Edge tangency controls

These are used to specify the way that the pairs of control net legs either side of the boundary are aligned. The Edge Tangency Options dialog (see Figure 3:111 below)is launched by double-clicking the Left Mouse button on the control, or clicking the Right Mouse button on it, and selecting OPTIONS… from the popup menu.

Figure 3:111. Edge tangency controls (near the cursor) and the associated dialog.

How the alignment of each pair of legs is performed depends on whether Tange into mating edge is checked. Checking it means that the patch edge is willing to tange into the edge of its neighboring patch. An edge that is willing to tange is displayed with arrowheads, otherwise arrow tails are used. Figure 3:112 below shows how the control points are moved depending on the willingness of either patch edge to tange.

Figure 3:112. The way that control net legs are aligned across a patch boundary: (a) neither edge willing to tange - no alignment. (b) one edge willing to tange - the control point on that patch moves. (c) both edges willing to tange - both patches’ control points move.

If Include patch corners is checked, then the legs labelled B to F are aligned (see Figure 3:113 below). If it is unchecked, legs C to E are aligned. This option is available because the control points of legs B and F also affect the tangency conditions across other boundaries.

If Sweep ribbon control points is checked, the positions of the control points adjacent to the boundary are first found (prior to any further aligning) by sweeping (forming a linear combination of vectors) between two end control net legs as shown in Figure 3:114 below. If Include patch corners is checked, the end control net legs are A and G, otherwise B and F are used.

Figure 3:113. Labelling scheme for adjoining pairs of control net legs across a patch boundary.

Figure 3:114. Sweeping the control points along the edge of a patch. In this diagram, the Include patch corners option has been enabled.

If Allow control points to slide during patch fitting is checked, the directions of the control net legs are locked when the patch geometry is next regenerated, although their lengths are free to change, shown in Figure 3:115 below. If this is not checked, both the directions and lengths of the legs (and hence the control point positions) are locked.

Figure 3:115. The effect of allowing control points to slide during patch fitting.

The options are applied to the edge when Apply is clicked. This causes sweeping and aligning to be performed immediately. Note that unchecking Tange into mating edge does not restore the edge to an unaligned state - this will only occur when the patch geometry is regenerated.