MANUAL

Purpose

To allow the user to manually select the position of the knots for the B-spline curve fit.

Syntax

Description

Manual selection of knots allows the user a large degree of control when fitting a Spline through scattered data by applying Least Square (LSQ) fit to facilitate the fairing process. By using manual selection, the user is able to identify the KNOT, KNUCKLE and TANGENT positions on the Spline for the whole curve or part of it by specifying a range through which the LSQ fit will be applied.

The user begins the MANUAL selection by issuing the MANUAL START command followed by any number of MANUAL KNOT, MANUAL KNUCKLE, MANUAL TANGENT or MANUAL POINT commands as required. When any of these options are issued the cursor will appear on the screen thus prompting the user to select the data points to be identified as either KNOTS, KNUCKLES, TANGENTS or POINTS. The cursor will remain active until it is cancelled by selecting the CANCEL CURSOR box.

Once all the points selected for the manual fit have been entered the user should then issue the command MANUAL END. The user should check that the manual fit has achieved the required result by displaying the curve before accepting it.

The PREFIT command must NOT be used with the MANUAL command as it re-fits the curve through the original points thus cancelling any MANUAL knot selection.

The user may also partly fit the spline with a LSQ by giving the range or ranges through which the LSQ fit will be made. In this case the user should issue the MANUAL START /RANGE command. The user is then required to define pairs of points defining the range of the LSQ fit. These points can be selected by using the cursor or by specifying their reference numbers.

Rules for Manual Knot Selection:

• Maximum number of user knots = Number of data points - 4.

• The user should not place any knots at the first and last two data points on the curve.

• If a knuckle (3 coincident knots) is inserted into the curve no knots should be inserted at either of the data points adjacent to the knuckle point.

• If a curvature (second derivative) discontinuity (2 coincident knots) is inserted into the curve a knot can only be inserted at the adjacent point on one side of the discontinuity point. The additional knot adjacent to the discontinuity is inserted on the side where the curvature is controlled.

Examples