From BlenderWiki

Curve Editing

Curve editing has less tools and options compared to its mesh counterpart. This does not mean it is easier or less powerful!

This page covers the basics of curve editing – more advanced topics, like retopology, or extrusion (and bevel and taper), are addressed in the next page.

Basic Curve Editing (translation, rotation, scale)

Once you have a selection of one or more control points, you can grab/move (G), rotate (R) or scale (S) them, like many other things in Blender, as described in the Manipulation in 3D Space section.

Note that Bézier control points contain three vertices (the central one and the two handles), so a whole selected control point is equivalent to three selected vertices for transform tools (i.e. you can rotate and scale it, unlike standard mesh or NURBS vertices).

Note that in general, Bézier curves are easier to edit than NURBS, as when you modify a control point, you only the two curve segments on both side of the point. With NURBS, when you move a vertex, the curve can be modified up to three point on both side, depending on the order of the curve… Moreover, a Bézier curve always pass through the center of all its control points – NURBS are far from being so simple!

You also have in Edit mode an extra option when using these basic manipulations: the proportional editing.

Advanced Transform Tools

The To Sphere, Shear, Wrap and Push/Pull transform tools are described in the Manipulation in 3D chapter.

The two other tools, Tilt and Shrink/Fatten Radius are related to curve extrusion.

Bézier Control Points Settings

This only concerns Bézier curves. As we saw in a previous page, these curves can have four types of handles (giving smooth or angled curve…). ⇧ ShiftH makes all selected control points automatic, H toggles between free and aligned, and V makes them vector. Follow above link for more details.

NURBS Control Points Settings

We also saw in the same previous page that NURBS control points have a weight, which is the influence of this point on the curve. You set it either using the big Set Weight button in the Curve Tools panel (after having defined the weight in the numeric field to the right), or by directly typing a value in the W numeric field of the Transform Properties panel.

Adding New Segments

Once a curve is created you can add new segments (in fact, new control points defining new segments…), either by extruding it, or placing new handles with CtrlLMB clicks. Each new segment is added to one end of the curve. A new segment will only be added if a single vertex, or handle, at one end of the curve is selected. If two or more control points are selected nothing is added (however, if you used the Extrude command, all selected control points are placed in Grab mode…).

Note that unlike with meshes, you can’t create a new curve inside the edited object by just CtrlLMB -clicking with nothing selected – to do so, you can cut an existing curve in two parts (by deleting a segment), copy an existing one (⇧ ShiftD), or add a new one (Add menu)…

Opening and Closing a Curve

This toggles between an open curve and closed curve. Only curves with at least one selected control point will be closed/open.

The shape of the closing segment is based on the start and end handles for Bézier curves, and as usual on adjacent control points for NURBS. The only time a handle is adjusted after closing is if the handle is an Auto one. (Open curve) and (Closed curve) is the same Bézier curve open and closed.

This action only works on the original starting control-point or the last control-point added. Deleting a segment(s) dosen’t change how the action applies; it still operates only on the starting and last control-points. This means that C may actually join two curves instead of closing a single curve!

Remember that when a curve is closed, it creates a renderable flat face.

Open curve.

Closed curve.

Closed curve (Solid).

Duplication

Well, this command just duplicates the selected control points, along with the curve segments implicitly selected (if any). The copy is selected and placed in Grab mode, so you can move it to another place.

Deleting Elements

The Erase pop-up menu of curves offers you three options:

Selected

This will delete the selected control points, without breaking the curve (i.e. the adjacent points will be directly linked, joined, once the intermediary ones are deleted). Remember that NURBS order cannot be higher than its number of control points, so it might decrease when you delete some control points… Of course, when only one point remains, there is no more visible curve, and when all points are deleted, the curve itself is deleted.

Segment

This option is somewhat the opposite to the preceding one, as it will cut the curve, without removing any control point, by erasing one selected segment.

This option always removes only one segment (the last “selected” one), even when several are in the selection. So to delete all segments in your selection, you’ll have to repetitively use the same erase option…

All

As with meshes, this deletes everything in the object!

Joining or Merging Curves

Joining curves is really the act of making a segment between the two curves. To join two separated use one end control point from each curve. The two curves are joined by a segment to become a single curve. (One curve joined) is the result of joining (Two curves). The segment, labeled “S”, is the new segment joining the two curves. F for creating curve segments… Well, in fact, the same shortcut creates edges (and faces) in meshes, so…

Two curves.

One curve joined.

You cannot close a curve by joining the curves, you must close the curve. You will get the error “Can't make segment” when you attempt to join using the starting and last control-point. For example, in (One curve joined) you must use close to close the curve. You will get the same error if you have not-end points selected, or too much end-points, etc.

Note that you can only join curves of the same type (i.e. Bézier with Bézier, NURBS with NURBS, …)!

Subdivision

Curve subdivision is most simple: using either the Subdivide entry in the Specials menu (W), or the Subdivide button of the Curve Tools1 panel, you will subdivide once all selected segments by adding a new control point exactly between each pair of selected ones.

Switch Direction

This command will “reverse” the direction of any curve with at least one selected element (i.e. start point will begin end one, and vice-versa). Mainly useful when using curve as path, or the bevel and taper options…

Other Specials Options

The Specials menu contains some additional options:

Set Goal Weight

This sets the “goal weight” of selected control points, also found as the Weight property in the Transform Properties panel. I don’t know the use of this setting, though…

Set Radius

This is a setting used by the Extrusion tool…

Smooth

This command has exactly the same effect as with meshes, it slightly moves the selected points to smooth the overall curve.

Smooth Radius

Well, it just smooths out the Radius value of the control points along the curve…

Conversion

Well, here we will talk about “internal” conversions, between Bézier, NURBS and poly.

There is also an “external” conversion, from curve to mesh, that only works in Object mode. It transforms a Curve object in a Mesh one, using the curve resolution to create edges and vertices. Note also it keeps the faces and volumes created by closed and extruded curves…

Back to our “internal” conversion. The three Poly, Bezier and Nurb buttons of the Convert group, in the Curve Tools panel, allow you to convert all (partly) selected curves to the chosen type. Note this is not a “smart” conversion, i.e. Blender do not tries to keep the same shape, nor the same number of control points… For example, when converting a NURBS to a Bézier, each group of three NURBS control points become a unique Bézier one (center point and two handles). When converting from poly to Bézier, each control point becomes a whole Bézier one (a vector type), tripling the total number of points, and so on…

Misc Editing

You have some of the same options as with meshes, or in Object mode. You can separate a given curve (P), make other selected objects children of one or three control points (CtrlP – note however that parenting to three control points has a strange behavior with curves…), or add hooks to control some points with other objects.

The Mirror tool is also available, behaving exactly as with mesh vertices, as well as the snap commands.