polydata surface border as polyline

Support
1

Hello, I’d like to obtain the border of a polydata triangulated surface as a polyline.

With the following code (where tri_surf is the triangulated surface polydata), I get all the edges on the border, but they are not ordered, so I cannot build a proper closed polyline representing the border of the surface from these scattered edges.

edges = vtk.vtkFeatureEdges()
edges.BoundaryEdgesOn()
edges.NonManifoldEdgesOff()
edges.FeatureEdgesOff()
edges.ManifoldEdgesOff()
edges.SetInputData(tri_surf)
edges.Update()

Thanks very much!

2

Off the top of my head you might want to try the triangle strip / polyline stripper filter (vtkStripper).

3

Hello, thanks for quick answer! It looks like vtkStripper is what I need, however the order of the lines in the polyline is still messed up. In this example, what I expect as border is in yellow and the polyline I get is in one.

image
image2198×1396 451 KB

This is my code:

def get_boundary_edges(self):
    border_edges = vtk.vtkFeatureEdges()
    border_edges.BoundaryEdgesOn()
    border_edges.NonManifoldEdgesOff()
    border_edges.FeatureEdgesOff()
    border_edges.ManifoldEdgesOff()
    border_edges.SetInputData(self)
    border_edges.Update()
    clean_border_edges = vtk.vtkCleanPolyData()
    clean_border_edges.SetInputConnection(border_edges.GetOutputPort())
    border_strips = vtk.vtkStripper()
    border_strips.SetInputConnection(clean_border_edges.GetOutputPort())
    border_strips.Update()
    border_polyline = vtk.vtkPolyData()
    border_polyline.SetPoints(border_strips.GetOutput().GetPoints())
    border_polyline.SetPolys(border_strips.GetOutput().GetLines())
    return border_polyline

Thanks!!!

4

Update: like this it works!

def get_boundary_edges(self):
    border_edges = vtk.vtkFeatureEdges()
    border_edges.BoundaryEdgesOn()
    border_edges.NonManifoldEdgesOff()
    border_edges.FeatureEdgesOff()
    border_edges.ManifoldEdgesOff()
    border_edges.SetInputData(self)
    border_edges.Update()
    clean_border_edges = vtk.vtkCleanPolyData()
    clean_border_edges.SetInputConnection(border_edges.GetOutputPort())
    border_strips = vtk.vtkStripper()
    border_strips.SetInputConnection(clean_border_edges.GetOutputPort())
    border_strips.Update()
    border_polyline = vtk.vtkPolyData()
    border_polyline.SetPoints(border_strips.GetOutput().GetCell(0).GetPoints())
    border_polyline.SetPolys(border_strips.GetOutput().GetLines())
    return border_polyline

I just changed the line

border_polyline.SetPoints(border_strips.GetOutput().GetPoints())

into

border_polyline.SetPoints(border_strips.GetOutput().GetCell(0).GetPoints())

I guess that the reason is that the cell called here is a polyline, generated by vtkStripper, that combines all the single lines in the correct order.

However, if the surface is composed of two disconnected parts, I get two polylines, and in this case I am still not able to add all the points properly, and I end up with just the border of the first part.

I thought I can append the points from the second cell (or more cells) like this

    for i in range(border_polyline.GetNumberOfCells()-1):
        border_polyline.GetPoints().AppendPoints(border_strips.GetOutput().GetCell(i+1).GetPoints())

But for some reason I don’t understand it does not work. It appears it replaces the points of the first part with those of the second one.

Suggestions? Thanks very much!

5

Rather than trying to manually construct the polydata, how about:

r_polyline = r_strips.GetOutput()

which will have both the polylines and associated points.

6

Thanks Will! Unfortunately this was the first option I tried, but this is the messed up, unordered border I get (in red).

image
image1502×955 294 KB

That’s why I was experimenting with the code above, that works in case of single-part surfaces, but yields just the border of the first part in case of multiple parts.

For this reason I was trying to add also the other parts.

Thanks!

7

Hello, now it works. Basically I am building the polygons that correspond to borders, using points in the order with which they appear in the polyline cells issued by vtkStripper.

In this way I get proper borders even in case of multi-part surfaces.

image
image1424×986 135 KB

This is the code:

def get_boundary_edges(self):
    border_edges = vtk.vtkFeatureEdges()
    border_edges.BoundaryEdgesOn()
    border_edges.NonManifoldEdgesOff()
    border_edges.FeatureEdgesOff()
    border_edges.ManifoldEdgesOff()
    border_edges.SetInputData(self)
    border_edges.Update()
    clean_border_edges = vtk.vtkCleanPolyData()
    clean_border_edges.SetInputConnection(border_edges.GetOutputPort())
    border_strips = vtk.vtkStripper()
    border_strips.SetJoinContiguousSegments(True)
    border_strips.SetInputConnection(clean_border_edges.GetOutputPort())
    border_strips.Update()
    border_polygons = vtk.vtkCellArray()
    border_polygons.SetNumberOfCells(border_strips.GetOutput().GetNumberOfCells())
    border_points = vtk.vtkPoints()
    points_in_border = 0
    for cell in range(border_strips.GetOutput().GetNumberOfCells()):
        polygon = vtk.vtkPolygon()
        polygon.GetPointIds().SetNumberOfIds(border_strips.GetOutput().GetCell(cell).GetNumberOfPoints())
        for point_in_cell in range(border_strips.GetOutput().GetCell(cell).GetNumberOfPoints()):
            point_in_border = point_in_cell + points_in_border
            border_points.InsertNextPoint(border_strips.GetOutput().GetCell(cell).GetPoints().GetPoint(point_in_cell))
            polygon.GetPointIds().SetId(point_in_cell, point_in_border)
        border_polygons.InsertNextCell(polygon)
        points_in_polygon = polygon.GetNumberOfPoints()
        points_in_border += points_in_polygon
    borders = vtk.vtkPolyData()
    borders.SetPoints(border_points)
    borders.SetPolys(border_polygons)
    borders.Modified()
    return borders

Please let me know if you find a more efficient or elegant solution!

Thanks!

2 Likes
8

This was just what I was looking for, I spent over an hour trying to write an algorithm that reordered the points but realized this must already be an anticipated/desired output structure

9

You’re welcome!