Manual Fairing

The method employed in manual fairing of patches is to move the control points of a surface patch, while assessing the surface quality via isophotes. The isophotes are updated in real time as the surface is altered. This gives the user of PACE an intuitive feel for the direction in which to move the surface control points. The first thing to do before moving any control points is to look not just at the patch that is about to be altered but also all the surrounding patches as well. It is important to observe the general flow of the isophotes and try to visualize how they should look. Imagine how you wish the isophotes to look globally, particularly as they cross the boundaries of patches. The behavior of the isophotes across the patch boundaries gives us information about the continuity across the surfaces.

• If the isophotes appear as broken curves at the boundary the patches have positional continuity only.

• If the isophotes suddenly change direction but remain unbroken as they cross the boundary then the patches are tangent continuous but not curvature continuous.

• If the isophotes cross the boundary smoothly then patches are curvature continuous.

These three points are fundamental in the evaluation of the quality of the surface.

When viewing a surface using isophotes to assess its quality it is important to vary the direction of the isophote light (see Patch Display Settings Dialog), because under certain circumstances discontinuities may not show up. This is particularly the case when manually fairing a surface. When fairing a patch, it is advisable to switch between two isophote directions after any significant changes have been made to the surface.

A control point can be manipulated in one of two ways. The first is via the arrow keys and the other by direct input of the position into a dialog box.

Use the arrow keys

• Highlight the desired control point with a left mouse click and use the arrow keys to nudge the control points up, down, left and right.

The control point is moved in a plane parallel to the screen. It is also possible to move the control point orthogonal to this plane by using the up and down arrow keys while clicking the shift key. The amount that the control point moves is determined by the setting of the nudge size (see Set Arrow Key Nudge Size Dialog). The other method for repositioning a control point is to highlight the control point with a left mouse click, then click the highlighted control point using the Right Mouse button then again with the Right button click the properties dialog and then finally fill in the dialog box with the three co-ordinate values (see Patch Control Point Dialog). Alternatively, double click on the control point with the Left Mouse button.

The decision on which control points to alter depends on which level of tangency is to be maintained (positional, tangential or curvature) (see Reference: Surface Manipulation using Control Points).

Figure 3:159. Patches lacking required level of continuity

Figure 3:160. Patches after fairing

Figure 3:161. Faired surface viewed with different isophote direction

From the above three figures we see firstly the patches that require fairing. It appears that we have tangent continuity but not curvature continuity, therefore we will manually fair the third row of control points. In the next figure we see the patches after a small amount of fairing. Now the isophotes flow more freely over the surface. The final figure is the faired surface viewed using a isophotes of a different direction, the surface still appears fair, so is acceptable.