Creating Blend Surfaces

This task explains how to generate a blend surface between surface edges or curves on surfaces.

The connecting edges may consist of several edges or curves. If no support is used, the result will be created with G0 continuity between blend and connecting surfaces. Additionally, curves or edges can be projected onto surfaces. In this case, the blend surface will be matched to the projected surface curves. If a support surface is selected, the transition quality may be position, tangent, curvature, or torsion continuity (G0-G3).

The beginning or end cross tangent of the blend surface can be aligned collinear to the beginning or end cross tangent of the adjacent surfaces. For the blend type 'Global', the iso-curves of the blend surface can be aligned by a moving frame. Depending on the transition quality of the adjacent surfaces between each other, the blend surfaces are matched to each other meeting tangent or curvature continuity. The segmentation of the blend surface depends on the segmentation of the edge selected first.

The start and end points of the connecting edges can be re-defined subsequently. Move the point manipulators at the start and end along the selected edges (see also Manipulators and contextual commands).
 

Creation of a blend surface (blend type 'Global')
In the following example, a blend surface will be created, one of the connecting edges consisting of several elements.
Open the Blend.CATPart document.

  1. Click the Blend Surface icon .
    The 'Blend Surface' dialog opens.
 
 

  1. Select an edge of Surface 1 as first connecting edge ('Side 1').
    Surface 1 is automatically selected as Support 1.

  2. Select edges of Surfaces 2 to 6 as second connecting edge ('Side 2').
    The surfaces 2 to 6 are automatically selected as Support 2.
 
 

  1. Click 'Apply'.
    The blend surface is calculated with G1 continuity.
 

  1. Move the point manipulator at start 'S' of 'Edge 1' along the edge.
    The connecting edge will be shortened, and the blend surface will be re-calculated accordingly.
 

  1. Click 'OK'.
    The blend surface will be created as a feature.
You can define the following options:
 
  • Blend type

    Single Creation of a blend surface with the options of the 'Options' and 'Coupling' tabs. 2nd Curve is automatically used as moving frame.
    Global The 'Moving Frame' tab is additionally available for aligning the iso-curves of the blend surface. The 'Coupling' tab is only available, if 2nd Curve is selected as moving frame type.
  • Tangent length: The control point distribution may be controlled via the inner tension manipulators. For computing the cross tangent lengths, the following modes are available:
    • Auto : Automatic computation of the tangent lengths.
    • Constant : Use of a constant length as master for all other tangent lengths.
    • Ratio : Adaptation of the second and all following control point segments (depending on the set continuity) to the first control point segment. This will be done by assuming that the length ratio between the first and the second control point segment is the same as the length ration between the second and the third one, and so on.
 
  • Side 1/Side 2:
    • Edge 1/2: Select 1st and 2nd connecting edge. By default, the edge selected first will be used as spine curve for defining the moving frame. You can, however, select any other curve as spine curve (see 'Moving Frame' tab).
      If an offset or iso-parametric curve is selected as input element, certain parameters of these curves can be modified via manipulators directly in the Blend Surface command (see Curve offset and isoparametric curve as input features).
    • Support 1/2: Blend patches into the inside of a surface can be created using this option. For this, existing curves from Edge 1 and Edge 2 are projected onto the supports in patch normal direction to generate surface curves. These curves are then used as connecting edges.
       
      Note:
      The creation of a surface blend into the inside of a surface requires the manual creation of an iso-curve (see Creating Associative Isoparametric Curves).
    If no support is used, the result will be created with G0 continuity. If a support surface is selected, the Continuity can be G0, G1, G2, or G3.
  'Options' tab
  For both Side 1 and Side 2 you can define the following options:
  • Continuity
    • G0, G1, G2, G3 (G3 in ISD only): Controls the transition quality between blend and adjacent surfaces for the two connecting edges.
      These options are only available if Support is activated.
      Continuity = G0

      		 Continuity = G1

      		 Continuity = G2

      		 Continuity = G3
 
bullet Alignment: These options can be used for controlling the distribution of the control points of the blend surface.
For the modes 'Edges Start/End', 'All Edges' and 'Auto', the start and end tangents can be modified via the manipulator. When using these options, the inner cross tangents will be propagated.
As the direction of the cross tangents is fixed when using the alignment modes 'Standard' and 'Linear', the start and end tangents cannot be modified.
  • Standard : Corresponding points on all blend edges at 'Side 1' and 'Side 2' are connected via a line. The direction of this line is projected onto the tangent plane at the specific position of the edge. This projected direction is used as a U tangent of the blend surface at the specific edge point.
    Edge continuity cannot be defined.
    Alignment: Standard
  • Edges Start/End : Existing cross edges of the support at 'Side 1' and/or 'Side 2' are continued. The direction of the cross tangents within the specific edge will be spread over the whole edge (with respect to isolines).
    Edge continuity can be defined.
    Alignment: Edges Start/End Alignment: Edges Start/End – Distribution of tangents


  • All Edges : All edges of the support crossing the blend edge are continued. The cross tangent directions will be propagated over the edge (with respect to isolines) until a segment border is reached.
    Edge continuity cannot be defined.
    Alignment: All Edges Alignment: All Edges – Distribution of tangents


  • Linear: Cross isolines will be continued if the edge is adaptable the U or V isoline of the support.
    Edge continuity cannot be defined.
    Alignment: Linear
  • Auto: Automatic distribution of the cross tangents.
    Edge continuity can be defined.
    Alignment: Auto
 
bullet Edge continuity (G3 in ISD only): Aligns the beginning and end cross tangents at the edges of the blend and adjacent surfaces.
Edge continuity can be defined for the 'Alignment' modes EdgesStart/End and Auto.
  • Start and End continuity OFF: The beginning and/or end cross tangents will be aligned coplanar only, i. e. there may be an angle between the cross tangents of the blend and adjacent surfaces.
  • Start or End continuity ON: The beginning or end cross tangent of the blend surface will be aligned collinear to the beginning or end cross tangent of the adjacent surfaces.
  • Start and End continuity ON: All cross tangents of the blend surface will be aligned collinear to the cross tangents of the adjacent surfaces.
 
  • Trim Type: The underlying support surfaces can be trimmed at the connecting edges of the created blend surface.
     
    • Creation of approximated surfaces.
    • Creation of trimmed surfaces.
Result without trimming Result with trimming
a) with option

b) with option
  See  'Moving Frame' tab (in ISD only)
  'Coupling' tab
 

The Coupling tab is only available in conjunction with the option 2n Curve on the Moving Frame tab.
It is used to influence the moving frame in order to obtain an optimal result independent from the structure of the original geometry.

  • Coupling: The coupling functionality can be switched on and off.
    • OFF: If edge 1 and 2 have the same number of segments, the segmentation is decisive for the Blend Surface calculation, i. e. the segment boundaries will be taken from the original geometry.

       

      If the segmentation is unequal, the total arc length of the edges will be used for the segmentation calculation.
    • ON: The segmentation is calculated according to the arc length. The arc lengths of edge 1 and 2 are determined and on edge 2, the segment boundaries are inserted at the points whose distance value from the start and end points of edge 2 have the same ratio to the total arc length as on edge 1.
      If no coupling is defined, the total arc length will be used for the segmentation calculation.

       

      If one or several couplings are defined, for the segmentation calculation will be used the arc lengths of the sections defined by the coupling vertices.



      Note: The coupling vertices can be selected arbitrarily. However, they should lie on the edges of the original geometry and be selected in succession of their positions.
  • Force G1 in edge direction :
    • OFF: The continuity of the selected supports and the coupling conditions imposed by the user will be inherited for the result surface.
    • ON: The surface blend attempts to override and remove any internal G1 patch discontinuities.
  • Display: The defined couplings are displayed as lines.
  • Coupling table:
    • X: A defined coupling can be deleted.
    • Coupling: Display of the defined couplings.
    • Edge 1/2: Display of the selected coupling vertices on edge 1 and 2.
      The selection can be changed after clicking onto a coupling vertex in the table.
  • Coupling Vertex: For the current coupling can be either selected existing points as coupling vertex or new points created via the contextual command.
See 'Approximation' tab
  In addition to the general options for the approximation, the following options are available on this tab:
  • Blend Surface Segmentation
    • Sep. Spine: The segmentation is taken from the curve selected under Separate Spine on the 'Moving Frame' tab.
    • Sep. Spine and Side 1: The segmentation is taken from the curve selected under Separate Spine on the 'Moving Frame' tab and Side 1.
    • Sep. Spine and Side 2: The segmentation is taken from the curve selected under Separate Spine on the 'Moving Frame' tab and Side 2.
    • All: The segmentation is taken from the curve selected under Separate Spine on the 'Moving Frame' tab and Side 1 and 2.
  See 'Output' tab
Click 'More Info' to display deviations and output results.
 
  • Display: Depending on the options selected, values are displayed in the graphics area.
  • Deviation (Side 1 and 2): Maximum deviation between the edges of the created surface and the connecting edges.
    • Check buttons: The maximum deviation can be displayed in the graphics for each side.
    • G0 - G3 (G3 in ISD only): Display of the maximum deviation in the dialog box.
  • U+V: Display of the UV vectors of curves and surfaces.
  • MFT: Display of the local coordinate system for the moving frame type.
  • Dynamic, Static, None: See Apply Modes
  • Output Result: See Output Result
 

Curve offset and isoparametric curve as input features

If an offset or isoparametric curve is selected as input element, certain parameters of these curves can be modified via manipulators directly in the command Blend Surface.

For offset curves, length manipulators are displayed at the curve’s start and end. For isoparametric curves, a point manipulator is shown directly on the support surface.

Explicit creation of input features

The following prerequisites apply to the use of these features as input elements:

  • The input features must have been created with the commands ISD Curve Offset or GSD Isoparametic Curve. 
  • The support surface for creating the blend surface must be the same used for the input feature. The size of the support surfaces must be identical.
  • The offset curve must have been created with Alignment: On Surface. The option Both Sides must be switched off.
Implicit creation of an isoparametric curve

Instead of creating an external isoparametric curve it is also possible to internally create such a curve inside the Blend Surface command which simplifies the workflow drastically.

After starting the function command, the selection of the edges and support elements will be activated. When the mouse pointer is positioned on top of a surface, the preview of an isoparametric curve is shown at the pointer position. By pressing the left mouse button, the curve will be created. This curve will then be added to the list of edges, and the selected surface to the list of supports.

The border curve of the support which is closest to the picked position will be taken to align the isoparametric curve (U or V direction).
 

Manipulators at input features

The following manipulators are available:
  Length manipulators for controlling the curve offset distance Point manipulators for controlling the isoparametric curve position
 

Manipulators and contextual commands

The created blend surface can subsequently be modified by point and continuity manipulators. At the manipulator handles, you can activate with the right mouse button contextual commands with the following options:
 
  • Edit : Modifies the relative manipulator position via dialog box 'Tuner'.
  • Keep this point: Creates a point at the current position of the manipulator.
  • Swap UV: The parameter direction of the curve is swapped.
  • Snap to border : The curve is snapped to the closer border in that parameter direction. 
  • G0, G1, G2, G3 : Sets the specified continuity.