NURBS > Fit Surfaces generates a NURBS surface in each patch.
On an object with contour patches, the NURBS tension in the contour patches and in the regular patches can be specified separately. This allows the geometry of the contour patches to carry more or less weight than the regular patches, which is desireable for some objects.
Before using this function, use Grids > Specify Sharp Contours to mark certain contour lines or contour points as sharp (if any). That action prevents Fit Surfaces from fitting a continuous NURBS surface across those lines or points.
When Fit Surfaces is complete, you can modify the surface by various functions on this menu, or click NURBS > Remove Surfaces and start over.
The parameters are:
Fitting Method - specifies a fitting technique:
Adaptive radio button - optimizes the number of control points used within each patch (using an m by n control point layout rather than m by m).
Constant radio button - builds an m by m control point layout, where m is specified by the Control Points parameter.
Parameters for Adaptive - (These values apply to regular patches, not to contour patches.)
Maximum Control Points integer field - specifies the maximum value of m or n. The actual value is computed.
Tolerance decimal field - specifies the target maximum distance that the NURBS surface may lie from the original polygon mesh. This target maximum is further affected by Outlier Percentage. This maximum may be impossible to achieve if Maximum Control Points is too low. You can analyze the actual tolerance with Analysis > 3D Compare.
Surface Tension decimal field (0.0 to 1.0) - (applies only to the NURBS control points that lie on patches outside the contour patches) adjusts the balance between accuracy and smoothness. Low tension maintains the shape closer to original data, while high tension generates a smoother surface.
Outlier Percentage decimal field (default 0.5%) - specifies the percentage of points on the underlying mesh that can be out of Tolerance while the NURBS surface can be considered within Tolerance. If the original scan data is relatively poor, this value might have to be raised to 2 or 3.
Parameters for Constant -
Control Points integer field - specifies the exact value of m, the number of control points along each side of the patch. The number you select is applied to every side, regardless of length or curvature.
Surface Tension decimal field (0.0 to 1.0) - (applies only to the NURBS that lie on regular patches) adjusts the balance between accuracy and smoothness. Low tension maintains the shape closer to original data, while high tension generates a smoother surface.
Evaluate Deviation checkbox - activates the Statistics group below.
Advanced Options checkbox - (only when both “regular patches” and “contour patches” exist) activates the following parameters which apply only in the contour patches.
• Extended Region Fitting group - affects contour patches only.
Control Points slider - (Low to High; only when Fitting Method is Constant) - adjusts the number of control points (the value of m) inside the contour patches.
Tension (0.0 to 1.0) - is the same as Surface Tension, but applies only to the NURBS in contour patches.
Statistics group (only when Evaluate Deviation is checked) -
Percent Out Of Tolerance indicator - the number of surfaces that could not be fit to the specified Tolerance.
Max. Deviation indicator - the highest deviation of any NURBS from the underlying surface.
Avg. Deviation indicator - the average deviation of all NURBS from the underlying surfaces.
Std. Deviation indicator - the standard deviation of all NURBS from the underlying surfaces.
OK button - saves the NURBS surface to the Shape object in the Model Manager and closes the dialog.
Cancel button - terminates the dialog without saving the NURBS surface.
Apply button - generates a preview based on the Control Points and Tension values.