Modeling Create geometry Analyze problems with a curve network > Analysis

Create and edit curve networks

How to create and edit surfaces defined by a network of intersecting curves.

Understand curve networks

Explains concepts and terminology used in describing curve networks.

To show the curve network toolbox, choose Surfaces > Curve networks .

Overview

Build a mesh of intersecting curves to describe a surface.
Choose the New network tool and pick all the curves.
AliasStudio interpolates between the curves to create surfaces automatically.
You can continue to reshape, add or delete the construction curves, and the curve network surface will update.
Add sculpt curves to reshape the whole curve network surface by mapping the shape of a free curve onto the surface.

Terminology

Refer to the diagram at left for illustrations of the following terms used with curve networks:

Curve: one of the curves in the network.
Intersection: where two curves in the network cross at the exact same point in space. In the interface this is also called a vertex.
Curve segment: a section of a curve defined by intersections (vertices) at both ends. Many of the curve network tools work on curve segments, including the continuity tools. In the interface, and sometimes in this chapter, this is called an edge.
Region: closed area of a curve network bounded by either three or four curve segments (edges).
Curve network surfaces: the surfaces are distinct from the curve network itself. In fact, the surface can be pulled away from the network by sculpt curves.
Curve network: all the curves defining the surfaces. AliasStudio will not build surfaces until all the curves in the network are valid.

Valid and invalid topology

AliasStudio creates one surface for each valid region of the curve network.

All curve network surfaces are grouped under a single node, as shown in the Object lister or SBD window.

Under certain conditions, you may be able to create a single surface from the resulting curve network surfaces, by using the Combine surfaces tool.

The following types of closed regions are valid

Four sided regions are fitted with standard NURBS surfaces.





Three sided regions are fitted with three NURBS surfaces.





Regions with T-junctions that can be "collapsed" into three-, four-, or five-sided regions are treated the same as regular three-, four-, or five-sided regions.

The following regions are not valid

Regions with more than five sides.





Regions with only two sides.





Regions with T-junctions that cannot be "collapsed" into three, four, or five sided regions.

Other rules

There can be only one curve segment between any two intersections.

General tips and notes

Curve networks do have a performance penalty. Do not use curve networks for surfaces that can be constructed just as easily with the normal surface creation tools.
Use more curves in areas with detail, and fewer in areas with little variation.
To design very complex shapes, create the network with a few defining curves, then add more curves and view the incremental effect on the network.

This lets you monitor continuity and topology requirements and identify problem curves quickly, which is more difficult when you add all the curves at once.

Try to construct the network from curves that all have the same degree and the same kind of parameterization.
Curve networks produce best results when every surface region is four-sided. This means a maximum of four surface regions meeting at any intersection.
If you must have triangular or five-sided regions, keep them as simple as possible. Use simple curves with little variation.
Avoid T-intersections by adding more curves to the network, or extending existing curves.
The history viewer can be used to return to a network for further modifications.

Work with curve networks

How to pick, update, and rebuild existing curve networks.

To show the curve network toolbox, choose Surfaces > Curve networks .

Pick a curve network

Choose CrvNet Tools > Pick network .
You must pick a curve from the network, not a surface.
Curve network tools will affect the picked network from now on.

Convert a curve network to normal surfaces without history

Pick the curve network and choose Delete > Delete construction history .

Create a curve network

How to create a surface from a network of intersecting curves.

To show the curve network toolbox, choose Surfaces > Curve networks .

Create a network of curves

Create a network of intersecting curves to describe a surface.

You can use any kind of curve (free curves, isoparametric curves, curves on surface, trim edges), of any parameterization and degree, rational or non-rational.
Make sure the curves intersect.
See Make curves intersect
The enclosed area of the network should have only three, four, or five sides.
Remember the restrictions on the topology of regions.
See Understand curve networks
Make sure the curves have smooth curvature.
See Find curve problems using the curve curvature plot

Remember that you do not need to describe the entire model with a single curve network. Try using several curve networks for different parts of the model.

Create a new curve network surface

Choose CrvNet Tools > New network .
Pick all the curves you want to define the new surface. It doesn't matter in what order you pick the curves.
Click Go.

The New network tool will let you create a curve network from invalid curves (for example, curves that do not intersect), but the actual surfaces will not build until you edit the curves to create a valid network.

Use CrvNet Tools > Analyze network to look for the problem that prevented the surfaces from building.

Add or subtract curves in a curve network

Add curves to, or remove curves from the mesh that controls the curve network surfaces.

Remember the rules for valid and invalid topology as you add or subtract curves.

See Understand curve networks

To show the curve network toolbox, choose Surfaces > Curve networks .

Add a curve to a curve network

Choose CrvNet Tools > Add curves to network .
Pick the curves you want to add to the network.

Subtract a curve from the curve network

Choose CrvNet Tools > Subtract curves from network .
Click the curves in the network you want to remove.

Change the continuity along network curves

Change the level of continuity across curves in the network.

Positional continuity is the default for a new network. If you need more or less smoothness between the different surfaces, use the continuity tools to change the continuity required across any given network curve.

To change the default continuity for new networks, choose Surfaces > Curve networks .

To change the continuity enforced on a curve

Click one of the Continuity tool icons (Positional continuity or Tangent continuity), or choose from the Continuity sub-menu of the CrvNet Tools palette menu.
Click the curve you want to change.

The continuity tool changes the continuity across the curve you clicked.

Changing the continuity for a curve changes the continuity for all the segments on this curve. In other words, the newly specified continuity is enforced between all surface pairs that are adjacent to each other across this curve.

The continuity requirements that you specify may not be achievable with the given curves.

Avoid using many different areas of continuity in a single network. The more consistent the continuity requirements, the better the surfaces.

Edit network curves while maintaining intersections

Edit a network curve using the normal curve editing tools, while maintaining the intersections with the other curves in the network.

Normally when you edit a curve in the network, you run the risk of moving or reshaping the curve out of intersections with the rest of the network. To prevent this you can lock the curve intersections so editing tools won't disrupt them.

To show the curve network toolbox, choose Surfaces > Curve networks .

Lock a curve to maintain intersections

Choose CrvNet Tools > Lock intersection .
Click a free network curve you want to lock.

The curve network surfaces become invisible and the curve highlights.

Use editing tools to edit the curve.

You can only lock one curve at a time.

Unlock a locked curve

Choose CrvNet Tools > Lock intersection .
Do one of the following:
Click the curve to unlock it. The surfaces reappear and are updated.
Click another curve to lock and edit that curve. This automatically unlocks the previously locked curve.

Notes

You can only lock a free curve. Isoparametric curves, trim edges and curves on surface cannot be locked.
You can only lock intersections on networks without sculpt curves. If you want to use the Lock intersection tool, you will have to remove any sculpt curves from the network first.
You cannot use curve network tools on the network while a curve is locked. For example, you cannot add or remove curves, or change continuity.

To edit the curve network, unlock the locked curve by choosing the Lock intersection tool and clicking the locked curve.

If you edit any curve other than the locked curve, the locked curve is unlocked and the curve network returns to its normal state.
If you save the file while a curve is locked, it will be unlocked when you open the file again.

Understand sculpt curves

Sculpt curves shape the curve network surface by pushing or pulling the surface away from the network.

A sculpt curve is like a handle on the surface which lets you push or pull the surface.

Sculpt curves allow you to make complex or subtle changes to surfaces such as adding bumps, indentations, twists, or cork-screw effects.

> How sculpt curves work

AliasStudio projects the sculpt curve onto the surface along the surface normals.
AliasStudio links the points along the sculpt curve to the points on the projected curve.
When a point on the sculpt curve moves, the corresponding point on the surface moves also.
The degree to which the point on the surface follows the point on the sculpt curve depends on the weight or weights along the sculpt curve.
The area of the surfaces which is pulled along is the region of influence of the sculpt curve.

Notes

To speed up rebuilds, keep the sculpt curves simple.
Just as if you were editing the surface manually, the sculpt curve can only reshape the surface if it has enough CVs to control it.

For example, mapping a sculpt curve to a degree 3 surface with one span in each direction will not allow detailed modifications of the surface. The four interior CVs in this example are not enough to make the sculpting effort worthwhile.

Sculpt curves can be applied only to four-sided regions in a curve network. Surfaces fitted to triangular or five-sided regions and regions with collapsible T-junctions remain unchanged by sculpt curves.
Once sculpt curves are mapped onto the curve network, any changes to the topology of the network (such as adding or removing curves) requires AliasStudio to remap the sculpt curves.

Because this operation can potentially take a long time, a dialog box will ask you if you really want to make the change.

Lock curve network edges

How to maintain continuity with other surfaces when adding sculpt curves

The edges of networks without sculpt curves are always fixed. When you add sculpt curves, the edges of the network can be pushed or pulled out of position. This is undesirable if those edges need to maintain continuity with other surfaces.

To prevent network edges from moving, use the Pin Edge tool to "pin down" some edges down so they are not influenced by sculpt curves.

To show the curve network toolbox, choose Surfaces > Curve networks .

Pin or unpin edges

Choose CrvNet Tools > Pin edge .
Click the curve segments you want to pin or unpin.
Click Go.

Add or remove a sculpt curve in a curve network

Specify curves to use as sculpt curves for a curve network.

To show the curve network toolbox, choose Surfaces > Curve networks .

Add a sculpt curve to a curve network

Choose CrvNet Tools > Add sculpt curve .
Click the free curves and/or isoparametric curves you want to use as sculpt curves, or drag a pick box around them.

If the sculpt curve does not map to any of the network surfaces, it is not added to the network and remains in its default color.

Remove a sculpt curve from a curve network

Choose CrvNet Tools > Delete sculpt curve .
Click the sculpt curves you want to remove from the network, or drag a pick box around them.

Change the influence of sculpt curves

Change the amount and area of influence a sculpt curve has on the network surfaces.

To show the curve network toolbox, choose Surfaces > Curve networks .

Make a sculpt curve have the same amount of influence along its entire length

Choose CrvNet Tools > Influence weights > Unit weight .
Click the sculpt curve you want to change.

The tool displays the current weight profile of the sculpt curve.

Drag left or right to change the weight of the curve, or type an exact value.
Click Go.

Set different amounts of influence at different points along a sculpt curve

Choose CrvNet Tools > Influence weights > Multi weight .
Click the sculpt curve you want to change.

The tool displays the current weight profile of the sculpt curve.

Do any of the following:
Click the profile comb, then click again to insert a new needle for editing.
Click one of the labeled needles, then drag the to change the weight at that point in the profile.
Click one of the labeled needles, then drag the to move the needle in the profile.
Click one of the labeled needles, then click the to remove the needle from the profile.
Click Go.

Change a sculpt curve's area of influence

Choose one of the region of influence tools:
CrvNet Tools > Region of influence > Large region
CrvNet Tools > Region of influence > Medium region
CrvNet Tools > Region of influence > Small region
Click the sculpt curve you want to change, or drag a pick box around them.

Reset sculpt curves

Choose CrvNet Tools > Reset sculpt curves mapping .

Analyze problems with a curve network

Displays information about the network, allows inspection of curve segments and sculpt curves, and recommends fixes for problems with continuity.

Show network information

Choose CrvNet Tools > Analyze network .

The Analyze network tool displays information about components of the current curve network.

Press and hold on a curve segment or sculpt curve to show more information, including a recommendation on how to edit the network if continuity could not be achieved.

Analysis

The Analyze network tool labels the curve network with the following information:

Network component	Indicated by
Intersections	Red dots
Continuity type for each curve segment	Label at the midpoint of the curve segment (Pos or Tan).
Curve segments with at least one adjacent surface	White lines
Curve segments with no adjacent surfaces	Green lines
Sculpt curves	Red lines
Pinned curve segments	Pin icon

If the tool fails to build a surface in a valid closed region of the network, the edges are displayed in red. When this happens, query the edge (see below) to obtain more information about the probable cause of the failure.

The Analyze network tool also lets you select curve segments and sculpt curves to get specific information:

Information	Description
Curve Type	Network or Sculpt.
Region of Influence (for sculpt curves)	Large, Medium, or Small.
Nature	Pinned or Free. Whether or not the surface is pinned to the curve segment.
Edge Type	· Interior: the curve segment is inside the network (the surfaces are on both sides of the segment). · Boundary: the curve segment is on the edge of the network (the surfaces are only on one side of the segment). · Orphan: the curve segment is not connected to any surface at all.
Continuity	Achieved or Not achieved. If continuity was not achieved, the Analyze network tool will give a recommended action.
Recommended Action (for curve segments where continuity was not achieved)	A recommendation for how to modify the network so that the curve network can achieve the requested continuity.

If the curve network could not achieve the requested continuity at a particular curve segment, the Analyze network tool will recommend a course of action to fix the problem and achieve the requested level of continuity:

The Recommended Action is only an educated guess. In many cases it is impossible to pinpoint the exact reason why continuity could not be achieved.

"Recommended Action"	Possible Cause	Suggestions
End point(s) belong to degenerate surface(s)	One or more of the surfaces meeting at one of the intersections that define the curve segment is a degenerate surface.	· The intersection causing the problem is indicated with an arrow. · Try to redefine the network with curves that do not require degenerate surfaces.
Check network topology at end point(s)	One of the intersections that defines the curve segment may be part of a region with more than four sides.	· The intersection causing the problem is indicated with an arrow. · Try to redefine the network with curves that do not require non-rectangular surfaces.
Conflicting surfaces incident at endpoint(s)	There may be a conflict between the normals or curvatures of the surfaces (outside the network) which meet at this intersection.	· The intersection causing the problem is indicated with an arrow. · Check the alignment of the surfaces at the endpoint of the curve segment, and reshape to provide required continuity.
Problem curve at endpoint(s)	One or both of the curves intersecting this curve segment do not have sufficient curvature quality.	· The intersection causing the problem is indicated with an arrow. · Use the Curve curvature measurement tool to check the curvature of the intersecting curves.
Align curves at edge end(s)	The curves intersecting this curve segment are badly misaligned.	· Check the alignment of both pairs of curve segments.
Smooth curve incident at ends	One or both of the curves intersecting this curve segment have some kind of quality problem.	· Turn on display of edit points and look for bunched up edit points on the intersecting curves. · Use the Curve curvature measurement tool to check the curvature of the intersecting curves. · Use the Smooth tool to smooth the intersecting curves.
Check edge curve for problems	The curves that this curve segment belongs to has some kind of quality problem.	· Turn on display of edit points and look for bunched up edit points. · Use the Curve curvature measurement tool to check the curve's curvature. · Use the Smooth tool to smooth the curve.
Check knot spacing on edge curve	The curve that this curve segment belongs to may have "bunched up" edit points or multi-knots.	· Edit or rebuild the curve to spread out the edit points.
Simplify triangle curves	The curves defining a triangular region are too complex.	· Use the Rebuild curve tool to reduce the number of edit points in the curves. · Use the smooth tool to remove sharp bends in the curves.