Introduce the implicit vtkStridedArray: a strided view on a buffer

The vtkStridedArray is a vtkImplicitArray providing a strided view on an existing memory buffer.

If you work with a multi-dimensional buffer of data, you can create a vtkStridedArray to manipulate a single dimension (that can be itself a scalar or a vector) of this buffer through the classical vtkDataArray API.

This is typically useful in the InSitu context as with the vtkConduitSource for Catalyst. In this case, we want to create vtkDataArray around existing memory with zero copy.

Let’s have this buffer

std::vector<float> localBuffer
{
      0, 1000, 2000, 3000,
      1, 1001, 2001, 3001,
      2, 1002, 2002, 3002,
      3, 1003, 2003, 3003,
      4, 1004, 2004, 3004,
      5, 1005, 2005, 3005,
      6, 1006, 2006, 3006,
      7, 1007, 2007, 3007,
      8, 1008, 2008, 3008,
      9, 1009, 2009, 3009
}

This may represent a one-component array (first column) interlaced with a 3-components array, both with 10 tuples.

We can now create a vtkStridedArray as follow:

  vtkNew<vtkStridedArray<float>> stridedArray;
  stridedArray->SetNumberOfComponents(3);
  stridedArray->SetNumberOfTuples(10);
  stridedArray->ConstructBackend(
    localBuffer.data(), /*stride*/ 3, /*offset*/ 1, /*components*/ 3);

This will provide a (zero copy) view on the last 3 columns of our initial buffer.

In the context of CatalystConduit, with the same buffer we can write the following:

  fields["vectorArray/values/x"].set_external(
    localBuffer.data(), nbPts, /*offset=*/sizeof(float), stride * sizeof(float));
  fields["vectorArray/values/y"].set_external(
    localBuffer.data(), nbPts, /*offset=*/sizeof(float) * 2, stride * sizeof(float));
  fields["vectorArray/values/z"].set_external(
    localBuffer.data(), nbPts, /*offset=*/sizeof(float) * 3, stride * sizeof(float));

The conduit source will then provide a vtkStridedArray as in previous example.