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PolyCAD

Constraint attributes are used to control the shape of the curve without specifically having to accurately position the control points. X-Topology curves are fundamentally based on BSpline Curve mathematics, however, the user does not manipulate the BSpline Curve definition directly. The X-Topology curve provides a set of points that can be positioned, attached to other curves and marked up using constraint attributes. The definition and the curve's relationship to others is analysed and a number of BSpline curve segments are generated conforming to the shape the user has requested. Much like IntelliCurve constraints, X-Topology curve constraints come in two forms. Those that effect a single point (Knuckle, Interpolate, Straight and Blend) and those that effect a whole curve (Polyline, Cubic Spline). For an illustrative example, see the X-Topology Tutorial.

Knuckle

Creates a corner at the selected point location. Applying the knuckle constraint to all the internal definition points (not the end definition points) is the same as applying the polyline constraint.

Interpolate Points

Makes the curve interpolate the selected point location.

Number of Contiguous interpolation points Effect on curve
Single interpolation point The curve interpolates the point with 2nd order continuity. However, maintaining 2nd order continuity places a heavy constraint on the curve shape which means that the adjacent control points cannot be placed too close because the shape of the curve becomes very sensitive to the adjacent control point position
Double interpolation points Two consecutive interpolation points creates a curve segment which blends between the tangents defined by the adjacent curve sections. Creating two interpolation points is the same as applying the blend constraint to the two points.
Multiple interpolation points The curve is fitted to interpolation points using a cubic spline fit. The end of this section of curve will be tangential to the adjacent curve segments.

Tangent throughPoint

Forces the curve to interpolate the selected point with the tangent defined on the submenu, either parallel with the X, Y or Z axes.

Straight Section

Creates a straight line between the selected points. If more than two points are selected when this constraint is applied the internal points are deleted.

Blend Section

Creates a blended curve segment between the two selected points based on the tangent defined by the adjacent curve sections. The blend constraint has the same effect as if the interpolation constraint was applied to both points.

Polyline Curve

The Polyline Curve constraint creates a straight line between consecutive points. This is the same as applying the knuckle constraint to all of the internal points.

Cubic Spline Curve

This Cubic Spline Curve constraint creates a curve which interpolates all of the definition points. This is the same as applying the interpolation constraint to all of the internal points.