July 24, 2020, 3:47pm
In viewing the latest excellent tutorials on “Better Living Through CNC” on YouTube, the topic of curve degree and curve spans has come up. The Alias Golden Rules was mentioned as a good reference for this and its rule
#3 discusses curve span. I can’t figure out how, in Rhino, how to determine and control how many spans a curve has. Can someone enlighten me?
A single-span curve has degree+1 control points. Any more than that, the curve is multi-span. So a degree 3 single-span curve will have 4 control points and a degree 5 single-span curve 6 control points.
July 24, 2020, 4:08pm
Thanks. See the attached. Same curve drawn 2 ways. Have I interpreted these correctly?
The first curve looks like two degree=3 curves that are joined.
See if this helps:
July 24, 2020, 4:23pm
The WHAT command on the top curve returns this:
ID: dd79e855-f64e-4b92-b46b-91bba3d9adec (8033)
Object name: (not named)
Layer name: Default
source = from layer
index = -1
Active visual analysis modes:
Curvature graph analysis
Open NURBS curve
start = (-1’-8 1/4",8 1/8",0")
end = (2 11/16",1’-2 1/4",0")
degree = 5
control points: non-rational, count=9
knots: non-uniform, domain = 0" to 3’-0 1/16"
clamped at start and end
How do I know how many spans there are from this information?
July 24, 2020, 4:29pm
it kind of doesn’t matter how many spans it has if it’s above single span (degree + 1 control point)
If it’s not single span, then it’s a multi span curve and the single span rules kind of go out the window.
July 24, 2020, 4:42pm
I guess I’m having trouble determining when I’ve gone from a single span curve with a lot of CV’s to a multi-span curve.
Did you read the page I linked about NURBS cuves?
If you are using degree 3 curves, more than 4 CV’s it’s multi-span…
Don’t forget the difference between two or more joined curves (a polycurve) and a single NURBS curve which may be single or multi-span…
July 24, 2020, 5:38pm
Greek to me but thanks anyway.
July 24, 2020, 5:59pm
Sorry John, I didn’t mean to be flip earlier. The page you sent me to was way over my head I’m afraid.
Nothing to be concerned about.
My push back point is this is the level of technical detail you need to understand the specific questions you’re asking.
Here’s a link to the same information in a more friendly accessible format:
rhino_basics.pdf (2.5 MB)
July 24, 2020, 6:45pm
Thanks John. Much more understandable.