"Flat index" for vtkDataObjectTree's iterators

Development
1

Hi everyone,

It’s my understanding that for composite objects the “flat index” refers to the “subdata” inside the composite, and that this index is 0-based as usual. Therefore, it looks like there might be a bug with vtkDataObjectTree’s iterator:

from vtkmodules.vtkCommonDataModel import vtkMultiBlockDataSet
from vtkmodules.vtkFiltersSources import vtkSphereSource


def main():
    root = vtkMultiBlockDataSet()

    leaf1 = vtkSphereSource()
    leaf1.SetCenter(0, 0, 0)
    leaf1.Update()
    root.SetBlock(0, leaf1.GetOutput())

    assert root.GetNumberOfBlocks() == 1
    # This next assertion fails, because GetCurrentFlatIndex() is returning 1
    assert root.NewIterator().GetCurrentFlatIndex() == 0


if __name__ == '__main__':
    main()

This is using VTK 9.1.0 from conda-forge on Windows. It looks like maybe the iterator’s flat index is being initialized to 0 and then immediately incremented to 1. I only did a cursory inspection of the code though.

Thanks!

2

It’s the index of the node in pre-order traversal of the tree. For example:

      A
    /   \
  B       C
 / \     /|\
D   E   F G H

For this tree, the pre-order traversal yeilds A, B, D, E, C, F, G, H, and their indices are 0, 1, 2,... correspondingly.

Since 0 identifies the root note, the and the iterator skips the root node hence it simply skips index 0.

3

Hi, thanks for the quick reply. What you say makes sense from the point of view of the tree, but then it sort of looks to me like the API is a bit inconsistent. Unless I’m misunderstanding the purpose of the flat index, I would expect this to work:

    assert root.GetNumberOfBlocks() == 1
    first = root.NewIterator().GetCurrentFlatIndex()
    assert root.GetBlock(first) is not None

… but since the index is effectively “1-based” the call does indeed return None.

For a bit more context, I found this issue when writing some picking code that uses the flat index to identify the subdata that was picked. So it looks like I should check whether the picked prop was an instance of vtkDataObjectTree and then subtract 1 from the returned index.

4

GetBlock doesn’t use the flat index, but simply an index for the immediate children of the vtkMutliBlockDataSet. Thus, in my example tree, GetBlock(0) on A will return a vtkMultiBlockDataSet for node B; and GetBlock(1) returns vtkMutiBlockDataSet for block C.

5

GetBlock doesn’t use the flat index

Thanks, I think that’s the bit I was missing. Do you know of a way that I can use this flat index to retrieve the “subitem”? The docs for vtkCompositeDataIterator::GetCurrentFlatIndex() seem to indicate this purpose:

Flat index is an index to identify the data in a composite data structure.

I suppose I could iterate again on the composite structure until I found the “matching” iterator, but that seems like a roundabout way of doing this.

Thanks again for the help.