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Improvements related to Polyhedra's cell storage

Mickael PHILIT requested to merge MicK7/vtk:vtkCellArray_in_unstructuredgrid into master

Storing faces as vtkCellArray

When storing face information for polyhedral cells, vtkUnstructuredGrid uses a vtkIdTypeArray in a special format that looks awfully similar to how vtkCellArray used maintain its internal storage. The new internal storage simply changes it to using vtkCellArray instead. That will allow us to use a non-interleaved storage for faces. Together with using a vtkCellArray for FaceLocations the one-2-many relationship can be easily preserved without resorting to custom layout used by the Faces array.

So here's the proposed structure:

  • Connectivity (vtkCellArray): simply stores point ids for all points for each polyhedral element
  • Faces (vtkCellArray): each element defines a polygonal face. The indices per element directly refer to point ids.
  • FaceLocations (vtkCellArray): each element identifies a polyhedral cell. The indices per element reference face defined in the ElementFace array.

Contrast this with how this information is currently stored:

  • Connectivity (vtkCellArray): simply stores point ids for all points for each polyhedral element (same as in the new proposal)
  • (Legacy)Faces (vtkIdTypeArray): an interlaved array of the form (numCellFaces, numFace0Pts, id1, id2, id3, numFace1Pts,id1, id2, id3, ...)
  • (Legacy)FaceLocations (vtkIdTypeArray): offset array into (Legacy)Faces array indicating where the faces for a corresponding cell are stored

Backward compatibility

To ensure a nice transition to the new storage, some old API are kept. More precisely the method void SetCells(vtkUnsignedCharArray* cellTypes, vtkCellArray* cells, vtkIdTypeArray* faceLocations, vtkIdTypeArray* faces); from vtkUnstructuredGrid is deprecated but can still be used. In this case a copy of faceLocations and faces data will be done. Another case are the GetFaces and GetFaceLocations methods. They are still available but an internal cache with the old internal layout is used. It means that they should be considered as read only methods and not be relied on to change internal data of vtkUnstructuredGrid. The caching process may impact a bit performance.

The FaceConn and FaceLocs internal arrays of vtkUnstructuredGridCellIterator are no longer available as vtkIdTypeArray. This may break your subclasses if you rely on them directly and not on the exposed API of vtkUnstructuredGridCellIterator. A solution to keep using them in subclasses is to define them directly in subclasses and handle their logic.

Compatibility with Conduit

The new layout is much closer to how Conduit store its polyhedral cell information. Thus, some gain should be expected on the long run.

Initial proposal: The proposal made here https://discourse.vtk.org/t/polyhedral-cells-storing-faces-as-vtkcellarray/5656

Edited by Spiros Tsalikis

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