Reference: Surface Manipulation using Control Points

Direct manipulation of the surface is carried out via movement of the control points. Altering the position of different surface control points affects the surface in varying ways. What is implied as "different" is their relative position to the boundary of the patch they belong to. The control points can be displayed as a mesh of points over the patch. Those control points at the edge of the patch will alter the position of the boundary (and therefore the tangency, curvature, ). Those control points next to the edge control points affect the tangencies (and therefore the curvature), repositioning these will not alter the position of the boundary though. The control points two rows or columns from the edge affect the curvature at the boundary but not the position or tangency. It is possible to create patches with varying numbers of control points but is advisable that in most cases this is kept low. Generally, for patches not using data points 6 by 6 control points are the most useful. When using data, try 10 by 10 (see General Page).

Figure 3:137. An example of control point meshes

With practice, patches can be faired quite quickly. If there are a few adjacent patches to be faired then it is best if these are considered a single set that must be faired together. The way forward is to imagine what the isophotes should look like over the set and keep this in mind when working on any of the patches. A control point can be moved by either dragging it using the Left Mouse button or by selecting it with the Left Mouse button and then nudging it with the arrow keys. Dragging a control point moves it in the plane of the window. With the arrow keys, the control point can be moved <Up>, <Down>, <Left> and <Right> in the plane of the window, if the <Shift> key is held down then the <Up> and <Down> keys will move the control point in and out of the plane of the window. The distance a single stroke of an arrow key will move the control point is the nudge size; see Set Arrow Key Nudge Size Dialog. Before altering the control point mesh it is helpful to view the mesh with the surface display turned off (see Surface Graphical Reference). A regular looking mesh is usually a prerequisite for a fair surface. While in the process of moving the control points, the isophotes display mode is the most useful.

Figure 3:138. Hull form before patch fairing

Figure 3:139. Hull form after patch fairing