From BlenderWiki

Surface Editing

Surface editing has even less tools and options than its curve counterpart – and has many common points with it… So this page covers (or tries to cover) all the subject, from the basics of surface editing to more advanced topics, like retopology.

Basic Surface 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.

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. Surfaces have no specific transform tools.

NURBS Control Points Settings

We saw in a previous page that NURBS control points have a weight, which is the influence of this point on the surface. 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 or Extruding

Unlike meshes or curves, you cannot generally directly add new control points to a surface (with CtrlLMB clicks), as you can only extend a surface by adding a whole U- or V-row at once. The only exception is when working on a NURBS surface curve, i.e. a surface with only one control point on each U- or V-row. In this special case, all works exactly as with curves.

Most of the time, only extrusion is available. As usual, once the tool is activated the extrusion happens immediately and you are placed into Grab mode, ready to drag the new extruded surface to its destination.

There are two things very important to understand:

• Surfaces are 2D objects – so you can’t extrude anything inside a surface (e.g. “inner” row), it wouldn’t make any sense!
• The control “grid” must remain “squarish”, which means that you can only extrude a whole row, not parts of rows here and there…

To summarize, the Extrude tool will only works when one and only one whole border row is selected – else nothing happens.

As for curves, you cannot create a new surface in your object out of nowhere, by just CtrlLMB -clicking with nothing selected. However, unlike for curves, there is no “cut” option allowing you to separate a surface in several parts, so you only can create a new surface by copying an existing one (⇧ ShiftD), or adding a new one (Add menu…).

Examples

Images (Selecting control-point) to (Complete) show a typical extrusion along the side of a surface.

In (Selecting control-point) and (⇧ ShiftR), a border row of control-points were highlighted by selecting a single control-point, labeled “C”, and then using the handy row select tool (⇧ ShiftR) to select the rest of the control-points.

Selecting control-point.

⇧ ShiftR.

The edge is then extruded using the E as shown in (Extruding). Notice how the mesh has bunched up next to the highlighted edge; the area in question is highlighted in a light-grey circular area. That is because the new extruded surface section is bunched up there as well.

Extruding.

Complete.

By moving the new section away from the area the surface begins to “unbunch”, as shown in (Complete). The direction of movement is marked with a white arrow, labeled “E”, and the new section is labeled “S”.

You can continue this process of extruding – or adding – new surface sections until you have reached the final shape for your model.

Opening or Closing a Surface

As curves, surfaces can be closed (cyclic) or opened. However, as surfaces are 2D, you can control this property independently along the U and V axis.

To toggle the cyclic property of a surface along one axis, use C and choose either cyclic U or cyclic V from the Toggle pop-up menu. The corresponding surface’s outer edges will join together to form a “closed” surface.

Duplication

Well, as with meshes and curves, this command just duplicates the selection. As usual, the copy is selected and placed in Grab mode, so you can move it to another place.

However, with surfaces there are some selections that can’t be duplicated, in which case they will just be placed in Grab mode… In fact, only selections forming a single valid sub-grid are copyable, let’s see this in practice:

• You can copy a single control point. From it, you will be able to “extrude” a “surface curve” along the U axis, and then extrude this unique U-row along the V axis to create a real new surface.
• You can copy a single continuous part of a row (or a whole row, of course). This will give you a new U-row, even if you selected (part of) a V-row!
• You can copy a single whole sub-grid.

Note that trying to duplicate several valid “sub-grids” (even being single points) at once won’t work, you’ll have to do it one after the other…

Deleting Elements

The Erase pop-up menu of surfaces offers you two options:

Selected

This will delete the selected rows, without breaking the surface (i.e. the adjacent rows will be directly linked, joined, once the intermediary ones are deleted). The selection must abide the following rules:

• Whole rows, and only whole rows must be selected.
• Only rows along the same axis must be selected (i.e. you can’t delete both U- and V-rows at the same time).

Also remember that NURBS order cannot be higher than its number of control points in a given axis, so it might decrease when you delete some control points… Of course, when only one row remains, the surface becomes a “surface curve”, when only one point remains, there is no more visible surface, and when all points are deleted, the surface itself is deleted.

All

As with meshes or curves, this deletes everything in the object!

Example

Before.

After.

In (Before) a row of control-points have been selected by initially selecting the control-point labeled “A” and using ⇧ ShiftR to select the remaining control-points. Then, using the Erase menu (X), the selected row of control-points is erased resulting in (After).

Joining or Merging Surfaces

Just like curves, merging two surfaces requires that a single edge, a border row of control-points, from two separate surfaces are selected. This means that the surfaces must be part of the same object. For example, you can’t join two surfaces while in Object mode – but you can of course, as with any objects of the same type, join two or more Surface objects into one object (CtrlJ) – they just won’t be “linked” or merged in a single one… Yes, it’s a bit confusing!

This command is equivalent to creating edges or Faces for meshes (hence it’s shortcut), and so it only works in Edit mode. The selection must contains only border rows of the same resolution (with the same number of control points), else Blender will try to do its best to guess what to merge with what, or the merge will fail (either silently, or stating that “Resolution doesn’t match” if rows with different number of points are selected, or that there is “Too few selections to merge” if you only selected points in one surface…

So to avoid problems, you should always only select border rows with same number of points… Note that you can join a border U-row of one surface with a border V-row of another one, Blender will automatically “invert” the axis of one surface for them to match correctly.

NURBS surface curves are often used to create objects like hulls, as they define cross sections all along the object, and you just have to “skin” them as described above to get a nice, smooth and harmonious shape. See this tutorial for a detailed workflow.

Examples

(Joining ready) is an example of two NURBS surface curves, not NURBS curves, in Edit mode, ready to be joined. (Joining complete) is the result of joining the two curves.

Joining ready.

Joining complete.

Subdivision

Surface 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 completely selected grids by subdividing each “quad” in four smaller ones.

If you apply it to a 1D surface (a “surface curve”), this tool works exactly as with curves.

Spin

This tool is a bit similar to its mesh counterpart – but with less control and options (in fact, there’s none!).

It only works on selected “surfaces” made of one U-row (and not with one V-row), so called “surface curves”, by “extruding” this “cross section” in a square pattern, automatically adjusting the weights of control points to get a perfect circular extrusion (this also implies closing the surface along the V axis), following exactly the same principle as for the NURBS Tube or NURBS Donut primitives.

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 exactly the same additional options as for curves – but I suppose Set Radius and Smooth Radius have nothing to do here…

Conversion

As there are only NURBS surfaces, there is no “internal” conversion here.

However, there is an “external” conversion available, from surface to mesh, that only works in Object mode. It transforms a Surface object in a Mesh one, using the surface resolution in both directions to create faces, edges and vertices.

Retopology

This tool, a restrained version of its mesh counterpart, behaves exactly as with curves.

Misc Editing

You have some of the same options as with meshes, or in Object mode. You can separate a given surface (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.