About Creating Midsurfaces

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 other side is automatically selected and a midsurface is generated 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:

• The offset ratio is relative to the order of faces and the midsurface is created at an offset from the main face.
  Note: For illustration purpose, the main side is color coded as red and the other side as green in the following images.

  Face pair	Midsurface with offset ratio= 0.3	Offset ratio= 0.5	Offset ratio= 0.7

• The shape of the midsurface depends on the order of face selection.
  • For face pairs P1-Q and P2-Q, the midsurface is continuous.
  • For face pairs P-Q1 and P-Q2, the midsurface is non-continuous.

  Face pairs	Midsurface

  Note: A face can belong to more than one pair.

When you are creating midsurfaces by offsetting faces, the minimum and maximum offset values are computed after you click Compute Offset. These values are displayed in the work area by the Min and Max indicators.

This representation is visible throughout the command's lifecycle. It disappears in the following cases:

• Modification in the list of faces to offset
• Change of the input support
• Change of creation mode.

The Compute Offset command displays the average offset value in the Offset box. This value is computed by calculating the average value between the detected pairs’ offset value, weighted by the smallest face area of each pair. The offset parameter is then measured by this value.

When a sub element contains several topological faces to form a pair, the Midsurface 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 Midsurface command automatically computes the pairing and accepts two pairs of cells.

In the Face Pairs area, you can edit various parameters of the selected face pairs from the context menu, as listed below:

• Remove selected pairs
• Swap selected pairs
• Replace other side face: Replaces the second face of the pair with the first one.
• Propagate selected pair: Creates a pair of adjacent faces automatically by propagation.
  Notes:

  • 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.
• Find invalid pairs: Finds and selects existing face pairs that are invalid for midsurface 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.

  You can then remove the invalid pairs by selecting Remove selected pairs.