Problem moving vertices of polyline

[Mase123y]

Im having an issue with a Polyline that was converted from a spline. Im trying to move the vertices and it works like I would expect at the top of the polyline but when I move the vertices toward the bottom of the polyline, the vertices that move are not the ones Im clicking on. Its confusing for me and Im not sure whats going on. This was created in Autocad LT 2013. This was first created a long time ago and Im not sure exactly how it was created so I can not recreate it with a new line because it the new line doesnt match up exactly. In the drawing are the fit points used to create the line. Can someone check to see if this happens in their own AutoCAD? I dont know if its the polyline or AutoCAD doing this.

 

Edit: Seems like I created this line starting off with a 2D Polyline then changed it using PEDIT and chose FIT and it still causes this issue with new lines.

 

Poly.dwg

Edited January 11, 2016 by Mase123y

[rkmcswain]

I get the same behavior as you here. (Civil 3D 2014, and AutoCAD 2016)

[guran]

The strange thing is that if you change the curve from "Curve fit" into any of the other options it works fine.

[Mase123y]

perhaps thats what ill have to do is change the fitting to something else so I can modify the vertices then change it back

[lrm]

Several years ago the only spline type available with AutoCAD was created by converting a polyline to a spline (technically, a B-spline). There are two options for the conversion via the PEDIT command. They are Spline or Fit. The Pedit Spline option used the vertices of the polyline to define the Control Vertices (CV’s) of the B-spline. CV’s are used by the B-spline math to define the shape of the spline but except for the first and last CV, they do not lie on the spline. The Fit option uses the vertices of the polyline to define “fit” points for the spline, that is, points which the spline passes through. In either case, the result in AutoCAD is a third order B-spline.

 

When you converted a spline to a polyline you created a polyline with many vertices. AutoCAD was approximating the smooth spline by a series of short straight line segments. Then when you used Fit you were asking AutoCAD to create a spline that passes through all of the vertices. The resulting B-spline must maintain continuity of slope for the curve as it passes through all the CVs thus the strange behavior when you move one of the fit points.

 

Note, AutoCAD would later add a new object type called Spline. It is based on NURBS (Non-Uniform Rational B-Splines). NURBS provide many benefits over plain B-splines. NURBS may have a discontinuity of slope (sharp corners), can exactly duplicate an arc (a B-spline cannot), and has other benefits. The “rational” part of a NURBS applies to the fact that the curve is defined by the ratio of two B-splines. “non-uniform” allows for random trimming without the redefinition of CVs. BTW, a B-spline can be thought of a series of Bezier splines laid head to tail.

 

Why are you converting the spline to a polyline? If you want a "nice" spline (assuming the original was not) I suggest creating a new spline (not a polyline spline) by using only a few of the vertices to define fit points (using the Fit Method) and then using SPLINEDIT to refine the spline's shape. I think the curve you show could be accurately define with 5 or 6 CVs but it depends on what your needs are.

 

 

~Lee

[Mase123y]

I need to use a polyline because I need to utilize this curve in an older version DXF which doesnt support splines very well if at all.

[lrm]

If you need to use a polyline I would suggest using the PEDIT Spline option with the polyline and not the Fit option. Check out the options of the PEDIT command. Depending on how you want to edit the resulting shape it might be better to create a new polyline with as few vertices as possible to reproduce the desired shape. In the image below 1 is a screen capture from your post, 2 shows a polyline (yellow) that I constructed over the shape using 9 CVs (Control Vertices) on a polyline that has been splined. Depending on your needs, I would prefer to delete the third from the top CV.

3 shows the vertices more clearly. The more CVs you use the more you increase the likelihood that you may introduce undesired wrinkles. The second and next to the last CV control the slope and radius of curvature at the ends of the spline.

~Lee