1. Curves and NURBS
Advanced modeling techniques, mostly suited for curved or wire-like structures that would prove tricky to model as a straight polygon mesh. Specifically, we are going to examine:
Metaballs/meta objects: Useful for modeling liquid objects like mud
Curves: Useful for modeling wire, especially of the bendable variety
Spin editing: Useful for modeling circular forms
NURBS (nonuniform rational basis spline): Useful for accurate modeling of curved surfaces

2. Metaballs
Metaballs (aka meta objects) are like ordinary primitives, but with a special ability to act like mud—squishing into each other in order to make new shapes.
1. Start with a new scene, select the default cube, and press X to delete it.
2. Add the metaball via Shift+A ➤ Metaball ➤ Meta Ball.
3. The new sphere looks ordinary enough so far, except that it has an extra ring around it. While you are in object mode, duplicate the metaball with Shift+D.
4. Drag the new metaball away from the first. As you do, notice how they meld together, as shown in Figure

3. Metaballs (Meta-Objects)
Experiment with how the meta objects interact with each other, including the other available types: Meta Tube, Meta Plane, Meta Cube, and Meta Ellipsoid
Be sure to transform (scale, rotate, and move) the shapes in order to observe the effects

4. How Meta Objects Work
If you observe a meta object in edit mode, you will notice it has a field of two circles: one red and one green. While in edit mode, you can RMB-select either of these circles and resize them using the S key (conversely, in object mode, resizing the meta object has the same effect as resizing the outer red circle).
The outer red circle is known as the selection circle.
The inner green circle marks the stiffness of the meta object

5. Making a mud monster
Exercise 1: Mud Monster

6. Curves
You can add curves by pressing Shift+A and accessing the Curve menu. There are three types of curves, as follows:
Bezier (curve and circle)
NURBS (curve and circle)
Path
They can produce some awesome organic-looking effects, such as growing vines or the tentacles shown in Figure

7. Bezier (Curve and Circle)
Circles and Bezier curves are controlled in a similar way to the paths in vector art programs such as Inkscape and Adobe Illustrator.
The control points are like seesaw lines that can be moved, rotated, or scaled to influence the positioning and curvature of the curve

8. NURBS (Curve and Circle)
NURBS curves and NURBS circles are similar to the circles and Bezier curves of the last section, except that the control handles act more like magnetic corners of a cage around the curve object, as opposed to actual points on the object itself.
As mentioned, NURBS stands for nonuniform rational basis spline, which is a technical description basically stating that the NURBS points control the shape of curves or surfaces in a gravity-like manner

9. Path
For a path, the controllers are joined as a line, with the path object flowing through the curve of the line; the object’s positioning is affected as an average of the values of the control points.

10. Modifying a Curve Path Editing Adding Points Deleting Points
Controller Handle Types
Align
Auto
Free
Vector
Making a Circle

11. 2D and 3D Curves
Main difference is that a 3D curve can have its control points positioned anywhere in the 3D space, whereas 2D curves are always flat on a 2D plane.

12. Modeling with a 2D Curve
Start with a blank scene (delete the default cube).
1. Add a Bezier circle (click Shift+A, and then select Add ➤ Curve ➤ Circle).
2. Add a second Bezier circle and shrink it down a little inside the first so the two circles form the outside and inside of the ring.
3. In object mode, Shift-RMB-select both circles.
4. Press Ctrl+J, and the two circles will become one object, looking something like Figure

13. Continue….
5. Go to the Object Data tab. By default, the curve type is probably set to 3D. Set it instead to 2D, and the space between the two circles will be filled in, as in Figure. This is because 2D paths form flat surfaces, filled in where possible.
A little further down the Object Data tab, in the Modification section, slide the Extrude value up to something like 0.2. This Extrude controller works much like pressing the E key to extrude polygon surfaces. When you increase the Extrude value, the circle is raised, forming a solid ring shape, as shown in Figure

14. The Blender logo, produced using a 2D path

15. Modeling with a 3D Curve Select the ring made from the 2D curve.
1. In the Shape area of the Object Data tab, click the 3D button. Notice that the caps of the ring will instantly disappear—they are not supported with the 3D version of curves. Now, if you use the Front and Back caps options in the Shape area of the Object Data tab, they will control whether the corresponding side has a bevel enabled.
2. Go into edit mode so you can manipulate the controller points.
3. Select a couple of controller points and move them upward (the Z direction) from their previously flat position.

16. Exercise 2: Curve Bugs

17. Spin
The Spin tool takes selected vertices of a model (in edit mode) and spins them around the 3D cursor to create either tubelike forms or a ring of repeating objects (the Dupli option determines whether the spin results in a ring or repeating objects in a circle— see Figure

18. NURBS
NURBS surfaces are curve based. They are highly accurate when modeling large curved areas like a bathtub, the keel of a boat, or the curved surfaces on a modern car, although they lack the control that polygon modeling offers at the individual vertex level.