Creating NURBS models in Autodesk Maya

Task

Procedure

Divide a bilaterally symmetrical mesh along its mid-sagittal plane and remove one half.

Polygons: Edit mesh>Cut Faces tool>Options; "Delete cut faces").

Mapping landmarks onto the 3D surface

Create a UV map

Polygons: Create UVs>Automatic Mapping; right-drag> "Object Mode" to return to object mode).

Use the 3D Paint tool to mark landmarks and contours on the mesh

Rendering: Texturing>3D Paint Tool>Options

Tool Settings>File Textures>Attribute to paint>Color; File Textures>Assign/Edit Textures>"Assign/Edit Textures"

Tool Settings>Flood>Color: "white"; Flood> "Flood Paint"

Tool Settings>Color>Color: "blue"

Adjust brush size (Tool Settings>Brush>Radius [U])

Creating contour curves

Make mesh "Live":

Modify>Make Live

Create NURBS curves using the Edit Points Curve Tool

Create>EP Curve Tool>Options; degree setting: "3 Cubic"; knot spacing: "Chord length"

Lofting to create NURBS shapes

Select all curves one by one moving circumferentially around the vertebral element

Toolbar>Select tool; Shift-click. (Note: although any curve may be selected first, it is important to select the curves sequentially moving around the surface of the template mesh.)

Loft curves to create a NURBS surface

(Surfaces: Surfaces>Loft>Options) using the following settings: Parameterization (controls how the interpolation is handled and whether the surface will be a closed loop): "Chord length"; "Auto reverse"; "Close"; Surface degree (the degree of the polynomials used to calculate the surface): "Cubic"; Output geometry (type of geometry created): "NURBS". All other settings were left at defaults (Figure 6).

Adjustments and extrapolation of missing surfaces.

Slide knots along the surface of the template mesh

Select curve, right-click and drag to select "Edit points"; select Edit point with Move tool and click and drag on the center of the Move manipulator (Figure 7).

Adjusted knots without snapping to mesh

Same as above, but drag arrows of Move tool rather than center.

Create closed facets

Duplicate edge curve

Surfaces: Edit curves> Duplicate surface curves; click to select open edge

Duplicate again

Edit>Duplicate

Scale one duplicate to a single point

Modify>Center pivot; Attribute editor: "Scale X: 0", "Scale Y: 0", "Scale Z: 0"

If necessary, move point to center of the open edge

Move tool

Loft new curves

Shift-click to select, Surfaces>Loft

Attach facet to NURBS shape

First select facet, then Shift-click to select NURBS shape (order is important!); Edit NURBS>Attach Surfaces>Options; Attach method: "Blend"; Blend bias: "0," "Insert knot," Insert parameter: "1.0") (Figure 9).

Mirroring and attaching

Group all elements

Shift-click to select; Edit>Group

Mirror

Edit>Duplicate; scale by a factor of -1 in the X direction (Attribute editor: "Scale X: -1", "Scale Y: 1", "Scale Z: 1")

Attach halves at mid-line

Shift-click to select; Surfaces: Edit NURBS>Attach Surfaces>Options: Attach method, "Connect"; Multiple knots, "Remove." (Figure 10)

Alternative techniques

Create polygonal models

Create>EP Curve Tool>Options, degree setting: "Linear"; knot spacing: "Chord length"; output geometry: "Polygons"

Partially automate knot insertion

Surfaces: Edit Curves>Insert Knots>Options>Insert location: "Between selections." "Multiplicity" setting determines how many knots will be inserted within each span