To change the constraint continuity of a blend point

1. Select one blend point.
2. Choose the BlendCrv Tools > Constraint Continuity > Blend Constraint G2 tool.

   

   The blend curve changes from a straight line to a smooth transitional curve between the two scanned lines. Note also that the value for Continuity has changed to a 2 (G2) in the Information Window. If the shape of the blend curve is not perfect, you will have an opportunity later in the tutorial to manipulate it.

3. Repeat Steps 1 & 2 for the remaining three blend points (adjusting continuity at each end of both blend curves).

To examine the proximity of the blend curve to the scan line

1. Choose Pick > Nothing
2. Zoom into the scene until you have a clear view of the blend curve and the scan line.
3. Select the blend curve.
4. Turn on the CVs (CV/Hull in the Control Panel).

   

The CV distribution of the blend curve is very good, but the blend curve does not fit to the scan line. How could you correct this problem?

Method 1: Change the position of the reference curve endpoints.

Method 2: Use the manipulator to change the shape of the blend curve.

Before you make a decision about how to correct this problem, you should stop to consider some aspects of the work you have completed.

The CV distribution of the blend curve is satisfactory – so, there’s no need to change it. That leaves you with the endpoints – because the top and rear curves still have their construction history, you should use small incremental movements to manipulate the endpoints.

To reactivate a tool’s option box using the Query Edit tool

1. Choose the Object Edit > Query Edit tool.
2. Select the rear curve.

   The Fit Curve Control window and the blue manipulators will appear again.

3. Move the upper cross down toward the end of the curve.

   Because the blend curve is still attached to the curve (with G2), it will follow.

4. While you move the cross of the fitted rear curve, pay attention to how close the blend curve comes to the scan line.

   

At this point in the tutorial, you should be comfortable with using your eyes to judge the deviation to the scan line. Within the confines of this tutorial, your work does not have to be precise, but for situations where your work must be more professional, you will require the assistance of a measurement tool, such as a Locator.

To use a Locator tool in measuring a curve-to-curve distance

The scan line has to be pickable.

1. Click and hold the Y-scan layer button to open its menu and choose Set State > Pickable.
2. Choose the Locators > Deviation > Curve to Curve tool.
3. Select the scan line and the curve(s) you want to measure. More than one curve can be selected.

   The deviation between the selected items will be measured and appear as a locator.

   NoteIn the Control Panel you can switch the deviation locator on or off by selecting/deselecting the Deviation checkbox (the curves need to be picked first).

   

4. Modify your curves and note how the deviation locator updates the deviation status.
5. After you are satisfied with the results, select and delete the deviation locator.

If you are still not content with the match between the scan line and the blend curve, you can explore another method for reshaping the blend curve.

Blend points are located at the ends of blend curves. Each blend point contains information that can be read in the Information window (Windows > Information > Information window).

To use the blend curve edit manipulator in the Blend curve toolbox

1. Choose Pick > Nothing .
2. Pick the upper blend point.
3. Open the blend curve toolbox.
4. Choose the BlendCrv Tools > Blend Curve Edit tool.

5. A new manipulator will appear that features a blue sphere and a blue rectangle.

6. With the , pick the sphere, hold down the mouse button, and move the mouse. Note the movement of the blend curve.
7. Repeat Step 5 using the rectangle.
   NoteWhen manipulating the rectangle, the second and third CVs move while manipulating the sphere will move just the third CV.

   

   Up to this point in the tutorial, you have learned two methods for fitting a blend curve to a scan line. The first method changed the reference points. If the first method does not produce sufficient results, as a last resort, use the manipulator to change the shape of the blend curve.

8. Repeat the entire sequence for the front blend.

In a real job environment where time is often a critical factor, when changing the connection quality of the blend point to CURVATURE quality (G2) you should simultaneously select both blend points to save time.

Take a moment to consider the work completed at this point in the tutorial.

You will recall that earlier in the tutorial you conducted a deviation control by hand and used your eyes to judge your work.

The next step is to use a curvature comb to check the smoothness of your curve connections.

To check the curvature of curves

1. In the Y-scan layer menu, choose Set state > Reference.
2. Select all of the curves.
3. Choose the Locators > Curve Curvature tool.

Locators will appear that show curvature combs connected to the selected curves. Curvature combs are locators, and as such, you can access information about them through the menu item Windows > Information > Information window.

In most cases though, the curvature comb locator will not provide you with all of the information you may require. To gain more information, select the Curvature shelf from the Control Panel and change the Comb Scale value.

To change the Comb Scale value

1. Select the Curvature button from the Control Panel to open the Curvature shelf.
2. Change the values for the Comb Scale and the Samples options.

   

Having changed the Comb Scale and Samples values, your curve should resemble the image below.

The comb shows a significant jump in values at the position of the blend curves (not a smooth transitional curvature comb). If you were to leave the curves in their current state, the end result would take into account the distortion of values as pointed out by the curvature combs. To remove any curvature distortions you should adjust the combs in a finite fashion. As the manipulation of the curves is a complex task, be careful to adjust the combs in small increments. In fact, it could take you a matter of years to perfect the art of manipulating curves. However, in the interests of time, there are some hints that should speed-up the process.

Factors to consider when manipulating curves

• Watch the curvature plot of the top curve. It has a clean arc because the CVs are placed at regular intervals.
• Try to increase the curvature at the end of the top curve. To accomplish this task, directly manipulate some of the CVs.
• Moving the second CV always has a significant impact. Manipulate it very slowly in order to keep the close deviation to the scan line that you have already achieved.

To model CVs directly

1. Pick the top curve.
2. From the menu, choose Delete > Delete Construction History .
3. You will be prompted to make sure that you want to delete the construction history for the curve. Click YES to confirm.

   The top curve will no longer have a Fit Curve history.

4. Select the highlighted CV shown in the image below.
5. At the bottom of the Control Panel, select the Move CV tool.
6. In the CV Move shelf, set the two values associated with Mode to SLIDE and CV.

   The selected CV will now appear with two arrows that point in the direction of the available movement associated with the CV. The direction will always point toward the next CV.

7. Pick one arrow and move the CV slightly.

   

8. Watch the curvature and how the curvature comb changes.

Remember, the objective of this exercise is to strike a balance between a smooth transitional curvature comb and a close fit to the scan lines. As you experiment with different strategies and methods for achieving the desired results, you may find that you will need to correct the endpoints of your first three curves.

Your finished curvature comb should resemble the image below. It may require a significant investment in time to achieve the same results but the experience you gain from the exercise will help you later in your surface modeling career.

To clean up your work

1. Choose the Delete > Delete All Locators menu item. All of the green curvature combs should disappear.
2. Choose Layers > New to produce a new layer.
3. Double-click the new layer, then type “Center Line CRV” and press (Windows) or (Mac).
4. With the new layer still selected, pick all of the new curves that represent the center line.
5. Click the new layer button, hold down the mouse button and move the mouse down to Assign. The selected geometry will be assigned to the new layer.
6. Using the same technique as Step 5, make the new layer’s contents invisible by moving the mouse down to Visible in the new layer menu. The check mark beside the option will disappear and the geometry assigned to the layer will become invisible.
7. From the menu, choose ObjectDisplay > Visible to make all of the Y-sections appear.
8. Repeat Step 6 to make the layer Y-scan invisible.
9. Save your work.

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

Summary

• Blend points have special information that can be accessed through the Windows > Information > Information window menu item, or can be changed via the options available in the Blend curve toolbox.
• Curve fitting is an intricate process that should lead to striking a balance between a smooth transitional curvature comb and a close deviation to the scan line.
• Curve positions are never fixed. Blend curves can change the endpoints of main curves.

Following the process detailed at the beginning of the tutorial, you can define the shape of the other two directions (the front surface, or X- scans, and the top surface, or Z-scans).

While fitting the new curves you will be introduced to some new methods. In general though, you will be able to use the same processes detailed in this section to complete the next step.