Curves
The foundation of a Nurbs surface is a curve. Surfaces are web of interconnected curves. The curves help you create and modify surfaces. Proficiency at drawing and editing curves is an important part of Nurbs modeling.
You can not render curves. With Nurbs curves ,you can position points on a curve or surface exactly where you want them and reshape the curve or surface by moving just a few control points that lie on or near the curve. Examples of NURBS primitives are spheres, cubes, cylinders, cones and planes.
A major advantage of modeling with NURBS in Maya is that you can convert any NURBS object to Subdivision Surfaces (also called "SubDs") or polygons at any time.
Using NURBS can have its drawbacks, however. When modeling a character that needs to be animated, you might notice seams, or even visible gaps, between NURBS surfaces .As a result of the way NURBS are constructed, you rarely come up with an object composed of a single surface, and the surfaces must meet together exactly to avoid these problems.
Elements of a Curve
The elements of a curve follow:
A curve point is an arbitrary point on the curve. It can have the same position as a CV or edit point.
CVs are control vertices ,These points often lie away from the curve.
Edit points are also called knots. You can move these points to reposition a specific point on the curve. You can't move the edit points of a surface.
The start of a curve is the first point of the curve created when you draw the curve.The end of the curve is the last point of the curve you create. You need to know which points are the start and end points for certain modeling operations.
The curve direction is displayed as a small letter u.
A hull is a network of straight lines that connects CVs. Hulls are mainly visual aids to see where interconnected CVs exist.
A span is the point of a curve between the two edit points. If you add extra edit points to a curve enhance your control of its shape, you increase the number of spans. you don't select and manipulate a curve span directly.
Fig 33 Elements of curve
Tools for Creating Curves
There are three tools for creating curves
 Create > CV Curve Tool
 Create > EP Curve Tool
 Create > Pencil Curve Tool
CV Curve Tool
This Tool creates the curve's CV exactly where you click them. It creates edit points based on where you create the CVs. When it turns white, you have created enough CVs to complete the curve. You must create at least four CVs when you use the default option settings.
EP Curve Tool
The EP Curve Tool creates the curve's edit points exactly where you click them. It creates CVs in positions based on where you create the edit points. EP stands for edit points. You must place at least two edit points to create the curve.
Pencil Curve Tool
The Pencil Tool lets you draw a curve by dragging the mouse or by using a digitizing pen and tablet. When you create a curve using the Pencil Curve Tool, you can not delete curve segments by pressing the Backspace key.
Maya creates a trail of edit points and sometimes several curves where you drag the mouse or pen. The CVs created with this tool might seem to pose efficiency and control problems, you can simplify the curves with Edit Curves > Rebuild Curve.
Attaching curves
Use Edit Curves > Attach Curves to create a single curve from two curves.
Attach Method
Connect joins the curves with minimal curvature smoothing at the joint point.
Blends smooths the curvature at the joint point based on the blend Bias value.
Detaching Curves
Use Edit Curves > Detach Curves to break a curve into two curves or to open a closed curve.
Construction Plane
Use Create > Construction Plane to create a plane as a construction aid. You can not rotate a construction plane, make it a live surface, then draw a curve on it. You can not render or animate a construction plane.
Select create > Construction Plane to open the option window.
Fig 34 Construction plane options
