How to Make Objects with NURBS Modeling

These are tips on using NURBS to create models.

Objects and Sub-Objects

• In 3ds Max, a NURBS model is a single, top-level NURBS object that can contain a variety of sub-objects. Get in the habit of creating a single object at the top level, then going immediately to the Modify panel and adding sub-objects by using rollouts or the NURBS Creation Toolbox.
• Sub-objects are either independent or dependent. Dependent sub-objects use relational modeling to build NURBS geometry that is related to other geometry. However, understand that the more dependencies a model has, the slower interactive performance becomes.
• In general, point curves and surfaces are slower than CV curves and surfaces. Trims are the slowest kind of dependency, and texture surfaces are the slowest kind of dependent sub-object.
• If a dependent sub-object doesn't change during animation, you can improve performance by making the sub-object independent after you finish creating it.
• You can use Surface Select to apply modifiers to a sub-object selection. However, before you do so make sure that Relational Stack is on; Relational Stack is on the General rollout for NURBS models. Otherwise, Surface Select can select only the Surface and Surface CV sub-object levels.

Converting Other Objects to NURBS

• Remember that you can collapse splines to NURBS objects. A spline Shape or a NURBS curve can be a good starter object for a NURBS model.
• Shapes with sharp angles collapse to multiple NURBS curves. You can control this before NURBS conversion by first converting the Shape to an editable spline. Modify the editable spline so that all its vertices are Bezier or Smooth vertices. Then when you collapse the spline to a NURBS curve, you obtain a single curve.
• If you want a single NURBS curve, don't change vertices to Bezier Corner vertices. These always convert to a junction between two different NURBS curves.
• Collapsing a primitive into a NURBS object is one of the quickest ways to start building a NURBS model. After collapsing the primitive, you can select various CVs and transform them. Other objects you can convert to NURBS are prisms, torus knots, lofts, and patch grids.
• You can also change the NURBS surface by applying modifiers. The modifiers act on the points or CVs of the surface, and not on the surface itself. After applying the modifiers, collapse the modifier stack. This removes the modifiers from the stack without changing the position of the modified points or CVs, making for a simpler and quicker model.
• Another way to create a NURBS surface object is to apply a Lathe or Extrude modifier to a NURBS curve. Set the modifier's Output Type to NURBS, and then collapse when you're done adjusting the parameters.

  (There are also NURBS lathe and extrude surface sub-object types, which you can apply to curve sub-objects.)

Shortcuts, Snaps, and User Interface Tips

• Remember to turn on the Plug-In Keyboard Shortcut Toggle. While it is on, you can use all the NURBS keyboard shortcuts.
• One of the most useful NURBS keyboard shortcuts is H, which opens the Select Sub-Objects dialog. This is a subset of the Selection Floater that you can use during sub-object creation as well as sub-object selection. This is handy when sub-objects are crowded or hard to see.

  A variant is +H, which also displays the Select Sub-Objects dialog, but lists only those NURBS sub-objects beneath the mouse cursor position.

• There are special NURBS Snaps in the Grid and Snap Settings dialog (right-click the 3D Snap toggle to display this). When you use NURBS snaps, turn off Options/Axis Constraints; otherwise, snaps work only in the current axis.

  Also, remember that snaps work in a viewport only when you have made the viewport active. And choosing your snap settings does not turn on snaps. You must also turn on the 3D Snap Toggle button (on the status bar).

  Snaps are especially important when you create the curves for building 1-rail and 2-rail sweep surfaces.

• Remember that without leaving the viewport, you can right-click to display a quad menu with shortcuts for changing the sub-object level, creating some sub-objects, and using some other edit commands.
• When you work with NURBS, there are a lot of rollouts in the Modify panel. Minimize the rollouts you don't need. For example, minimizing the Modifiers rollout helps unless you're applying Modifiers, and minimizing the Surface Common rollout is useful when you're creating U loft, UV loft, and 1-rail or 2-rail sweep surfaces.
• Don't set viewports to display edged faces. Displaying edges is almost twice as slow as displaying a simple shaded viewport.

Creating Curves

• When drawing a CV curve, click three times to get a sharp corner.

  Be aware, however, that multiple CVs increase the amount of calculation and therefore reduce the performance and stability of your model. However, if you want to use the curve to construct a U Loft, and so on, this is the best technique.

• You can also create sharp corners by fusing the ends of two separate NURBS curve sub-objects. This is the recommended method if you aren't using the curves to construct a surface.
• While creating curves, you can turn on the Draw in All Viewports toggle. This lets you draw curves in 3D. Begin drawing a curve in one viewport, go to another viewport, and continue drawing.

  If your mouse has a middle mouse button, + lets you use Orbit to change a viewport's orientation while you are creating the curve.

• To create a transform curve along a specific axis, turn on the appropriate axis constraints, and then +move a copy of the transform curve.

Curves and Direction

• NURBS curves show their direction in viewports. A small circle indicates the first vertex. If the curve is closed, a plus sign (+) indicates the direction of the curve.

  Be aware of curve direction when you use curves to construct blend surfaces, U loft and UV loft surfaces, and 1-rail and 2-rail sweeps. If the curves don't have the same direction, you can get strange twisting. Make sure curves have the same direction before you construct the surface. On the Curve Common rollout, the controls Reverse and Make First let you control the direction of the curve, and where its starting point or CV is located.

  Another good way to make sure curves are aligned is to draw one curve and then use +Clone to create the others. After creating the aligned curves, you can transform CVs to vary the curves on which the surface will be based.

Curves for Sweeps

• Besides expecting cross-section curves to be all in the same direction, 1-rail and 2-rail sweep surfaces work best if the cross sections intersect the rail or rails. To achieve this, draw the rails first, then draw the cross sections using the NURBS Snaps Curve End and Curve Edge turned on.
• 2-rail sweeps have the additional requirement that the endpoints of the first cross section intersect the endpoints of the rails. Again, NURBS Snaps help you do this.

  If the endpoints of the first cross-section don't coincide with the rail endpoints, the resulting surface might not follow the rails.

• While you're editing a sweep, the Edit Curve button lets you directly transform the CVs of a rail or cross section, without changing the sub-object level. Edit Curve also gives you access to all the rollouts that control the curve. You can use Refine or Make First, for example, without changing levels.

Curves on Surfaces and Projected Curves

• You can use a viewport to draw a curve on a surface (COS), but this works only for visible portions of the surface. To see the entire surface and the curve or curves on it projected into a flat plane, use Edit Curve.

  If your mouse has a middle mouse button, + lets you use Orbit to change a viewport's orientation while you draw the curve on surface.

• Neither curves on surfaces nor projected curves can cross the edge of a surface. This includes the seam on surfaces with fused CVs. If you try to project across the seam, only part of the curve's projection is created.

Creating Blend Surfaces

• You can blend between curves or between surface edges. (You can't blend from a trimmed edge. In that situation, you are blending from the curve that trimmed the surface.)
• If you want a controllable tangent or tension, you must blend to a surface edge or a curve on a surface. Adjusting tension changes the flatness or "bulginess" of that end of the blend.

  When a curve and a surface (or two surfaces) are near each other, sometimes it can be hard to tell which edge you are selecting. To assist you, the currently selected surface turns yellow, and the edge that will be used for the blend turns blue. Make sure you have selected the right surface before you choose the edge.

• If the edges you are blending have different numbers of points (usually due to different surface approximation settings), then sometimes rendering shows gaps between the blend and the original surface. If this happens, go to the Surface Approximation rollout and increase the value of Merge until the gaps disappear when you render.

  The Merge setting affects only the production renderer. It has no effect on viewport display.

Lofts

• If you need a surface between only two curves, use a ruled surface instead of a U loft. This is faster.
• If loft creation seems slow, make sure the Display While Creating checkbox (in the U Loft Surface rollout) is turned off.
• If the U loft doesn't come out as you expected, try reparameterizing the curves. Click Reparam. at the Curve sub-object level. This button is on the CV Curve rollout. In the Reparameterize dialog, choose Chord Length reparameterization.

  If a curve is dependent or a point curve, first you will have to make it independent (this also improves performance).

  Curves that are made of two joined curves have this problem more often than others. If you have a joined curve as one of the curves to construct the loft, reparameterize it before you create the loft, or set the curves to reparameterize automatically.

• The Edit Curve button lets you directly transform the CVs of a curve within a U loft or UV loft, without changing the sub-object level. Edit Curve also gives you access to all the rollouts that control the curve. You can use Refine or Make First, for example, without changing levels.
• To close a UV loft, you can pick the first V curve again to make it the last curve in the loft. Sometimes a seam is visible at this location in the UV loft.

Multisided Blend Surfaces

• If 3ds Max doesn't create the multisided blend, fuse the CVs at the three or four corners. Snapping CVs to each other doesn't always succeed, because of rounding off.

Multicurve Trimmed Surfaces

• Multicurve trimmed surfaces are the only way to create a trimmed hole that contains sharp angles.

Displacement Mapping

• In general, the default tessellation settings aren't suitable for displaced surfaces. With these default settings, displacement mapping can create an extremely high face count, which performs very slowly. Change the surface approximation to the lowest necessary resolution. A good rule of thumb is to start with Spatial approximation and an Edge value of 20. If that is too low, reduce the Edge value until the model looks as it should.
• Use the Displace NURBS world space modifier to convert the displacement map into an actual displaced mesh so you can see the effect of displacement in viewports. To make a displaced mesh copy of the NURBS model, use Snapshot.

Connecting an Arm to a Shoulder

• The easiest approach is to create a CV curve on surface or normal projected curve on the shoulder. Then create the arm as a U loft. For the last curve of the U loft, select the CV curve on surface or the normal projected curve. Then turn on Use COS Tangents, which makes the loft surface tangent to the other surface where the arm joins the shoulder.
• If the blend appears twisted, use the Start Point spinner to change the location of the first point of the curves that make up the U loft surface.
• Another way to connect a U loft to another surface is to project the last curve in the U loft onto the other surface. Click Make COS to convert the projected curve into a curve on surface, and then on the U Loft Surface rollout click Insert to make the new curve on surface the last curve in the U loft. You can scale the curve on surface or move its CVs to get the curvature and blending you want.