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View Full Version : What is the difference btw CV e EP Curve?


Dias
07-10-2004, 02:21 AM
Hi

I've started to study Nurbs surfaces to improve my car modeling tecnique, and the first think that I want to ask you is: can you explane me the difference between CV and EP curve tools?

I don't understand very well how EP Curve Tool works, the curve is go anyway bot not the way I want. :shrug:

So I use CV Curve Tool because it's more easy to control, but I want to know advantages/disadvantages of one on another. :)

Thanks

Charkins
07-10-2004, 02:29 AM
F1 :)

I tend to use EP for mechanical stuff (1 degree extrudes, etc) and CV for organic stuff (lofts, patches, etc).

You can find a tutorial ($) at www.digital-tutors.com (http://www.digital-tutors.com) and it goes through a tut and tells you pros/cons.

Dias
07-10-2004, 02:38 AM
F1 didn't helped me. :hmm:

Now I try digital-tutorial, I hope to find somethink free. :)

rock
07-10-2004, 03:28 AM
From Maya 6 doc,

Use EP curve tool if you want the curve to pass through certain points. It calculates the positions of CVs based on where you want the edit points.

The Pencil Curve tool creates a curve with a large number of data points. Use Edit Curves > Rebuild Curve (http://localhost:4446/Maya6.0/en_US/NURBS/editcurvesrebuildcurve.html) to simplify the curve.

In theory, moving edit points would be an excellent way to edit a curve, since they lie on the curve itself. Unfortunately, it doesn't work out that way. This is because the shape of the curve determines the positions of edit points, not the other way around.

Maya does allow you to move edit points by "reverse engineering" the curve from the edit point. When you move an edit point, the Move tool tries to find a curve which passes through the new edit point location.

Because this process is time-consuming and has an infinite number of solutions, the tool must place constraints on how moving the edit point affects the curve. Because of these constraints, you usually cannot make major changes well by moving edit points. Moving edit points is best for small scale reshaping.

Even though it is slightly less intuitive, the only way to reshape the curve with complete power is by moving CVs.

CVs (control vertices) control how the curve is "pulled" from a straight line between edit points. They are the most basic and important means for controlling the shape of a curve. Lines between consecutive CVs form the control hull.

The number of CVs is equal to the degree of the curve plus one. So, for example, a degree 3 curve has four CVs per span. To increase the number of CVs to gain more control over curve shape, you can increase the number of spans by inserting edit points or increase the curve's degree.

Maya draws CVs differently to let you tell the difference between the start and the end of a curve. The first CV (at the start point of the curve) is drawn as a box. The second CV is drawn as a small "U", to show the increasing U dimension from the start point. All other CVs are drawn as small dots.

You can tell when a curve is made from multiple spans in several ways. One is to look for edit points on the curve. Edit points mark the connection point between two spans. Maya draws edit points as small Xs.

Unlike the on-curve control points of Bezier curves (used in many 2D illustration programs), NURBS edit points are not usually used for editing curves. CVs control the shape of a NURBS curve, and edit points are just indicators of how many spans a curve has.

There are, however, a few tasks that use edit points:

If you want more control in a curve, you can insert an edit point to increase the number of spans in the curve and give you more CVs to work with.
You can also delete edit points to decrease the number of spans in a curve (and probably change the shape of the curve).
It is possible to move edit points to change the shape of a curve, but you should avoid doing this except for minor adjustments.

Emil3d
07-10-2004, 03:42 AM
There is no difference in the final result, just a different way of drawing the same thing. All curves have CVs and EPs components. To see them select a curve and go to Display>NURBS components>CVs and Edit Points. When you are clicking with the EP tool you are defining the Edit Points through which the curve passes. With the CV tool you are defining the Control Vertices which draw the tangent lines (hulls) of the curve. Thatís all about understanding. About getting used to using them is simply a matter of practicing and I donít thing a tutorial can help.

Emil3d
07-10-2004, 04:14 AM
I didnít see the Rockís message when I posted mine, but now I would add that I answered the question in your message: whatís the difference between the EP and CV Tools while Rock answered the question from your title which is more about understanding curves.

Dias
07-10-2004, 10:13 AM
Wow, thank you very much guys, it helps me. :)

Now I can start modeling my Z4. :buttrock:

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