Difference between Edit points and CVs

[Anonymous]

Hi, I want to ask for help, any advice what is the difference between Edit points and CVs?  I know that Edit point is between two Spans  and thanks to CVs I can modify lines, curves in Alias but this I can do with Edit points too isnt it? Thank you all  for help. Joseph

[Anonymous]

You can edit curves by modifying both CVs and EPs, but CVs are more common to modify in Alias. You correctly pointed out the connection of two spans at the EP location. For more info have look here:

https://knowledge.autodesk.com/support/alias-products/learn-explore/caas/CloudHelp/cloudhelp/2017/EN...

https://knowledge.autodesk.com/support/alias-products/learn-explore/caas/CloudHelp/cloudhelp/2017/EN...

https://knowledge.autodesk.com/support/alias-products/learn-explore/caas/CloudHelp/cloudhelp/2018/EN...

I just asked Google for that information and it is all summarized quite well on the knowledge center of Autodesk. If you´re not looking for more precise answers, this should help. Otherwise you should form your question differently.

Cheers!

[rauscht]

When creating a new curve of a certain degree from the scratch, you have basically two options:

1. Create the curve by specifying the Control Vertices (CVs)
   => Use "Curves" / "New Curves" / "New CV Curve"
2. Create the curve by specifying some Edit Points (EPs) the curve should run through
   => Use "Curves" / "New Curves" / "New Edit Point Curve"

(Both tools have an option dialog where the intended degree can be specified. For now, I assume this degree been set to 3.)

In the attached screen shot, you will find two planar curves (z = 0) to the top, both using four input points at almost identical positions (mirrored by y-axis). In the left curve these four points are interpreted a CVs, while for the curve to the right the giving point are taken as EPs.

1. The curve has degree d = 3 (of course) and consists of a single span (as degree 3 Bezier curve is defined by 4 CVs).
   Addition: If the user would have used (d + s) many input CVs the yielded curve would consist of s spans.
2. This curve has also d = 3 (see above) and runs through the specified 4 points. It has 6 CVs (which are needed / implied as to meet the requirement to run through these EPs); consequently, it has 3 spans.

Please note that EP curves should be considered to be CV curves too, and vice versa. Actually, from the math point of view there is no difference: all NURBS curves are defined by their knot vector (which defines degree and parametrization) and their CVs.

To illustrate this, please have a look at curve 3 in the attached screen shot. It's a degree 3 curve again having 6 CVs. At first glance, one might think it has been defined as an EP curve running through the four points on that curve (similiar to curve 2). Actually, I have defined this curve using "New CV Curve" tool. Hence, the displayed points on all three curves (which each curve passes through) are EPs and they are available even if one introduces the curves as "CV curve" (remember: every curve is a CV curve!)

• curve 1 has two EPs: start and end point
• curve 2 and 3 has four each: start and end point, and two in the middle

curve 2 and 3 both have 3 spans; the transition points between spans are exactly those EPs.

Hth

Thomas

[c3systems]

@Anonymous

I assume you are referring to a multi-span curve.

Editing the curve using CVs will "locally" affect the curve.

Editing the curve using Edit Points will "globally" affect the curve.

When using the edit point modification, the curve is continuously generated/evaluated so that it passes through all the edit points in the most smoothest manner. Then again this "smoothness" and resultant placement of CVs will depend on whether the parameters are Chord based or Uniform.

In practise, editing a curve using Edit Points might be difficult, especially if the edit points are not "faired" properly. Try moving the Edit Points of a curve created using CV method and observe how the curve (and CVs) changes shape in a global manner.