Learning Objectives

In this section you will learn how to:

• Create curves and fit them to the scan lines.

After you have fashioned your model from the foam cube, scan the model’s surface to produce a series of scan lines that represent your creation. If the scan lines do not look exactly the same as the foam model, keep in mind that the model was handcrafted, and as a result, small surface degradations may have been created during the model’s construction. It is also entirely possible that the scanning device made errors due to a discrepancy in stated construction tolerances. Regardless of the cause of the errors, it should be understood that the primary role of the scan lines is to provide a guide for the dynamics of the model. Before beginning the surface modeling, use the scan lines to garner information about the model.

Similar to sketching on the foam cube, the first step in the surface modeling process is to create a center line based on the scanned input.

To view all of the scan lines

1. Open the wire file Curve Fitting.wire.
2. Pick all of the scan lines.
3. Choose the View > Look At tool.
   NoteThe scan lines used in the tutorial are degree 1 curves. When using the Pick > Component tool, the scan lines can only be selected as curves, not sections.

With all of the scan lines displayed, you might find that the model is cluttered. To make your work appear cleaner, isolate the scan line that represents the center line.

To isolate the center line

1. Click and hold the X-scans layer button to open the menu.
2. Turn off the Visible option.
3. Repeat Steps 1 and 2 for the Z-scans layer. In the end, only the Y-scan lines should remain visible.
4. Choose Pick > Object and select the scan line that represents the center line.
5. Choose ObjectDisplay > Hide Unselected . All unselected scan lines are hidden - only the selected center scan line is visible. (To make the hidden lines visible again, choose ObjectDisplay > Visible from the menu).

   

Commit these steps to memory – later in the tutorial you will be asked to isolate a particular scan line or make a layer visible or invisible.

You may recall that when you were in the workshop you found it difficult to shape the center line in one step. To complete the model, you shaped three main surfaces (the front, middle, and rear) and completed the model by adding two blends. Following the workshop example, fit a curve to the top part of the center scan line. Use an automatic fitting algorithm, such as the Curve Edit > Fit Curve tool, to undertake this part of the project.

To fit the top part of the center line using the Curves > Fit Curve tool

1. On the ViewCube, click the Left face, or in the Viewing Panel, select the middle arrow in the bottom row of arrows to switch to the side view .

   

   NoteThe ViewCube viewing control displays by default, but you will use the View Panel in these tutorials. To switch view controls, select Preferences > General Preferences ❒, click Viewing, and select View Panel. For information on the ViewCube, see Use the ViewCube tool and NavBar.
2. Double-click the Curve Edit > Fit Curve tool to produce the options box.
3. Select the scan line that represents the center line and set the options in the Fit Curve Control as shown in the image below.

   

   NoteThe Curve Edit > Fit Curve tool selects the entire scan line, but initially you will be interested in selecting only the top part of the curve.
4. Pick the cross that lies adjacent to the blue manipulator and move along the scan line until only the top portion of the curve is selected.

   

   Your curve should resemble the above image. As you gain more experience with surface modeling, you will learn to judge the placement of the CVs. Later in the tutorial, the term “Hull / CV distribution” will be used to describe the placement of CVs.

   

   

   By moving the crosses and changing the fitting method, you can influence the CV distribution.

   Another method for achieving a good CV distribution is to pick the CVs and move them to the desired position. However, this method is not recommended at this point in the modeling process as moving CVs will destroy the construction history of the curve.

5. Repeat Steps 2 – 4 for the front and the rear portions of the center line.

After fitting the three parts of the center line, your curve should resemble the image below.

Next, blends will need to be created between the three main curves. Another semi automatic tool, the Blend curve, will be utilized to perform this task. Blend curves can be intimidating to the beginner, so for this reason you will be asked to use a limited number of the simpler options.