Skin

The Surface->Skin command in the Model module allows you to create a 3D object from a series of curves or faces that define cross-sections of the object.

You can use relational modelling concepts on the objects created using this command. Always make sure that the Preferences->Create Modelling Relation command is activated first.

For an overview of relational modelling, see the Relational Modelling section of the Modelling User's Guide.

Order of Selection

For all curve types, the order of selection in skinning affects the final result. For example, say you are skinning between four curves, c1, c2, c3, and c4:

If you select in this order:

c1 c2 c3 c4 c5

the result is different than if you select in this order:

c1 c2 c4 c3 c5

Note:

If you make a mistake in the picking order, middle-click on a curve to unpick all subsequent curves, something like "clear all starting from this curve."

The following describes the basic procedure. If you want more details of the conditions for skinning curves, see the Skinning Over Curves section of the Modelling User's Guide.

Procedure

1. Draw a series of curves or faces representing the object to be reproduced at different depths (cross-sectional stages). Curves must be either all open or all closed.
2. If you use Linear, Bezier, Cardinal, and B-Spline curves:

• Draw the curve counterclockwise (or bottom to top).
• All curves must have the same number of points.

Tip:
It is easiest if you duplicate the first curve, position the duplicate, and edit (without removing) the points to build the new shape.

3. If you use NURBS curves:

• Draw the curves clockwise (or top to bottom).
• Use curves of the same degree (Linear, Quadratic, or Cubic).
• You can use curves with different number points and parameterizations. However, if you want to avoid complexity in the resulting surface, apply Uniform parameterization to all the curves and count the number of control points as before.

4. Choose the Surface->Skin command.
5. Use the left mouse button to pick each curve in the desired order. As each curve is selected, it is highlighted.
6. If you make a mistake:

• Left-click to unpick the current curve.
• Middle-click on any curve to clear all selections from that point.

7. When you have selected all the curves, right-click to display the Skinning dialogue box.
8. Set the parameters for the resulting object. Not all parameters are used, depending on the type of 3D object you select.

• If you skin non-NURBS curves, you can select Polygon or Patch for the resulting object type, but not a NURBS surface.
• If you skin NURBS curves, you can select Polygon or NURBS for the resulting object, but not a Patch surface.

You can choose a degree and parameterization for the resulting NURBS surface which is different than that of the component curves.

Tip:
If you want to preserve the interpolation of the curves, select
Non-uniform parameterization for the skin surface. If you want a specific parameterization for the skin surface, you can do so, but the curves may not interpolate in the same way.

9. Click Ok to accept the settings or Cancel to exit the dialogue box.
10. If you made a mistake in the drawing direction of the curves and wish to reverse the direction of the surface normals, select the resulting skin and use the Effect->Inverse command.

The result is a 3D object of the type selected, either open or closed.

The curves remain displayed on the screen after the new skinned object has been created, and can be used again to create more objects. Notice that the original curves and the skinned object are on the same spot.

If you are using relational modelling, as you edit, transform, or scale the generator curves, you will see the object simultaneously move and respond to your changes. The only operations that can be performed independently on the generated object are transformations and inversion of the object's surface normals.

Parameters

Polygon

The resulting object is a polygon mesh object. The following two parameters are related only to polygon mesh skin objects.

• Sub sets the number of subdivisions between each control points of the generator curves.
• Cap closes both ends of the object.

Patch

The resulting object is a patch surface. This option is dimmed when skinning NURBS curves.

• Linear creates a Linear patch.
• Cardinal creates a Cardinal patch.
• B-Spline creates a B-Spline patch.

NURBS

The resulting object is a NURBS surface. This option is dimmed when skinning Linear, Bezier, Cardinal, or B-Splines.

• Linear, Quadratic or Cubic degree specifies the degree of the resulting surface. For more information, see the Degree and Continuity section of the Modelling User's Guide.
• Non-Uniform, Chord Length, Centripetal or Uniform specifies the parameterization of the resulting NURBS surface.

Step

Sets the resolution of the skinned surface. The higher the value, the smoother the surface will be. This parameter is available only when the resulting object is a patch or NURBS surface.

Close

Joins the curve ends, interpolating between the first and last curve.

Last updated 02-apr-1998