Fairing

Before a spline surface can be faired, non fairness must be detected. This is best achieved by viewing the surface via either isophotes, reflection lines or Gaussian curvature These viewing methods can be used to examine the faired surface (as described in the earlier sections). To aid the visualization of the surface patch or patches it may be useful to refine the facets of these patches by altering the facet parameter referred to as Normal Tolerance (see Options Dialog - Faceter Page). Reducing this will improve the definition of the isophotes etc by increasing the number of facets.

Note:
This refinement should only be applied to the patches of interest and not the entire model as this would place a high demand on memory. A tolerance of 7 degrees is suggested.

When fairing a surface, naturally the surface itself is altered, but there is a requirement to limit the extent of any alterations. It is a usual requirement for the position of the boundary of a patch to be maintained, it is also often required that the tangency or curvature be maintained across a boundary. It is a natural requirement that the fairing process should not move the surface more than a predetermined distance from the original surface. It will be shown below how these constraints can be applied by constraining the surface control points (CP).

If a non fair surface is detected, then the first step is to display the CPs over the surface. Use the following procedure in this scenario.

Fairing a Non-Fair Surface

1. Display the control points (CPs) over the surface by opening the Surface Control Manipulation dialog (see Mesh).

   1. Select SURFACES \ MESH....

   2. Select Extract to show the control points of any patch selected from the screen using the mouse.

2. Use the options in the Control Point Panel to select individual CPs, rows, or columns. Hold down CTRL and SHIFT to select multiple rows, columns, or individual CPs. All selected CPs will be highlighted.

   Any selected CPs will not be allowed to move during the fairing procedure.

   Selecting the CPs along a boundary fixes the position of that boundary. The CPs along the boundary and the adjacent row/column are also selected then both the position and tangency of that boundary are fixed. Furthermore, if the three rows/columns of CPs adjacent to a boundary are selected, the curvature across that boundary will also be fixed.

3. Enter the maximum distance the surface is allowed to move in the Max Offset field.

4. When all constraints have been applied, click Fair, then select the patch with the mouse. The surface CPs are faired.

5. At this point the CPs have been faired but not attached to the surface. To do this, click Apply then select the patch with the mouse.

   Now, the faired surface exists and can be viewed.

It must be noted at this stage that although a maximum movement may have been stipulated this is an upper limit on the movement of the CPs but the actual movement of the surface is almost always less than this.

Sometimes when fairing a surface patch, it is desirable not to constrain one of its boundaries. This is done to obtain greater fairness. This will result in this free boundary moving away from a neighboring patch. This is not a problem, the neighboring patch can be made boundary continuous, then tangent continuous with the faired patch. Then, if the neighboring patch is to be faired, its tangency at the boundary can be maintained by the fixing of two rows/columns of CPs.

An isolated region of a patch can also be faired by selecting all CPs except those corresponding to that region. To aid this, it is sometimes useful to use the function of dragging the cursor over the whole patch, while keeping the SHIFT key depressed. This has the effect of selecting all the CPs. Now if individual CPs are selected twice while holding down the SHIFT and CTRL keys, these individuals will be deselected. This will result in all but a few CPs being fixed.