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Figure 16-3: Standard primitives, like the sphere on the left, can be converted to NURBS surfaces.

Editing NURBS

You can edit and model NURBS curves and surfaces into desired shapes using the rollouts in the Modify panel, using the tools in the NURBS Creation Toolbox, or by working with the NURBS subobjects.

Attach and Import

When a NURBS curve or surface is selected, the General rollout includes buttons to attach and import NURBS. The Attach Multiple and Import Multiple buttons let you select from a dialog box several objects to attach or import. When attaching NURBS, you have the option of reorienting the attached object.

Display options

The General rollout, shown in Figure 16-4, also includes a Display section where you can select which elements get displayed. For curves, the options include Lattices, Curves, and Dependents. For surfaces, you have further options to display Surfaces, Surface Trims, and Transform Degrade, and you can also choose the surface display to be a Tessellated Mesh or

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a Shaded Lattice. Next to the Display section is the NURBS Creation Toolbox button. This button opens a floating window of buttons that make working with NURBS easy. You find out more about the NURBS Creation Toolbox buttons in the next section.

For NURBS surfaces, the Display Line Parameters rollout lets you specify the number of U and V isoparms to use to display the NURBS surface. These isoparms are the lines that make the NURBS object visible in the viewport. You can also select to display Iso Only, Iso and Mesh, or Mesh Only.

Caution

You can set the number of U and V Isoparms to 0, which makes the NURBS surface invisible.

Figure 16-4: The General rollout includes options for determining what is displayed in the viewports.

NURBS

Creation

Toolbox

Surface and Curve Approximation

Using the Curve Approximation rollout, which becomes available when you choose the CV Curve option, you can set the Interpolation Steps value. The Optimize and Adaptive options automatically reduce the number of points required for the curve.

When you’re working with NURBS surfaces, the Surface Approximation rollout lets you control the surface details for both the viewport and the renderer. For Base Surface, Surface Edge, and Displaced Surface, you can set the Tessellation Method. The three Tessellation Presets are Low, Medium, and High. These presets set the parameters for the various tessellation

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methods, with Low representing the values that produce the lowest-quality surface. You can also select which tessellation method to use — Regular, Parametric, Spatial, Curvature, or Spatial and Curvature. Each of these methods uses a different algorithm to compute the surface.

The Merge value determines the space between surfaces that should be combined to eliminate gaps when the surface is rendered. In most cases, the default value is acceptable for eliminating surface gaps. The Advanced Parameters button opens an additional dialog box of parameters, shown in Figure 16-5, where you can choose the tessellation method to use. The options include Grid (which divides the surface using a regular grid), Tree (which divides the surface using a binary tree), and Delaunay (which subdivides the surface into equilateral triangles). The selected tessellation option is used by the Spatial, Curvature, and Spatial and Curvature tessellation methods. Use the Clear Surface Level button to eliminate all Surface Approximation settings.

Figure 16-5: The Advanced Surface Approximation dialog box lets you specify subdivision levels.

The Curve Approximation rollout lets you select the number of interpolation steps to use. You can also select the Optimize or Adaptive option. These options define the number of segments that are used to represent the curve.

If you have several NURBS surfaces whose approximation settings you’d like to change at once, you can use the Surface Approximation Utility found in the Utility panel. This utility includes the same settings found in the Surface Approximation and Surface Display rollouts, but can be applied to multiple NURBS objects at once.

The NURBS Creation Toolbox

Clicking the NURBS Creation Toolbox button in the General rollout opens the toolbox shown in Figure 16-6. Clicking the button a second time closes the toolbox. This toolbox

has three sections: Points, Curves, and Surfaces.

Note	You can also open the NURBS Creation Toolbox by using the Ctrl+T keyboard shortcut.

Each of these sections includes buttons that create dependent subobjects. Dependent subobjects are objects that depend on other points, curves, or surfaces. When the parent object is changed, the dependent subobjects are changed also.

The Points section includes buttons for creating dependent NURBS points. Table 16-1 describes these point types and their respective buttons.

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Figure 16-6: The NURBS Creation Toolbox lets you work with NURBS points, curves, and surfaces.

Table 16-1: Points NURBS Creation Toolbox Buttons

Toolbar Button	Name	Description
	Create Point	Creates a free independent point
	Create Offset Point	Creates a point that is offset from another point
	Create Curve Point	Creates a point that is on a curve
	Create Curve-Curve Point	Creates a point that intersects two curves
	Create Surf Point	Creates a point that is on a surface
	Create Surface Curve Point	Creates a point that intersects a curve
		and a surface

Creating freestanding or dependent NURBS points gives you another way to build curves. When one of these buttons is selected, the cursor changes when it is positioned over a place where the point can be created. For example, clicking the Create Surf Point button causes the cursor in the viewport to change when it is over a NURBS surface.

The Curves section includes many more buttons than the Points section. You can use these buttons to create dependent NURBS curves. Table 16-2 describes each of these buttons.

Table 16-2: Curves NURBS Creation Toolbox Buttons

Toolbar Button	Name	Description
	Create CV Curve	Creates a CV curve
	Create Point Curve	Creates a point curve

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Toolbar Button	Name	Description
	Create Fit Curve	Creates a point curve that fits the selected points
	Create Transform Curve	Creates a copy of a curve that is transformed
	Create Blend Curve	Blends or smoothly connects the ends of two
		NURBS curves
	Create Offset Curve	Creates a copy of the original curve that is larger
		or smaller and moved to one side according to
		the distance setting
	Create Mirror Curve	Creates a mirrored copy of the original curve in
		the selected axis at a user-set distance
	Create Chamfer Curve	Creates a bevel where two curves meet
	Create Fillet Curve	Creates a radius line to make a smooth transition
		between two curves that cross each other
	Create Surface-Surface	Creates a curve along the edge created when
	Intersection Curve	two NURBS surfaces intersect each other
	Create U Iso Curve	Creates a dependent curve from the U isoparm
		that make up the NURBS surface
	Create V Iso Curve	Creates a dependent curve from the V isoparm
		that makes up the NURBS surface
	Create Normal	Projects a curve on a NURBS surface by
	Projected Curve	projecting along a surface normal
	Create Vector Projected Curve	Projects a curve on a NURBS surface by
		projecting along a vector
	Create CV Curve on Surface	Enables the user to create a CV curve directly on
		a NURBS surface
	Create Point Curve on Surface	Enables the user to create a point curve directly
		on a NURBS surface
	Create Surface Offset Curve	Creates a curve that is offset from a surface curve
	Create Surface Edge Curve	Creates a curve that lies on the surface edge

You can create dependent curve subobjects from points, curves, or surfaces. The cursor indicates when these can be created. Some dependent curves require two objects. For example, the Create Blend Curve button can attach two curves together. Selecting the first and then selecting the second does this. Each curve is highlighted blue as it is selected. The curves must be part of the same object.

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The Surfaces section includes buttons for creating dependent NURBS surfaces. Table 16-3 describes each of these buttons.

Table 16-3: Surfaces NURBS Creation Toolbox Buttons

	Toolbar Button	Name	Description
		Create CV Surface	Creates a CV surface
		Create Point Surface	Creates a point surface
		Create Transform Surface	Creates a copy of a surface that is transformed
		Create Blend Surface	Connects one surface to another with a smooth
			surface between them
		Create Offset Surface	Creates a copy of the original curve that is
			moved to one side according to the distance
			setting
		Create Mirror Surface	Creates a mirrored copy of the original surface in
			the selected axis at a user set distance
		Create Extrude Surface	Creates a NURBS surface at right angles to the
			construction plane
		Create Lathed Surface	Creates a NURBS surface by rotating a curve
			about an axis
		Create Ruled Surface	Creates a straight surface that joins the edges of
			two separate surfaces; one edge can be curved
			and the other straight
		Create Cap Surface	Creates a surface that closes the edges of a
			closed surface
		Create U Loft Surface	Creates a surface by linking multiple closed
			curved contours along the U axis
		Create UV Loft Surface	Creates a surface by linking multiple closed
			curved contours along the U and V axes
		Create 1-Rail Sweep	Creates a surface using an edge defined by one
			curve with a cross section defined by another
		Create 2-Rail Sweep	Creates a surface using an edge defined by two
			curves with a cross section defined by another
		Create a Multisided	Creates a surface by blending several curves
		Blend Surface	and surfaces
		Create a Multicurved	Creates a surface that is trimmed by several
		Trimmed Surface	curves that form a loop
		Create Fillet Surface	Creates a surface with rounded corners where
			the surfaces meet