As you may have already learnt in the previous sections in this training manual, the closer you move control points together the more rapid the change in curvature. A discontinuity in a curve or surface can be modelled using this principle: when certain conditions are satisfied, a knuckle can be created by moving two control points on top of each other:

In this section you will learn how to use compacted control points to model chines or knuckles in a surface and what conditions have to be satisfied in order to be able to do this.

 This video shows how a knuckle can be created in a curve by compacting control points. This video also shows the advantage of grouping compacted control points together. [Web | Disk]

Compacting Control Points on a spline or curve

First we will use a simple spline to illustrate the effect of compacting control points.

Ø  Open the CompactCP_Curve_Start.msd design in the Profile window

Ø  Switch Data | Precision to highest.

You should see something like below:

Ø  Move the two middle control points closer together and watch what happens to the shape of the curve.

Tip:

Hold down the shift key while dragging. This will restrict the control point movement to move only horizontally or vertically.

From this you can see that where the control points are close together a knuckle is forming, but when you zoom in to that point it is still a smooth transition. Note that where the curve touches the control point net, both lines are tangential to each other. Let’s see what happens when you now put the control points directly on top of each other.

Ø  From the two control points in the middle select the one on the left first and then the one on the right while holding shift key down.

Ø  Now use the Controls | Compact command (or use the Ctrl+K shortcut key on your keyboard)

You should see the control point on the right being moved on top of the one on the left.

The control point that is selected 2^nd will move on top of the control point that was selected 1^st.

Ø  Click anywhere in the screen to clear the current selection.

Ø  Select the compacted control point by left clicking on it (don’t use a selection box)

Ø  Drag it upwards

Notice that the compacted control points –while they shared the same coordinates - are still being treated as individual points. Often it is convenient to group a number of compacted control points so that they get treated as one entity or group. In Maxsurf you can group control points using the Group command:

Ø  Using Ctrl+Z, undo the drag move until both control points are compacted again.

Ø  Now use a selection box to select both points

Ø  And use Controls | Group or Ctrl + K to group the control points

Ø  Now try to drag just one control point (you won’t be able to).

You’ll notice that the control points are now treated as one and cannot be moved separately anymore. You notice that the colour of the control points has changed to red.

Tip:

Ctrl + K = Compact control points
Ctrl + G = Grouping control points

Over Compacting

When using compacted control points to model a discontinuity, there is a very important relationship between compacted control points and surface stiffness.

Ø  Use the curve from the previous steps or the CompactCP_Curve_Compacted.msd model

Ø  Set Longitudinal curvature from the surface properties for this curve to 4.

You will now see something like this in Profile view:

The knuckle in the curve has disappeared after increasing the stiffness to 4.

Ø  Insert a control point just to the left of the grouped control points (red).

You’ll see that this situation is very similar to the situation before where the curve is tangential to the control point net where they touch.

Ø  Select the grouped control points first followed by the newly inserted control point while holding shift key down

Ø  Ctrl + K and Ctrl + G to compact and group

You have now compacted 3 control points on top of each other to create a knuckle in the curve with a stiffness of 4.

For more information on compacted control points, see the Compacted control points section in the Maxsurf manual.

 View a video that shows how to model a chine or knuckle line in a surface by compacting control points. [Web | Disk]

Using Compacted Control Points to Model a Chine

You are now ready to use compacted control points on the design created in the “Starting a Design from Scratch” section of this training manual and transform it from a round bilge into a chined hull design.

Ø  Open the MyFirstDesign.msd file

Ø  Save it as… MyFirstDesign_knuckle.msd

Ø  To make it easier to model a knuckle, reduce the transverse surface stiffness to 3 (this way you only have to compact 2 control points).

Ø  Switch to body plan view,

Ø  Hide the net, Unlock the surface, Display | Half

Ø  Use the Control box to switch to the aft most column of control points

Ø  Insert a row control point in the position as displayed in the image below:

Ø  Use a selection box to select the control points that are close together,

Ø  Compact and Group the control points

Ø  Switch to Rendered Perspective view to check your work:

Ø  Back in Body Plan view, use the control box to switch to the column in front of the Aft most column:

Ø  Repeat the select à Compact à Group commands for all columns in the design except for the front most column (we don’t want to create a knuckle in the bow profile).

Ø  Switch to Plan view and show the Net,

Ø  Display | Contours | Feature lines or use the  toolbar button to display the knuckle you have just introduced.

And in Perspective view:

Ø  Save your design as MyFirstDesign_Knuckle.msd

Exercise: insert another knuckle line so that you get something like in the image below:

You will see that you have to add an additional 2 rows of control points. Having so many control points close together in the bow now, makes it quite difficult to maintain a reasonably fair bow area. Inserting an extra column in the bow may give you more control over this area:

This design is available for download as MyFirstDesign_DoubleKnuckle.msd

Continue with the section on Using Linear Surfaces to Model a Chine.