To increase the degree of a surface

1. Choose Pick > Nothing .
2. Select the top surface.
3. In the Control Panel, turn on the CVs (Cv/Hull).
4. In the Control Panel, increase the Degree value (in the V direction) from 1 to 2.

   

5. Press (Windows) or (Mac).
6. You will be prompted to confirm that you really want to delete the history. Press YES to confirm.
7. Press the Accept button.

The top surface now has a Degree value of 2 in the V-direction and ensures the flexibility required to direct model the surface. To execute the direct model process, move the hulls and single CVs in such a manner that the green cross-section fits flush to the scan lines. At the same time, the second draft surface still has history and will always update when changes are applied to the top surface. When modeling the top surface, remember to check for symmetry issues.

To direct model the surface

1. Choose the Object Edit > Align > Symmetry Plane Align tool.
2. Pick the top surface on the edge that meets the center line. A selection box will appear because Surface detects the surface and the center line curve at the same time. Choose the surface.

   You should see a collection of blue lines that highlight the surface as being symmetrical to the plane at the Y0 position. The Object Edit > Align > Symmetry Plane Align tool created history that will maintain the symmetry of the surfaces. If you move the CVs of the first two hulls in the Z-direction, the corresponding CVs will follow. Before using the tool, make certain that the Project for Position option is activated in the Symmetry Plane Align tool’s option box.

3. Move the opposite hull down to achieve a better fit to the scan line.
4. Move the middle hull along the Y-direction to find a better fit.
5. To fit the second draft surface to the scan lines, choose single CVs and move them via the Move CV tool at the bottom of the Control Panel, with Mode set to SLIDE.

   

The objective of this section of the tutorial is to find the theoretical line by using simple techniques involving simple surfaces. The first priority should be to fit both surfaces to the scan lines, and then examine the theoretical line from a number of views.

To examine the theoretical line

1. Save your work.
2. To make the comparisons easier, duplicate one surface edge to create a curve that represents the theoretical line. (Use the Curve Edit > Create > Duplicate Curve tool).
3. Isolate the curve using the ObjectDisplay > Hide Unselected menu item.
4. Turn on the CVs of the curve (Cv/Hull in the Control Panel).

Compare your theoretical line to the views detailed below.

The theoretical line looks good from the side view.

However, the theoretical line does not look very good from the top view. Remember the criteria of the CV distribution in the curve-fitting section.

If the side and top views are acceptable, this is not a guarantee that the Perspective view is acceptable as well. As a basic rule, you must examine the theoretical line by tumbling the line in the Perspective view similar to the images below.

In the image below, note that the viewing position makes the theoretical line appear concave.

In the image below, note that the viewing position makes the theoretical line appear convex.

When tumbling between the concave and convex extremes, there must be a specific position where the view of the theoretical line has planar characteristics. See the image below for an example.

To achieve planar characteristics for the theoretical line that resemble the image above, pressure the curve using the Curve Edit > Curve Planarize tool.

If technical restrictions do not allow for the use of the Curve Edit > Curve Planarize tool, proceed with your existing theoretical line. The technique of pressuring curves is not a fixed rule, but rather, a guide for helping to define the feature lines that dominate the shape of a model.

Criteria for the theoretical line

• The surfaces that build the theoretical line must fit to the scan lines.
• From the top view, the theoretical line must have an even shape (criteria of CV distribution).
• From the side view, the theoretical curve must be fluid and properly aligned.

Solutions for meeting the theoretical line criteria

• Fit the surfaces, used to build the theoretical line, to the scan lines.
• Pressure the theoretical line in a plane. (A special function will be introduced to help you complete this task).
• Correct the theoretical line in each view without losing the planar character to achieve a better fit of the surfaces to the scan line. (Use the Move CV tool in the Control Panel, set to SLIDE or NUV).

A theoretical line is more than just a curve. A theoretical line, in a wider sense of the definition, relates to the edge of a surface. When scan lines are absent, it is a difficult task to model from a curve because the theoretical line will have no scan lines to act as guides. When working with surfaces that are close to the scan lines, the edge of the surfaces will show the theoretical line.

At this point in the tutorial, your duplicated curve should be fairly close to the theoretical line. However, the planar criterion is not fixed and it will require a new function to correct the issue.

To planarize a curve

1. Double-click the Curve Edit > Curve Planarize tool to produce the option box.
2. Set the options as follows.

   Set this parameter...	To this value
   Default projection plane	Best
   Lock ends	on
   Keep originals	off

   

3. Press the Go button.
4. Pick the curve representing the theoretical line.

   The Curve Edit > Curve Planarize tool produces a new curve that differs slightly from the original in that it fits correctly to the plane. If required, modify the new curve using the Control Panel > Evaluate > Move CV tool. Because the new curve was generated with the Curve Edit > Curve Planarize tool, the curve will remain planar.

   

5. Create a new layer, name it “Theoretical line” and assign to it the planarized curve.
6. Save your work.

Establishing a fundamentally sound theoretical line is crucial to the success of the surface modeling project. The theoretical line provides the foundation for the construction of the surface model. As well, remember that it is equally important to maintain a smooth theoretical line in order to construct quality fillets.

Another method for creating a theoretical line involves the use of blend curves. While blend curves are a fast procedure and offer a high degree of history, they also lead to errors in the model. Because blend curves produce multi-span curves, the fundamental base for the entire surfacing process can be damaged. As a general rule, the geometry, or number of spans in a model, should always be kept to a minimum to ensure professional results.

Fit curves to some scan lines. Extend the curves so that they intersect.

Create geometry points on the intersections. Double-click the Curve Edit > Curve Section tool and set the options as follows.

Create a blend curve by selecting all geometry points. Check the blend curve in the Control Panel to determine the number of spans and degrees. A multi-span curve has been produced. The blend curve is the theoretical line.

Examine the curvature plot of the blend curve, and then planarize the curve.