2.3 Edge Commands

The following commands are available on the Geometry/Edge subpad.

The following sections describe the purpose and operation of each of the Edge commands listed above.

2.3.1 Create Edge

The Create Edge command button allows you to perform the following operations.

The following sections describe the procedures and specifications required to execute the commands listed above.

The Create Straight Edge command allows you to create one or more straight edges between any two or more existing vertices. The created edges can be real or virtual.

• Two or more vertices that comprise the endpoints of the edge(s) to be created
• Edge geometry type (real or virtual)
• Host entity (optional)

Creating a Single Edge

To create an edge by means of the Create Straight Edge command, you must specify two vertices that comprise the endpoints of the edge. GAMBIT defines the sense of the edge based on the order in which the vertices are specified. The edge sense points from the first (start) vertex to the second (end) vertex (see Figure 2-30).

Figure 2-30: Straight edge specifications

If you specify more than two vertices on the Create Straight Edge form, GAMBIT creates multiple edges from a single operation. The order in which the vertices are specified determines the locations and connectivity of the created edges. For example, if you specify (in order) three vertices labeled vertex.1, vertex.7, and vertex.3 for a Create Straight Edge operation, GAMBIT creates two edges defined as follows.

Note that the two edges defined in the table above are connected to each other by means of vertex.7.

The Create Straight Edge command allows you to create either real or virtual edges. The geometry type (real or virtual) of the created edge is subject to the following constraints for vertex specification:

• To create a real edge, you must specify two real endpoint vertices.
• To create a virtual edge, you can specify any combination of real or virtual endpoint vertices.

If you create a virtual edge by means of the Create Straight Edge command, GAMBIT allows you to specify whether the virtual edge is hosted or unhosted. Hosted edges possess guest-host relationships with existing volumes, faces, or edges. Unhosted edges exist on their own and do not possess guest-host relationships with any other entities in the model.

Using the Create Straight Edge Form

To open the Create Straight Edge form (see below), click the Create Straight Edge command button on the Geometry/Edge subpad.

The Create Straight Edge form includes the following specifications.

The Create Real Circular Arc command allows you to create a real edge in the shape of a circular arc.

The following sections describe the specifications required to create a circular arc edge by means of the methods listed above.

Method 1—Center Vertex and Two Endpoint Vertices

To create a circular arc edge by means of Method 1, you must specify one center vertex and two endpoint vertices. The endpoint vertices must be equidistant from the center vertex (see Figure 2-31). You must also specify whether the edge is to be created along the longer or shorter of the two circular arcs that can be constructed between the endpoint vertices.

Figure 2-31: Circular arc edge specifications—Method 1

To create a circular arc edge by means of Method 2, you must specify three vertices each of which lies on the circle that defines the size and shape of the arc. GAMBIT constructs the edge from the first specified vertex through the second specified vertex to the third specified vertex (see Figure 2-32). Therefore, the first and third specified vertices constitute the endpoints of the edge.

Figure 2-32: Circular arc edge specifications—Method 2

To create a circular arc edge by means of Method 3, you must specify the following parameters (see Figure 2-33):

• Radius
• Start Angle and End Angle that define the arc sweep


• Center vertex(optional)


• Plane in which the arc lies

Figure 2-33: Circular arc edge specifications—Method 3

The Radius, Start Angle, and End Angle specifications define the size and sweep of the arc. The Center vertex specification defines the global location of the arc. (NOTE: If you do not specify a Center vertex, GAMBIT locates the center of the arc at the center of the currently active coordinate system.) The Plane specification defines the coordinate plane in which the arc lies.

To open the Create Real Circular Arc form (see below), click the Create Real Circular Arc command button on the Geometry/Edge subpad.

The Create Real Circular Arc form includes the following specifications.

Method: contains three radio buttons that allow you to specify the method by which the arc is created. The methods are briefly described as follows:

• Method 1—Specify one vertex that constitutes the center of a circle containing the arc and two other vertices that constitute the endpoints of the arc


• Method 2—Specify three vertices that lie on the arc


• Method 3—Specify the arc radius, angle, center, and plane

The center section of the Create Real Circular Arc form varies according to the method selected to construct the arc edge. The specifications available on the center section of the form are as follows.

Vertex:	---------------------------------------
Center	specifies the vertex that constitutes the center of the arc.
End-Points	specifies the vertices that constitute the endpoints of the arc.
Arc:	allows you to specify whether the created edge represents the shorter or longer arc between the endpoints.
Label	(both methods) specifies a label for the new edge. (See Section 2.1.1.)

Method 2

When you specify Method 2 for the creation of a circular arc edge, the middle section of the Create Real Circular Arc form appears as shown below.

When you specify Method 3 for the creation of a circular arc edge, the middle section of the Create Real Circular Arc form appears as shown below.

Create Real Full Circle

The Create Real Full Circle command allows you to create an edge in the shape of a full circle.

GAMBIT provides two methods for creating an edge in the shape of a full circle. Both methods require you to specify three existing vertices to define the size and location of the circle. The methods are defined as follows:

• Method 1—specifies one vertex that constitutes the center of the circle and two vertices that lie on the circle
• Method 2—specifies three vertices that lie on the circle

Method 1 (Figure 2-34(a)) requires that the two vertices that lie on the circle itself are equidistant from the vertex at the center of the circle. Method 2 (Figure 2-34(b)) requires only that the three specified vertices are not collinear.

Figure 2-34: Full-circle edge specifications

Using the Create Real Full Circle Form

To open the Create Real Full Circle form (see below), click the Create Real Full Circle command button on the Geometry/Edge subpad.

The Create Real Full Circle form includes the following specifications.

The center section of the Create Real Full Circle form varies according to the method selected to construct the circle. The specifications available on the center section of the form are as follows.

Method 1

Method 2

When you specify Method 2 for the creation of a full circle, the middle section of the Create Real Full Circle form appears as shown below.

Create Real Elliptical Arc

The Create Real Elliptical Arc command allows you to create an edge in the shape of an elliptical arc.

To create an elliptical arc edge, you must specify the following parameters (see Figure 2-35):

• Center vertex—located at the center of the ellipse
• Major vertex—defines the length of the major axis
• On Edge vertex—defines the dimensions of the ellipse
• Start angle and Stop angle for the edge—define the length and position of the edge along the elliptical arc

Figure 2-35: Elliptical arc edge specifications

The Center, Major, and On Edge vertices define the shape and size of the full ellipse of which the arc edge is a part. The Start angle and Stop angle define the length of the edge as well as its angular position relative to a reference vector constructed from the Center vertex to the Major vertex. (NOTE: The three vertices that define the ellipse must not be collinear.)

Using the Create Real Elliptical Arc Form

To open the Create Real Elliptical Arc form (see below), click the Create Real Elliptical Arc command button on the Geometry/Edge subpad.

The Create Real Elliptical Arc form includes the following specifications.

Create Real Conic Arc

The Create Real Conic Arc command allows you to create an edge in the shape of a conic arc.

To create a conic arc edge, you must specify the following parameters (see Figure 2-36):

• Start vertex—specifies the start endpoint
• Shoulder vertex—specifies the apex of the arc
• End vertex—specifies the end endpoint
• Shape Parameter—specifies the general shape of the arc (elliptical, parabolic, or hyperbolic)

Figure 2-36: Conic edge specifications

Specifying the Start, Shoulder, and End Vertices

The Start, Shoulder, and End vertices specify the location and sense of the conic arc edge. The sense of the edge points from the Start vertex to the End vertex.

The Shoulder vertex constitutes the apex of the conic arc, and its position of the Shoulder vertex determines whether or not the conic edge is symmetric with respect to the Start and End vertices. If the Shoulder vertex is equidistant from the Start and End vertices (as shown in Figure 2-36) then the conic arc edge is symmetric.

Specifying the Shape Parameter

The Shape Parameter specifies the shape of the conic arc edge. Its allowable values range from 0.01 to 0.99. The relationship between the Shape Parameter and the arc shape is as follows (see Figure 2-36).

Using the Create Real Conic Arc Form

To open the Create Real Conic Arc form (see below), click the Create Real Conic Arc command button on the Geometry/Edge subpad.

The Create Real Conic Arc form includes the following specifications.

Create Real Fillet Arc

The Create Real Fillet Arc command allows you to create a fillet edge between two existing edges.

To create a fillet edge, you must specify the following parameters:

• Two existing edges that define the fillet
• The edge selection points (u values)
• The fillet radius
• Whether or not the edges that are used to define the fillet are trimmed in the creation of the fillet edge

Specifying the Defining Edges

When you create a fillet edge, GAMBIT creates a circular arc edge between two existing edges that define the fillet. The defining edges may be straight or curved, but they must be coplanar. GAMBIT locates the fillet edge such that the circle that contains it is tangent to both edges. Furthermore, GAMBIT creates the fillet edge such that it constitutes the smaller of the two circular arcs that can be constructed between its endpoints (see Figure 2-37).

Figure 2-37: Fillet edge definition

Specifying Edge Selection Points

When you create a fillet edge between two existing edges, you must specify a selection point (u value) for each edge. The selection point is a dimensionless length parameter that corresponds to the distance between the selection point and one of the endpoints on the selected edge.

If an edge pair provides more than one possible location for a fillet of a specified radius, the selection points determine the location at which GAMBIT constructs the fillet edge. The effect of the selection point location varies according to whether or not the defining edges intersect each other. The general rules that apply to the effect of the selection point location are as follows.

Non-intersecting Edges

If a pair of edges does not intersect but does provide more than one possible location for a fillet, GAMBIT constructs the fillet edge nearest the selection point for one of the two edges. As an example, consider the two edges shown in Figure 2-38. In Figure 2-38(a), the selection points are both near the leftmost ends of the defining edges, therefore GAMBIT locates the fillet edge on the left. Similarly, in Figure 2-38(b), the selection points are near the rightmost ends of the defining edges, therefore GAMBIT locates the fillet edge on the right. In Figure 2-38(c), one of the selection points is near the left end of one edge, and the other selection point is near the right end of the other. In such cases, the final location of the fillet edge depends on the orientations and shapes of the defining edges.

Figure 2-38: Fillet location—non-intersecting edges

Intersecting Edges

When the edges that define the fillet intersect each other, the fillet location depends on the following two factors:

• The location of the selection points with respect to the edge endpoints
• The location of the selection points with respect to the point of intersection between the edges

Figure 2-39 illustrates the general effect of selection point locations for two perpendicular straight edges.

Figure 2-39: Fillet location—intersecting edges

Specifying the Fillet Radius

When you create a fillet edge, the location of the fillet depends, in part, on the fillet radius. Figure 2-40 shows the effect of fillet radius on the location of a fillet edge constructed between two curved edges. Note that both fillet edges shown in Figure 2-40 are located such that the circles that contain them are tangent to the defining edges at their points of intersection.

Figure 2-40: The effect of fillet radius on fillet edge location

If you specify a fillet radius that is either too small or too large to result in points of tangency between the defining edges, GAMBIT does not create the fillet edge.

Trimming Edges

When you create a fillet edge, GAMBIT allows you to specify that both of the defining edges are trimmed when the fillet edge is created. If you select the trim-edges option, GAMBIT deletes two of the four edge segments that exist on either side of the fillet edge endpoints. Figure 2-41 shows the difference between trimmed and untrimmed edges when creating a fillet edge according to the specifications shown in Figure 2-37.

Figure 2-41: Fillet edge—effect of trimming edges

The Effect of Selection Point on Trimmed Edges

When you specify that the defining edges are to be trimmed, GAMBIT uses the locations of the selection points to determine which edge segments to retain. As a general rule, GAMBIT retains the edge segments that contain the selection points.

Figure 2-42 shows the effect of edge trimming on two non-intersecting edges possessing shapes and orientations identical to those shown in Figure 2-38.

Figure 2-42: Fillet edge trimming—non-intersecting edges

Figure 2-43 shows the effect of edge trimming on two perpendicular intersecting edges identical to those shown in Figure 2-39.

Figure 2-43: Fillet edge trimming—intersecting edges

Using the Create Real Fillet Arc Form

To open the Create Real Fillet Arc form (see below), click the Create Real Fillet Arc command button on the Geometry/Edge subpad.

The Create Real Fillet Arc form includes the following specifications.

Create Real Edge From Vertices

The Create Real Edge From Vertices command allows you to create a NURBS edge the shape of which is defined by a set of vertices.

When you create an edge by means of the Create Real Edge >From Vertices form, GAMBIT forms the edge in the shape of a general NURBS curve of degree n. A NURBS curve of degree n is a piecewise rational polynomial function wherein the numerator and denominator are non-periodic B-splines of degree n. By default, GAMBIT employs a value of n = 3 and applies natural boundary conditions at the endpoint vertices. That is, the NURBS curve is created such that its second derivative is zero at the endpoints.

To create a NURBS edge, you must specify the following parameters:

• Two or more real vertices to be used in constructing the edge
• The method by which the edge is to be constructed

Specifying Vertices

The Create Real Edge From Vertices operation is subject to the following rules and restrictions:

• All specified vertices must be real.
• The sequence in which you specify the vertices determines the shape of the edge. (NOTE: It is possible to create more than one edge shape from a single set of vertices.)
• When GAMBIT constructs a NURBS edge, the first and last specified vertices become the endpoint vertices of the edge. Therefore, when you delete a NURBS edge and specify the Lower Geometry option on the Delete Edges form, GAMBIT deletes only the endpoint vertices when it deletes the edge.

Specifying the Curve Construction Method

GAMBIT provides two methods for constructing a NURBS edge (see Figure 2-44):

• Interpolate
• Approximate

The Interpolate method forces the edge to pass through all specified vertices. The Approximate method creates an edge that passes near to all interior vertices to within a specified tolerance. In both cases, the new edge begins and ends at the first and last specified vertices. (NOTE: The Interpolate method is equivalent to the Approximate method with zero tolerance.)

Figure 2-44: NURBS curve methods

Using the Create Real Edge From Vertices Form

To open the Create Real Edge From Vertices form (see below), click the Create Real Edge From Vertices command button on the Geometry/Edge subpad.

The Create Real Edge From Vertices form includes the following specifications.

The Revolve Vertices command allows you to create one or more real circular arc edges by revolving existing real and/or non-real vertices about a specified axis.

The Revolve Vertices command requires the following input parameters:

• One or more vertices to be revolved


• The axis of revolution


• The angle through which the vertices are revolved about the axis

When you create an edge by revolving a vertex, GAMBIT sweeps the vertex through the specified angle of revolution to create a circular arc edge (see Figure 2-45). (NOTE: If you revolve a non-real vertex, GAMBIT makes a real, in-place copy of the non-real vertex and revolves the real copy to create the edge.) The vertex specified for revolution (or real, in-place copy) constitutes the start endpoint of the created edge, and the edge sense points in the direction of revolution.

Figure 2-45: Revolve Vertices operation

You can specify any number of real and/or non-real vertices for the Revolve Vertices operation. As noted above, if you specify a non-real vertex, GAMBIT copies the virtual vertex to create a real vertex at the same location and revolves the real vertex to create a real edge. As a result, the Revolve Vertices command always creates a real edge regardless of the geometry type of the specified vertex.

To specify the axis of revolution, you must define the axis by means of the Vector Definition form. For a description of the Vector Definition form and its operation, see "Using the Vector Definition form," in Section 2.1.4.The conventions regarding the angle of revolution for the Revolve Vertices operation are identical to those described in "Rotating an Entity," in Section 2.1.4.

To open the Revolve Vertices form (see below), click the Revolve Vertices command button on the Geometry/Edge subpad.

The Revolve Vertices form includes the following specifications.

The Project Edge On Face command allows you to create a real edge that represents the projection of an existing real or virtual edge onto the surface of an existing real or virtual face.

When you execute the Project Edge On Face command, GAMBIT creates a real edge the shape of which represents the projection of the specified (projection) edge onto the surface of a specified (target) face (see Figure 2-46). The shape of the created edge follows the contours of the projection face, but the created edge is not connected to or topologically associated with the face.

Figure 2-46: Project Edge On Face-edge-project operation

If the projection of the edge in the direction of the face results in a curve that extends beyond the boundaries of the face, GAMBIT truncates the created edge at the face boundaries. In addition, if the projection of the edge crosses the face boundaries more than once, GAMBIT creates multiple edges from the Project Edge On Face operation (see Figure 2-47).

Figure 2-47: Project Edge On Face-creation of multiple edges

The Project Edge On Face command includes the following specifications:

• The edge to be projected (projection edge)
• A face that constitutes the projection surface (target face)
• A vector that defines the direction of projection

The projection edge and target face can represent any combination of real and/or virtual entities. For example, it is possible to project a real edge onto a virtual face or vice versa. Regardless of the geometry type(s) of the projection edge and face, however, the Project Edge On Face operation creates a real edge on the projection surface.

To specify the direction of projection, you must define the direction vector by means of the Vector Definition form. The Vector Definition form allows you to define the projection vector by means of either an existing edge, two points, two existing vertices, or a direction relative to any currently defined coordinate system. For a description of the Vector Definition form and its use, see "Using the Vector Definition Form" in Section 2.1.4.

Using the Project Edge On Face Form

To open the Project Edge On Face form (see below), click the Project Edge On Face command button on the Geometry/Face subpad.

The Project Edge On Face form includes the following specifications.

2.3.2 Connect/Disconnect Edges

The Connect/Disconnect Edges command button allows you to perform the following operations.

The following sections describe the procedures and specifications required to execute the operations listed above.

NOTE: The Specify Color Mode command button on the Graphics/Windows Control toolpad allows you to display model colors based on entity connectivity rather than topology. For a description of the use of the Specify Color Mode command button, see the GAMBIT User's Guide, Section 3.4.2.

Connect Edges

The Connect Edges command allows you to connect two or more edges. (NOTE: If you connect two or more meshed edges, and the numbers of mesh nodes on each edge are identical to each other, GAMBIT preserves the meshes when connecting the edges.)

To connect edges, you must specify the following parameters:

• Two or more edges to be connected
• The connection type

Specifying the Edges to Be Connected

The edges to be connected can be real or virtual, but they are subject to certain restrictions imposed by the connection type (see below).

Specifying the Connection Type

GAMBIT allows the following types of edge-connect operations:

• Real
• Virtual (Forced)
• Virtual (Tolerance)
• Real and Virtual (Tolerance)

The following sections describe the basic features of each connection type.

Specifying a Real Connection

The Real option allows you to connect coincident real edges--that is, two or more real edges that possess identical orientations and the endpoint vertices of which are coincident to within a global tolerance value of 10^-6. When you connect real edges and specify the Real option, GAMBIT deletes all but one of the specified edges and connects the remaining real edge to any and all faces to which the deleted edges were connected.

Specifying a Virtual (Forced) Connection

The Virtual (Forced) option allows you to connect real or virtual edges, regardless of their proximity to each other. When you connect edges and specify the Virtual (Forced) option, GAMBIT replaces the specified edges with a virtual edge. If a specified edge constitutes part of a face, GAMBIT overlays the face with a virtual face and shapes the virtual face according to the position of the new virtual edge. If the face is connected to a volume, GAMBIT overlays the volume with a virtual volume.

Specifying a Virtual (Tolerance) Connection

The Virtual (Tolerance) option allows you to specify that only those real and/or virtual edges that are near to each other to within a specified tolerance are connected. There are two ways to express the tolerance value:

• Tolerance
• Shortest Edge%

The Tolerance specification represents the tolerance value as expressed in absolute distance units. The Shortest Edge% specification represents the tolerance value expressed as a percentage of the length of the shortest edge in the model.

If you specify the Virtual (Tolerance) option, you can also specify the T-Junctions option. The T-Junctions option allows the creation of T-junctions during the edge connect operation (see "Specifying the T-Junctions Option," below).

Specifying a Real and Virtual (Tolerance) Connection

When you specify the Real and Virtual (Tolerance) option, GAMBIT performs the following two operations in sequence:

1. Real connect operations for edges that are coincident to within the global tolerance value
2. Virtual (Tolerance) connect operations (including the T-Junctions option) for unconnected, specified edges that are near to each other to within the user-specified tolerance

When you employ the Virtual (Tolerance) or Real and Virtual (Tolerance) option, GAMBIT allows you to select the T-Junctions option. If you specify the T-Junctions option, GAMBIT performs virtual T-connect operations where appropriate to connect edges the endpoints of which are near to the virtual edge to within a specified tolerance. (For a description of virtual T-connect operations, see Appendix A of this guide.)

As an example of the use of the T-Junctions option, consider the edges shown in Figure 2-48(a). If you specify the T-Junctions option when performing a Virtual (Tolerance) or Real and Virtual (Tolerance) connection operation involving edge.1 and edge.2, GAMBIT splits edge.2 and creates a T-connection at v_vertex.5 (see Figure 2-48(b)).

Figure 2-48: Connecting edges—T-Junctions option

Preserving the First Edge Shape

If you connect a set of edges using a Virtual(Forced), Virtual (Tolerance), or Real and Virtual (Tolerance) operation, you can determine the shape of the resulting edge by means of the Preserve first edge shape option. When you select the Preserve first edge shape option, the edge that results from the connect operation retains the shape of the first edge listed in the Edges list. If you do not select the Preserve first edge shape option when connecting the edges, the shape and location of the edge that results from the connect operation represents an average of the shapes and locations of the edges to be connected.

Preserving the Split Edge Shape in T-Connection Operations

When you perform a T-connection operation, GAMBIT allows you to specify the Preserve split-edge shape option. When you select the Preserve split-edge shape option, GAMBIT retains the shape of the edge to be split during the T-connection operation.

As an example of the effect of the Preserve split-edge shape option, consider the two edges (edge.1 and edge.2) shown in Figure-2-49(a).

• If you do not specify the Preserve split-edge shape option, GAMBIT creates the edges shown in Figure 2-49(b).
• If you do specify the Preserve split-edge shape option, GAMBIT retains the shape of edge.1 during the T-connect operation and creates the edges shown in Figure 2-49(c).

Figure 2-49: Effect of the Preserve split-edge shape option

NOTE: The Preserve split-edge shape option takes precedence over the Preserve first edge shape option.

Using the Connect Edges Form

To open the Connect Edges form (see below), click the Connect command button on the Geometry/Edge subpad.

The Connect Edges form includes the following specifications.

Disconnect About Real Edge

The Disconnect About Real Edge command allows you to disconnect topological entities that share a common real edge.

When you disconnect faces or volumes about a common real edge, GAMBIT creates new edges for all but one of the entities to which the specified edge is connected. For example, if the specified edge is shared by three faces, GAMBIT creates two new edges that are coincident with the specified edge and connects them to two of the three faces. The original edge is connected to the remaining face.

Specifying Endpoint Vertex Options

GAMBIT provides the following options with respect to the treatment of endpoint vertices for the disconnected edge:

• Edge + Vertices
• Edge Only
• Edge + Select Vertex

The following table describes the effects associated with each option.

As an example of the difference between the options described above, consider the configuration shown in Figure 2-50, in which two faces are connected by means of a common edge (edge.4) the endpoint vertices of which (vertex.3 and vertex.4) are also common to both faces.

Figure 2-50: Two faces sharing a common edge

If you disconnect the faces about edge.4, GAMBIT creates a new edge that is coincident with edge.4 and connects it to one of the two faces—for example, face.2. The original edge (edge.4) remains connected to the other face (face.1).

The manner in which GAMBIT treats the endpoint vertices varies according to the option type as follows.

Using the Disconnect About Real Edge Form

To open the Disconnect About Real Edge form (see below), click the Disconnect About Real Edge command button on the Geometry/Edge subpad.

The Disconnect About Real Edge form includes the following specifications.

2.3.3 Modify Edge Color/Label

The Modify Edge Color/Label command button allows you to perform two operations.

The following sections describe the procedures and specifications required to execute the operations listed above.

Modify Edge Color

The Modify Edge Color command allows you to change the displayed color of the geometry and/or mesh nodes associated with one or more edges.

Using the Modify Edge Color Form

To open the Modify Edge Color form (see below), click the Modify Color command button on the Geometry/Edge subpad.

The Modify Edge Color form includes the following specifications.

For specific instructions on setting the Geometry or Mesh colors, see Section 2.2.4.

Modify Edge Label

The Modify Edge Label command allows you to change the label associated with any edge.

Using the Modify Edge Label Form

To open the Modify Edge Label form (see below), click the Modify Label command button on the Geometry/Edge subpad.

The Modify Edge Label form includes the following specifications.

2.3.4 Move/Copy/Align Edges

The Move/Copy/Align Edges command button allows you to perform two operations.

The following sections describe the procedures and specifications required to execute the operations listed above.

Move/Copy Edges

The Move/Copy Edges command allows you to reposition and/or reorient one or more edges or to create copies of edges. For a general description of the procedures and specifications required to move and/or copy entities, see "Moving an Entity" and "Copying an Entity," respectively, in Section 2.1.4.

Using the Move/Copy Edges Form

To open the Move/Copy Edges form (see below), click the Move/Copy command button on the Geometry/Edge subpad.

For a complete description of the specifications available on the Move/Copy Edges form, see "Using Move/Copy Forms" in Section 2.1.4.

Align Edges

The Align Edges command allows you to reposition and/or reorient an edge so that it coincides with another edge or is aligned with a vector drawn between two vertices. You cannot plane-align an edge, because edges are one-dimensional entities. (For a general description of the procedure and specifications required to align an entity, see "Aligning an Entity" in Section 2.1.4, above.)

Using the Align Edges Form

To open the Align Edges form (see below), click the Align command button on the Geometry/Edge subpad.

For a complete description of the specifications available on the Align Edges form, see "Using Align Forms" in Section 2.1.4.

2.3.5 Split/Merge Edges

The Split/Merge Edges command button allows you to perform the following operations.

The following sections describe the procedures and specifications required to execute the operations listed above.

Split Edge

The Split Edge command allows you to split an existing edge into two real or virtual edges. (NOTE: If you split an edge that is linked to one or more edges, GAMBIT splits every edge in the set of linked edges in addition to the specified edge.)

The Split Edge command includes the following input parameters:

• The edge to be split
• The split type
• The split tool

GAMBIT allows you to split real or virtual edges but places the following restrictions on the type of edges that can be created from the split:

• If you split a real edge, you can create two real or virtual edges.
• If you split a non-real edge, you must create two virtual edges.

There are three types of edge split operations:

• Real connected
• Real disconnected
• Virtual connected

When you split a real edge, you must specify whether the two edges that replace it are real or virtual. If you replace a real edge with two real edges, you must specify whether the resulting edges are connected (Real connected) or disconnected (Real disconnected) at the split point. If you replace an edge with two virtual edges, GAMBIT connects the edges at the split point by means of a virtual vertex (Virtual connected).

NOTE: GAMBIT does not allow you to employ the Real disconnected option when splitting edges that are associated with higher-topology face or volume entities.

Specifying the Split Tool

When you split an edge, you must specify the location at which the edge is to be split. To do so, you must designate a split tool-that is, the means of locating the split point. There are three types of split tools:

• Point
• Vertex
• Edge

If you split a real or virtual edge using a point as the split tool, you must specify a U Value parameter that identifies the location of the point on the edge. The U Value parameter represents the fraction of total edge length and is equivalent to the u value used when creating a vertex on an edge. (For a detailed description of the u value, see "Create Vertex On Edge," in Section 2.2.1.)

When you specify the Point option and pick the edge from the graphics window using the mouse, GAMBIT highlights the edge and shows the location of the split point. You can slide the split point along the edge until you release either the Shift key or the left mouse button-at which time GAMBIT fixes the position of the split point. To change the position of the split point, either re-pick (Shift-middle-click) the edge and move the point to another location, or input the desired u value in the U Value text box on the Split Edge form.

• To perform a real split operation, you must use a real vertex as the split tool.
• To perform a virtual split operation, you can use either a real or non-real vertex as the split tool.

Specifying the Split-edge Position

When you split an edge with a split-tool vertex that is not coincident with the edge, GAMBIT allows you to determine the final configuration of the edge shape and vertex location by means of the Split-edge Position specification. The Split-edge Position specification includes three options (see Figure 2-51):

• Interpolate
• Preserve vertex location
• Preserve edge shape

Figure 2-51: Split-edge Position options

Specifying an Edge as the Split Tool

The Edge option allows you to split an edge at a location defined by the point(s) of intersection (or closest approach) between the specified (target) edge and an edge that serves as a split tool. If the target edge intersects or approaches the split-tool edge at more than one location, GAMBIT splits the target edge at each location.

GAMBIT provides three suboptions for the Edge split tool option:

• Retain
• Bidirectional
• Tolerance

The Retain suboption specifies that the split-tool edge is not deleted upon the completion of the Split Edge operation. (NOTE: If you do not select the Retain option, GAMBIT deletes the split-tool edge upon completion of the operation.)

The Bidirectional suboption splits both the target edge and the split-tool edge at their point(s) of intersection or closest approach. The target edge and split-tool edge are joined together at the split location. That is, the vertices created at the edge split locations are connected after the split operation is complete.

The Tolerance suboption performs the split operation at points of near intersection between the split-tool edge and the target edge. For the purposes of the edge-split operation, edges are considered to nearly intersect if they approach each other to within the user-specified Tolerance value on the Split Edge form.

Using the Split Edge Form

To open the Split Edge form (see below), click the Split command button on the Geometry/Edge subpad.

The Split Edge form includes the following specifications.

The lower section of the Split Edge form allows you to specify parameters related to the Split With option selected (Point, Vertex, or Edge).

Point Option

When you specify the Split With:Vertex option, the middle section of the Split Edge form appears as shown below.

When you specify the Split With:Edge option, the middle section of the Split Edge form appears as shown below.

Merge Edges (Virtual)

The Merge Edges (Virtual) command allows you to merge two or more real and/or virtual edges into a single virtual edge. (NOTE: If you merge edges that possess identical boundary-zone type specifications, GAMBIT assigns the specification to the edge that results from the merge operation.)

To merge edges by means of the Merge Edges (Virtual) command, you must specify the following parameters:

• The set of edges to be merged
• The merge type

Specifying the Edges to Be Merged

GAMBIT allows you to merge sets of two or more real and virtual edges but applies the following rules with respect to the set of edges to be merged (see Figure 2-52):

• Each edge in the set must be connected at one or both of its endpoints to another edge in the set
• None of the connected endpoints are allowed to be connected to more than two edges

Figure 2-52: GAMBIT edge-merging rules

Specifying the Merge Type

When you merge edges, you must specify the merge type. There are two types of edge-merging operations:

• Virtual (Forced)
• Virtual (Tolerance)

When you specify a Virtual (Forced) merge, GAMBIT merges all of the edges in the specified set, regardless of their respective lengths or angles with respect to each other. When you specify a Virtual (Tolerance) merge, GAMBIT performs the merge operation only if all edges in the set meet specified tolerance criteria.

Specifying Tolerance Criteria