Some characteristics of X-scans

• X-scans run perpendicular to the center line, whereas Y-scans run parallel to the center line.
• modeled X-scans are symmetrical, in that one half is the same as the other half.

Because of the X-scan characteristics, it is possible to model half the model and mirror the other half.

Mirroring saves time, but it can also produce a problem that must be addressed. Running along the Y-plane at the Y0 position, whole surfaces, such as the top surface, are seamless. On the other hand, when employing the “half-side-modeling” method, two surfaces and a connection seam are created. Connection seams lead to modeling problems with curvature combs.

For example, the image below shows a surface that covers the right half of the model. The left half of the model was mirrored from the right half. Note that the curvature combs have a seam crossing the center line. This seam can be modified, but special attention will have to be paid to the manipulation of the surface.

In contrast, the image below shows one surface covering the entire top of the model. The curvature combs crossing the center line are smooth, and as a result, you will never have to check the appearance of the curvature comb crossing the Y0 position.

To avoid the problems created by mirroring, all surfaces that cross the center line should be constructed as one surface.

To fit a curve that crosses the center line

1. Make the scan lines from the X-scan layer visible and pickable. (Use the Visible and Set State > Pickable menu items for the X-scan layer).
2. Select one scan line from the middle portion and isolate it. (Use the ObjectDisplay > Hide Unselected menu item).

   

3. To fit a curve to a scan line, double-click the Curve Edit > Fit Curve tool to produce the option box.
4. Set the options as shown in those in the image below.
5. Use the Curve Edit > Fit Curve tool in the same manner as detailed in “Establishing and creating the center line” section.

   

   

   Once you have created the curve and matched it to the scan line, you can use it as a guide.

6. Choose Pick > Nothing .
7. Select the new curve.
8. From the menu, double-click Edit > Duplicate > Mirror to produce the Mirror Options box.
9. Set the Mirror Across option to “XZ” and press Go.

   

   Two points have now been produced, one at either side of the model, each with a corresponding position relative to the center line. The next step in the procedure will be to create the real curve.

10. Double-click the Curves > New Curves > New Edit Point Curve icon to produce its option box.
11. Set the options as shown here.

    Set this parameter...	To this value
    Knot spacing	Uniform
    Curve degree	2
    Create guidelines	off

    

12. Create a straight curve between the two endpoints.
    TipUse the Magnet tool (upper right corner of the display) to snap the new CVs exactly to the endpoints.
13. Choose Pick > Nothing .
14. Delete the helper curves.
15. Select the middle CV and move the CV in the Z-direction to match the shape of the scan line.

As you manipulate the CV in the Z-direction, you may notice that you cannot model the shape of the scanned line with just one free CV. Eventually, you will need more CVs, but not before giving some thought to the maximum number of CVs you will require. To tackle this question, always start with the lowest degree (number of CVs) and increase the value in small steps. Employing a lowest-to-highest degree strategy should ensure that you do not overbuild your model.

To increase the number of CVs of a curve

1. Make sure that the curve is selected.
2. Turn on the CVs (Cv/Hull) in the Control Panel.
3. In the Degree text box located in the middle of the Control Panel, type in the value 4 and press (Windows) or (Mac).
4. Click the Accept button in the lower right corner of the window.

You now have more CVs to help you achieve the shape of the scan line.

With the increase in CVs there is also an increase in the freedom of movement. When moving CVs you should remember to always move corresponding CVs at the same time, otherwise, you will loose the symmetry of the curve.

Going forward with this exercise, you will encounter a problem when trying to change the position of the marked CV in the Y-direction. One solution to this problem is to select the CVs you intend to move on both sides of the center line and then center the pivot point. You will then be free to move the CVs using a non-proportional scale.

To move CVs using a non-proportional scale

1. Choose Pick > Nothing .
2. Select the CVs highlighted in the image above.
3. Choose the Transform > Local > Center Pivot tool. Note that the green pivot point jumps closer to the center CV.
4. The pivot point marks the balance point of the selected CVs. Your local source point for the next movement will be placed at the new location of the pivot point.
5. Choose the Transform > Non-p Scale tool and use the middle mouse button to move both CVs in the Y-direction, away from the Y0 position.

   

You will now be able to fit the curve to the scan line, but don’t consume too much time measuring the deviations between the scan line and the curve. Instead, concentrate on trusting your eyes and developing an understanding of the CVs role.

After completing the top curve, continue creating the missing parts of the X-scan line. Use the same methods you learned when creating the center line (changing the endpoints of the curves, and modifying the blend curve using the manipulator).

Remember to check the curvature comb of your X-curves by selecting all the curves and then using the Locators > Curve Curvature tool.

Also, take care when polishing the curvature plot that you limit your movement to the non-proportional scale of the CVs on the top curve. To avoid destroying the symmetry of the curve, you should not use the SLIDE command if the curve runs across the Y0 position.

Your efforts should resemble the curve in the image above.

To save your work

1. Select all of your new curves and save them to a new layer named “X CRV”.
2. Make sure that all the X-scans are visible again.
3. Make the X CRV layer and the X-scan layer invisible.
4. Save your work.

At this point in the tutorial, you should have an empty screen.

Summary

• In this section you were introduced to the curvature plot and its relationship to half-modeling techniques.
• As well, you were taught how to avoid a curvature peak in the plot that crosses the center line by building curves and surfaces across the middle.
• But perhaps most importantly, you were instructed to always move CVs in groups of corresponding pairs to avoid losing the symmetry of the curve. The best practice for moving multiple CVs is to select the CVs as a pair, center the pivot point, and use the non-proportional scale for horizontal movements.