compute_arc_length: does not behave as expected

[pvanhauw]

compute_arc_length when applied to a slice does not provide a continuous output. It seems like the algorithm does not account for the fact that the line are not ordered.

• Is that the algorithm to blame ?
• Is there a way to re-order the polyline as a pre-processing using a function from pyvista ? I guess I can always code that myself. But I also need to carry cell_data (does not apply in the exemple below)

---

To Reproduce

#!/bin/python
import pyvista as pv

mesh = pv.Sphere()

mesh_sliced = mesh.slice(normal=[0, 1, 0])
print(mesh_sliced.n_lines)

# checking that no merging of point is required since each point is used twice per line segment
pointIndex = mesh_sliced.lines.reshape(0 + mesh_sliced.n_lines, -1)
print(pointIndex)

# compute_arc_length is computed but it sounds like the order of the lines is not re-arange to obtain an arc_length that continuously increase
arc = mesh_sliced.compute_arc_length()

p = pv.Plotter()
p.add_mesh(mesh, opacity=0.1)
p.add_mesh(arc, scalars="arc_length")
p.show()

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Screenshots

---

import pyvista as pv
print(pv.Report())

--------------------------------------------------------------------------------
  Date: Thu Dec 16 09:55:30 2021 CET

                OS : Linux
            CPU(s) : 16
           Machine : x86_64
      Architecture : 64bit
               RAM : 62.8 GiB
       Environment : Python
        GPU Vendor : NVIDIA Corporation
      GPU Renderer : NVIDIA GeForce GTX 1070 Ti/PCIe/SSE2
       GPU Version : 4.5.0 NVIDIA 470.82.00

  Python 3.8.10 (default, Jun  4 2021, 15:09:15)  [GCC 7.5.0]

           pyvista : 0.32.1
               vtk : 9.0.3
             numpy : 1.20.2
           imageio : 2.9.0
           appdirs : 1.4.4
            scooby : 0.5.7
            meshio : 4.4.3
        matplotlib : 3.3.4
         pyvistaqt : 0.5.0
             PyQt5 : 5.9.2
           IPython : 7.22.0
          colorcet : 1.0.0
             scipy : 1.7.1
              tqdm : 4.60.0

  Intel(R) oneAPI Math Kernel Library Version 2021.2-Product Build 20210312
  for Intel(R) 64 architecture applications

[github-actions[bot]]

Hi and welcome! Thanks for posting your first issue in the PyVista project! Someone from @pyvista/developers will chime in before too long. If your question is support related, it may be automatically transferred to Discussions.

[pvanhauw]

changing

mesh_sliced = mesh.slice(normal=[0, 1, 0]) into : mesh_sliced = mesh.slice(normal=[0, 1, 0]).strip() did the job.

It sounds like a good idea to document that in compute_arc_length Anyway, that issue can be solved with some pre-processing

[pvanhauw]

reopening because cell data cannot be recovered correctly.

For exemple the cell_data below is of dimension 1 ( and contained as "field data" instead of "cell_data"), whatever "join" is set to True of False.

#!/bin/python
import pyvista as pv

mesh = pv.Sphere()
mesh = mesh.compute_cell_sizes()
print(mesh.field_data)
print(mesh.cell_data)

mesh_sliced = mesh.slice(normal=[0, 1, 0]).strip(join=True, pass_cell_data=True)
print(mesh_sliced.field_data)
print(mesh_sliced.cell_data)

mesh_sliced = mesh.slice(normal=[0, 1, 0]).strip(join=False, pass_cell_data=True)
print(mesh_sliced.field_data)
print(mesh_sliced.cell_data)

pyvista DataSetAttributes
Association     : NONE
Contains arrays : None
pyvista DataSetAttributes
Association     : CELL
Active Scalars  : None
Active Vectors  : None
Active Texture  : None
Active Normals  : None
Contains arrays :
    Length                  float64  (1680,)
    Area                    float64  (1680,)
    Volume                  float64  (1680,)
pyvista DataSetAttributes
Association     : NONE
Contains arrays :
    Length                  float64  (1,)
    Area                    float64  (1,)
    Volume                  float64  (1,)
pyvista DataSetAttributes
Association     : CELL
Active Scalars  : None
Active Vectors  : None
Active Texture  : None
Active Normals  : None
Contains arrays : None
pyvista DataSetAttributes
Association     : NONE
Contains arrays :
    Length                  float64  (1,)
    Area                    float64  (1,)
    Volume                  float64  (1,)
pyvista DataSetAttributes
Association     : CELL
Active Scalars  : None
Active Vectors  : None
Active Texture  : None
Active Normals  : None
Contains arrays : None
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