Segments>>Subdivide /* This should only increase the number of control points */ The NURBS object is no longer a circle! All NURBS objects have there shape change by Curve>>Segments>>Subdivide.

Bastien Montagne commented

2015-04-10 23:17:03 +02:00

Added subscriber: @Sergey

Bastien Montagne commented

2015-04-10 23:17:03 +02:00

OK, see what you means now… This sounds more like a TODO than a bug, though. Sergey might be interested too?

humphries boyce commented

2015-04-11 20:29:32 +02:00

A circle should stay a circle.
A sphere should stay a sphere.
etc.
That NURBS surface you work so hard to get right should keep its shape.
This is a bug!

A circle should stay a circle. A sphere should stay a sphere. etc. That NURBS surface you work so hard to get right should keep its shape. This is a bug!

Sergey Sharybin commented

2015-04-20 16:09:04 +02:00

@humphries, technically it is a TODO since current subdivision algorithm is working as it was designed to. Doesn't mean it should not be improved tho, but improvements are a part of TODO/ongoing development.

P.S. Seems you're familiar with the curves, maybe you'll be interested in preparing patch for the subdiv operator in blender? :)

@humphries, technically it is a TODO since current subdivision algorithm is working as it was designed to. Doesn't mean it should not be improved tho, but improvements are a part of TODO/ongoing development. P.S. Seems you're familiar with the curves, maybe you'll be interested in preparing patch for the subdiv operator in blender? :)

Chris Kohl commented

2021-06-23 00:00:00 +02:00

Added subscriber: @ckohl_art

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NURBS subdivision wrong. #44332