Parallel Curve

This function creates a curve that is parallel to another curve in the surface, including shell profile trace curves. The necessary parameters are defined in a form. The method is iterative and uses a spline function to create the directrix of a cylinder by which the surface is intersected.

The form has the following contents:

Figure 2:42. Form for creation and modification of parallel curve.

The Distance field: The distance between the existing curve and the new curve measured in the surface along the curves defined by the field Method below.

The Distance End 2 field: A second distance making unequal displacement possible. The two distances are measured at the two points where the original curve intersects the limiting box, or at the end point if it is inside the box. If not given, the displacement is equal along the curve and given by Distance above.

The Direction field: If Distance End 2 is used, this field defines the direction used to define end 1 and end 2 of the original curve. This field tells how to interpret "end 1" and "end 2". For example, FOR means that the curve should be thought of as running towards the foremost end of the ship, meaning "end 1" is the aftmost end of the curve and "end 2" is the foremost end.

Note: that the setting of the Direction field does not have to coincide with the direction of the curve geometry as stored in the object. The alternatives irrelevant for a specific curve are automatically excluded from this field.

The Method field: X, Y or Z means that the displacement is made along curves created by intersecting the surface with principal planes perpendicular to the given coordinate axis. Perp means that the curves are created by the intersection of the surface and planes that perpendicular to the original curve in a certain point. Using Perp will produce a developed plate that has a constant distance between the two edges made from the two parallel seams. Used in two directions, this can create a rectangular (or close to rectangular) developed plate.

The Side field: In which direction to move the curve. Only an approximate direction is needed as the displacement is done in the surface. Irrelevant alternatives are automatically excluded from the field list.

The Excess End 1 and Excess End 2 fields: These fields may be used to temporarily extend the curve at both ends before the parallel move is done. For example, consider a curve in the planar side of the ship, generated in such a way that the curve is sloping so that the aftmost end is lower than the for end. If this curve were to be moved upwards with the Method field set to Perp, the parallel curve would not only move upwards, but also towards the aft end of the ship. The addition of excesses means that it will be possible to create the parallel curve in its full length despite of this fact. Please note that the original curve will not be affected by the use of excess.

The Approximate Plane toggle: When set, an approximately parallel displacement is done, with the restriction that the resulting curve is planar. In this case only three points are displaced, and then a plane is created using these points. The points are the end points and the midpoint of the original curve intersected with the limiting box.

The Tolerance field: The tolerance used in measuring the accuracy of the displaced curve created as a spline. This value has to correspond to the accuracy of the surface. The default tolerance is 3 mm.

The Iterations field: Maximum number of iterations used to reach a displaced curve with the accuracy given by Tolerance above. By default the number of iterations are set to 20.

The Ok button confirms the input and creates the curve while the Cancel button exits the function.

The button Modify Curve: This button lets you change the definition of the curve that this curve is based upon. This button is only displayed when modifying an existing curve.

The button Modify Box: This button lets you change the extension of the curve. This button is only displayed when modifying an existing curve.

The button Select Type: Change the type of the curve to any of the curve types described in this section.