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Edit Points, Isoparms and Spans

A span is a section of continuous (smooth) NURBS geometry.

Edit points and isoparms define the span boundaries and show the number of spans in a curve or surface.

Single-span (Bezier) geometry

Single-span surfaces are preferred for production modeling because of the inherent smoothness within a span. Single-span NURBS geometry is the same as Bezier geometry. The two terms are interchangeable in the Alias interface.

  • On curves, a single-span has an edit point at each end. There is also a CV at the same location.

  • On surfaces, a span has a boundary isoparm (isoparametric line) at each edge. These are sometimes called natural edges to distinguish them from trimmed edges.

    Additional visible isoparms can be drawn on the surface using the Patch Precision tool. This can help to visualize the shape of the surface.

Multi-span geometry

Multi-span curves and surfaces can be used to create more complex shapes with more CVs.

  • On curves, there is a span between each pair of edit points.

  • On surfaces, there is a span between the interior isoparms in each direction.

    The visibility of the interior isoparms is controlled in the Parameterization section of the Control Panel.

Note: Although the word spans is sometimes used in Subdivision tools, these are more correctly called divisions as they are unrelated to the NURBS spans described here.

Tools and Workflow

The recommended method for drawing curves is to use a New Edit Point Curve and place the start and end. This creates a single-span curve that is smooth.

Then, use Pick > CV and the Transform tools to shape the curve and increase the Degree if more CVs are needed.

The following tools control the display of spans:

To create a curve by placing edit points:

  • New Edit Point Curve

    New curves are created with edit points displayed by default. New surfaces are created with interior isoparms displayed by default.

To select edit points on Curves and on Curves-on-Surface, they must first be displayed using Display > Control

To insert additional edit points or isoparms:

  • Insert
  • Snapping: Use Ctrl to snap to edit points or isoparms, Alt to snap to halfway between edit points or isoparms.

Deep dive: Internal continuity for multi-span geometry

When building multi-span curves or surfaces, the internal continuity achieved across a span boundary, such as edit point or isoparm, is determined by the degree of the curve or surface.

Deep dive: Working with edit points on curves

It is not recommended to use Pick > Edit Point and the Transform tools to modify the shape of a curve.

This is because the shape of the curve is created by the CV locations and determines the positions of edit points, not the other way around.

Alias does allow you to move edit points by "reverse engineering" the curve from the edit point. When you move an edit point, the Move tool tries to find a curve that passes through the new edit point location. Since this process is time-consuming and has an infinite number of solutions, the tool must place constraints on how moving the edit point affects the curve. These constraints make it difficult to make major changes well by moving edit points.

Edit points also have a stronger impact on a curve than CVs:

  • For a single-span curve, moving one of the end edit points will flatten the curve to a straight-line.
  • On multi-span curve, the part of the curve that will be influenced when a CV or edit point is moved is shown in white. Edit points have wider influence across the curve than CVs making it difficult to predict the shape change.

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