Line curvature, fit crv

[jackhui328 (Jack)]

hi,

Im trying to build a surface with a nice curve. but when I turn on the curvature graph it is really messy. I try to use fitcrv to smooth it out and minimize the change of shape. But it change too much. I don’t understand the term “fitting tolerance” and “angle tolerance”.

Here is another question

What is the difference between control point, knot point and kink?
I feels kink should be just a point breaking the curve into 2. But I have no idea of difference of the other. but somehow they perform difference in curvature graph

And also the I found after I use the the add control point or knot command the curvature graph is no longer smooth. Some very sharp angle appear in the curvature graph. Unless I delete the point I originally added. How should I decide to add control point or knot?

[Tom_P]

Dear @jackhui328

i recommend that you read the following:

What are NURBS?

https://docs.mcneel.com/rhino/7/help/en-us/index.htm#commands/insertknot.htm?Highlight=insertknot

https://docs.mcneel.com/rhino/7/help/en-us/index.htm#commands/insertcontrolpoint.htm?Highlight=insertControlpoint

also some topic already cover most of your questions:

What is really the knot of a NURBS curve?

What is really the knot of a NURBS curve? There is a lot of explanations about it, but I can’t understand it actually in practice usage. So when I’m drawing a curve I intuitively can understand what is the control points and edit points responsible for because I can move them and observe the curve reaction. I don’t use knots when drawing a new curve but can insert/remove them afterward. Why do I need to bother about knot at all? I have readen about knots but it’s a mathematical thing, how I sho…

Nurb Surfaces, Control points and weights

Yep. Anytime you force NURBS to do stuff that is not ‘smooth’ you will likely get surfaces/curves with internal fully multiple knots (kinks). They might be useful under certain circumstances, the user should simply be aware of what they are doing. Rhino will actually make stuff like this automatically, for example extruding a rounded rectangle with SplitAtTangents=No. That will make a single surface with internal G1 joints, and if you move the control points at those spots, you will also pr…

fitting-tolerance
the maximum distance between the input and the new output curve / geometry

angle tolerance
if you fit a curve with kinks (G0-continuity), for example a polyline, polygon …
The Fitting algorithm will consider kinks below this threshold as smooth.

for example if you start with a hexagon (6-side polygon) and set angle Tolerance to 61 Degree and Tolerance really wide / big / high(?) you should get something like similar to a circle…

hope this helps.
maybe you can check above resources and narrow your questions ?

kind regards. -tom

[pascal (Pascal Golay)]

@jackhui328

Some very sharp angle appear in the curvature graph.

Hello- keep in mind that a kink in the curvature graph is not a link in the curve - a degree 3 curve with more than one span will generally show a graph like this but that is a break in the rate of change of curvature, not in curvature.
Try drawing your curves as degree 5 (at least six points must be placed.) and look at the graph of those.

-Pascal