Creating Mid Surfaces

This task shows how to create a mid-surface from a pair of faces that are offset from each other. This functionality is useful for simulating medial surface representation of thin solids.

Following topics are discussed: 
 

Face Pair Selection 
  You can select a pair of faces belonging to a solid feature, volume or body. Within a pair, faces are ordered as the 'main side' and the 'other side' and are always offset from each other (that is, they do not share an edge or vertex). After you select the main side, the Mid Surface command automatically selects the other side and generates a mid-surface according to the specified offset ratio. If you select a face whose other side cannot be located in the 3D geometry, a message appears informing about the selection. 
  Some considerations for selecting a pair of faces:
 
  • Offset ratio is relative to the ordering of faces and the mid-surface is created at an offset from the selected face.
    For illustration purpose, main side is color coded as red and the other side as green in the following images.   
 
Face Pair Mid Surface with Offset ratio=0.7 Offset ratio=0.5 Offset ratio=0.3
 
  • The shape of the mid-surface depends on the order of face selection.
    • For face pairs P1-Q and P2-Q, the mid-surface is non-continuous.
    • For face pairs P-Q1 and P-Q2, the mid-surface is continuous.
 
A face can belong to more than one pair.  
 

Multi-cell Management
  When a sub-element contains several topological faces to form a pair, the Mid Surface command performs automatic pairing of faces. A pair is accepted only if the number of paired faces is equal to the number of faces of the sub-element with the lowest number of faces.
 
  Above illustration shows a pair with two multi-cell surfaces. The first one has three faces (blue) and the second one has two faces (green). The Mid Surface command automatically computes the pairing and accepts two pairs of cells.
 

Contextual Edition
   
  In the Face Pairs area, you can edit various parameters of the selected face pairs from the contextual menu.
   
 
  • Select Replace other side face to replace the second face of the pair with the first one.
  • Select Propagate selected pair to automatically create a pair of adjacent faces by propagation.
    • The face pair is propagated radially according to the continuity criterion. A pair is kept if its offset value is within the specified threshold.
       
    • In some cases, propagation might be done on the other side of the selected volume, resulting in swapping of some face pairs. You will need to swap back these pairs manually.
  • Select Find invalid pairs to find and select existing face pairs that are invalid for mid-surface creation, such as:
    • Pairs whose input topology is not part of the selected input support
    • Incomplete pairs
    • Pairs with input face in error (lost B-REP)
    • Duplicate pairs
    • Pairs whose input faces are not offset from each other.
     
  After finding the invalid pairs, you can remove them by selecting Remove selected pairs.
The Find invalid pairs option is available only when the feature is in error.
 
All of the contextual selection and edition options are also available in the 3D area. You can right-click the face pair to select these options.  
   
 

Create a Mid Surface 
 

  1. Click Mid Surface in the Surfaces toolbar (Offsets sub toolbar).

    The Mid Surface Definition dialog box appears.
     

  2. In the Support box, select the solid feature, volume feature or body that contains a pair of faces.

  3. In the Creation Mode list, select Faces Pairs and select one or more pairs of faces from which the mid-surface will be extracted.

  4. The Face Pairs area becomes available only after you select a feature in the Support box.

    In the Offset ratio box, type the offset value or use the arrows to change the value.

    The value of the offset ratio should be between 0 and 1.

  5. Click OK to generate the mid-surface.
  Create a Mid Surface by Offsetting Faces (Non-Associative Offset) 
  You can create a mid surface from a set of faces (or extract features) and an offset. This is useful for complex models which may not contain parallel faces to form a pair.
The created mid surface is automatically extrapolated and relimited to the selected support.
  1. Click Mid Surface in the Surfaces toolbar.
    The Mid Surface Definition dialog box appears.
     
  2. In the Support box, select a solid feature, volume feature or a body.
  3. In the Creation Mode list, select Faces to Offset.
  4. In the Faces To Offset box, select either the faces to offset or the extract features, as required.
    • Face selection: Select one or more faces in the work area.
      Note: You can click to see a list of selected faces. You can then click Replace or Remove to modify the list, if needed.
    • Extract feature selection: Perform one of the following actions from the context menu:
    • Select Insert > Operations > Extract. The Extract Definition dialog box appears. For more information, refer to Extracting Geometry.
    • Select Insert > Operations > Multiple Extract. For more information, refer to Extracting Sub-Elements.
  • You can access commands through the context menu in the 3D by right-clicking anywhere in the empty space.
  1. In the Computation Mode list, select Manual.
  2. Click Compute Offset beside the Offset box.
    • Offset faces from the selected faces are detected.
    • Minimum and maximum offset values are computed and displayed in the work area with the Min and Max indicators.
       
    • Average offset value is computed and displayed in the Offset box.
  3. Optional: Click Min or Max indicator in the work area to evaluate the offset parameter.
    The value in the Offset box is refreshed accordingly. 
  4. Optional: Right-click Min or Max indicator in the work area and select Remove Min or Remove Max, respectively from the context menu.
    The Min or Max indicators are removed from the current face. They re-appear on the next sequential face with the recomputed Min or Max value.
    Note: The indicators do not appear after they are removed from the last face.
  5. In the Offset ratio box, type the offset value or use the arrows to change the value.
    Note: Value of offset ratio must be between 0 and 1.
  6. Click OK.
    The mid surface is generated.

  Create a Mid Surface by Offsetting Faces (Associative Offset)

You can create a mid surface from a set of faces (or extract features) and an associative offset.

  1. In the Support box, select a solid feature, volume feature or a body.
  2. In the Creation Mode list, select Faces to Offset.
  3. In the Faces To Offset box, select either the faces to offset or the extract features, as required.
  4. In the Computation Mode list, select Automatic. This mode uses 5 rays to compute the offset faces: 4 rays projected from each corner of the selected face and one from the center.
  5. In the Computation Method list, select one of the following options:
    • Maximum: takes only the maximum applicable offset faces for mid surface computation.
    • Minimum: to takes only the minimum applicable offset faces for mid surface computation.
    • Average: to takes the average of applicable offset faces for mid surface computation.
  6. Specify a Thickness Threshold. The thickness threshold is a length parameter. It represents the maximum distance from the selected faces under which the offset faces are considered for computation.
     
  7. In the Offset ratio box, type the offset value or use the arrows to change the value.
    The value of the offset ratio must be between 0 and 1. 
  8. Click OK.
    The mid surface is generated.

  9. Vary the Thickness Threshold and click OK.

     The mid surface is generated at another position.
    Be aware that only 5 rays (4 corner + 1 center) are used for the calculation of the offset faces. So, in the figure below, offset faces at 40, 60 and 70 are recognized while offset faces at 90 and 120 are ignored. 
       
  Create a Mid Surface by a Thickness Threshold (Automatic Mid Surface Extraction)
  You can create a mid surface according to a thickness threshold.
  1. In the Support box, select a solid feature, volume feature or a body.
  2. In the Creation Mode list, select Automatic.
  3. Clear the Automatic Thickness Threshold check box and specify a Thickness Threshold. This length parameter acts as maximum distance for the selection of pairs of faces to create the mid surface.
    You can select the Automatic Thickness Threshold check box to automatically compute the thickness threshold. The computed value appears in the Thickness Threshold field.
  4. Click OK.
    The mid surface is generated.

  5. Vary the Thickness Threshold and click OK.

Manage Ribs
  You can manage the mid-surface creation of ribs using an existing connex mid-surface.
 

  1. Perform above steps 1 to 4 to define the main shell mid-surface.

    The Mid Surface Definition dialog box appears.

  2. Select the Create a new rib mid surface after OK check box.

  3. Click OK.

    The Mid Surface Definition dialog box appears again with Limiting mid surface box containing the main shell mid-surface.

  4. In the Face Pairs area, select a pair of faces from the rib.

    During the mid-surface creation of ribs, offset ratio is set to 0.5 by default. 

  5. Click OK .

    The rib mid-surface is generated with the main shell mid-surface acting as a relimiter.