FitCrv degree2 creates S curve?


#1

Hi,
V5
attached the curve before and after, the lower is the before, based on what was a paper plan for a deg2 curve, it in fact has two shallow S curves so it seems when analysed with CurvatureGraph.

Thus proving yet again that the deg2 data I am presented with can trip one up !

I applied tolerance 1 for angle (default value) and fitting and chose degree 2 and hit enter and yet the graph goes both sides of the result, which must be an S curve, correct me if I am wrong ?

choosing deg 2 should force out any S curves should it ?

this is a generic question, not project related !

deg2 sees S curve with FitCrv.3dm (25.5 KB)

Steve


#2

I’m afraid you didn’t understand what was explained about degrees etc… Degree 2 curves can have “S” bends if they have more than 3 points. ONLY degree 2 curves with exactly 3 control points (a single span) can be guaranteed to not have an “S” shape.

–Mitch


#3

Hi Mitch,

ooops sorry, I need to revisit that explanation.

Means I need to call S curves as S curves, not degree 2 curves !

How would one use FitCurve to kill any S bend ? I saw the degree option and hoped that was it !

I need to retain start and end points, kill any S bends, follow original curve as best as possible. keep planar.

something with a set of sliders so that the curvature graph changed on the fly would be nice.

  1. pick points curve must pass through
  2. tick box kill S curves
  3. tolerance to original shape, maybe a radius of effect beyond area of change option.

drag slider to see on the fly changes.

I would love to find a set of exercises that cover all the curve editing one might experience, well at least most, covering what the commands are capable of.

A day online in curve class or having downloaded McNeel exercises or watched a few videos I would well make use of.

I still long for the handle curves…I shall visit my other post to see what was the answer there, as Rhino22 also shared my need.

Curve editing is not easy…for me. The handle curve method in Freehand was so user friendly, that with accuracy in Rhino would be good. Unless the handle crossed the curve one had no S bend, smooth transition through points, and could move curve at those points to a new location/point and all behaved itself.

I will when I get time import a few tests from Freehand and see if the CurvatureGraph like the results. No doubt that isnt final seal of approval some other reason for unacceptable, but it would be interesting to see if any S bends occur with the handles all outboard of the curves.

How do others find Rhino curve editing, I would like to know.

Steve


#4

Steve, do you realise that you can use curvature and control points together, so setting your nudge key increments appropriately and nudging the control points will tidy up the s-curve.


#5

Steve1 -

I suggest you do the Level II coursework

http://www.rhino3d.com/download/rhino/5.0/Rhino5Level2Training/

-Sky


#6

Basically, you can’t… There is no command that I know of in Rhino that will automatically eliminate inflection points (S-Bends) in existing curves. (aside from rebuilding into something that by definition can’t have an S-Curve, such as the aforementioned degree 2+3 points curve - basically an arc or parabola - or a degree 1+2 points curve - which is a line…)

The tools you have are Fair (to try and smooth out noisy curves) and Rebuild (to increase/reduce the number of points and redistribute them), plus CurveDeviation to see how far off you went and CurvatureGraph for a visual gauge of the resulting curvature smoothness. Using those tools is “all manual” and requires a trained eye and some experience.

–Mitch


(David Cockey) #7

Manual editing of control points while watching the curvature graph. It can be tedious but it works cut patience is required. A number of small adjustments of control points works better than trying a few large adjustments.

Also needed is an understanding of how much of a curve will be affected by moving a control point, and how the relative positions of control points to each other affect curve shape.


#8

Hi ,
Taking on board lots, with Graph on, been nudging, to retain tangents and also lessening the length of the fronds relative to each other in Curv Graph, rebuilding, matching, Fair, Smooth, really grateful to now know all this.

CurveGraph tells all, any S bends can be dealt with.

May I ask, hopefully not a messy question…nudge does vert or horiz, any trick for nudging a control point perp to the curve ? towards or away ? At the mo I nudge 10 right then 10 up, not quite perp but approx.

      Manual editing of control points while watching the 

curvature graph. It can be tedious but it works cut patience is
required. A number of small adjustments of control points works better
than trying a few large adjustments.

Also needed is an understanding of how much of a curve will be
affected by moving a control point, and how the relative positions of
control points to each other affect curve shape.

Practice makes perfect. Knowing when to nudge following control point line and when to nudge perp to curve.

Mitch …cheers…CurveDeviation…thats a new one for me…will explore as the Mk1 eyeball may not be enough.

Steve


(David Cockey) #9

“Page up” and Page down" keys move the cursor normal to the current CPlane.

Holding Shift key while nudging increases the step size. Holding Ctrl key while nudging decreases the step size. Step sizes can be adjusted in Properties > Modeling Aids > Nudge.


(David Cockey) #10

In addition to CrvDeviation which displays the deviation between two curves, PointDeviation will show the deviation between a set of points and a curve or surface.