# Accurately cutting loops out of 3D PolyData

**URL:** https://discourse.vtk.org/t/accurately-cutting-loops-out-of-3d-polydata/8625
**Category:** Support
**Created:** [May 29, 2022, 9:19pm UTC](https://discourse.vtk.org/t/accurately-cutting-loops-out-of-3d-polydata/8625 "2022-05-29T21:19:34Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![whophil](https://discourse.vtk.org/user_avatar/discourse.vtk.org/whophil/32/2857_2.png) [@whophil](https://discourse.vtk.org/u/whophil)
#### Post date: [May 29, 2022, 9:19pm UTC](https://discourse.vtk.org/t/accurately-cutting-loops-out-of-3d-polydata/8625/1 "2022-05-29T21:19:34Z")

</div>

I am trying to accurately cut loops out of a 3D PolyData with a loop which lies on the surface (or very close to it). Similar topics have been discussed a number of times before:

- [Punching a polygonal hole into a triangulated surface](https://discourse.vtk.org/t/punching-a-polygonal-hole-into-a-triangulated-surface/7061/) demonstrates how to do this for 2D using `vtkImprintFilter`
- [Splitting cells on a PolyData](https://discourse.vtk.org/t/splitting-cells-on-a-polydata/5021/) Extrudes the loop into a skirt and uses `vtkIntersectionPolyDataFilter` scribe the loop onto the surface. However, with that approach I have not been able to “cut out” the surface patch which is inside the loop - I don’t know how to select only the points/cells which are inside. Also, I have encountered simple cases where the intersection fails entirely.

Is there any updated guidance on this? It feels like the new `vtkImprintFilter` has the capacity to do this, but I have been unsuccessful getting it to work on 3D geometries. CC @will.schroeder who implemented this filter.

An example is shown below, using the `surface.vtp` and `boundaryline.vtp` files provided by @cobo on [this post](https://discourse.vtk.org/t/punching-a-polygonal-hole-into-a-triangulated-surface/7061/8). This example produces the correct result when the target mesh and loop are co-planar, but not generally.

```python
#!/usr/bin/env python
import vtk
import pyvista as pv
import numpy as np

target = pv.read("surface.vtp")
loop = pv.read("boundaryline.vtp")

# # make the data planar - the imprint works fine if these lines are un-commented!
# target.points[:, 2] = 0.
# loop.points[:, 2] = 0.

# ray-cast the loop points onto the target
origins = loop.points
direction = np.array([0, 0, 1])
length = 100.
intersections = []
for p in loop.points:
    # uses `vtkOBBTree.IntersectWithLine`
    intersection, _ = target.extract_surface().ray_trace(p + direction * length,
                                                         p - direction * length,
                                                         first_point=True)
    intersections.append(intersection)
intersections = np.array(intersections)

# update point data
loop_on_target = loop.copy(deep=True)
loop_on_target.points = intersections

# extract contour loop from loop polydata
loopExtract = vtk.vtkContourLoopExtraction()
loopExtract.SetInputDataObject(loop_on_target)
loopExtract.Update()

# clean
cleanLoop = vtk.vtkCleanPolyData()
cleanLoop.SetInputConnection(loopExtract.GetOutputPort())

# imprint
imp = vtk.vtkImprintFilter()
imp.SetTargetData(target)
imp.SetImprintConnection(cleanLoop.GetOutputPort())
imp.SetTolerance(0.01)
imp.BoundaryEdgeInsertionOn()
imp.TriangulateOutputOn()
imp.SetOutputTypeToImprintedRegion()
imp.Update()

# get result of filter pipeline
res = pv.filters._get_output(imp)

# plot
p = pv.Plotter()
p.add_mesh(target, style='wireframe')
p.add_mesh(res, color='green')
p.add_mesh(res.copy(), color='w', style='wireframe')
p.add_mesh(loop_on_target, color='red', style='wireframe', line_width=5)
p.view_xy()
p.show()

```

Correct result when target mesh and loop are co-planar. The green shaded region is the imprinted region.

 ![Screen Shot 2022-05-29 at 9.29.43 PM](https://discourse.vtk.org/uploads/default/original/2X/3/37954e9cb640ebee7f97f94fd8406931fa599a42.jpeg)

Wrong result when the target mesh is 3D and the loop is projected onto the mesh.

 ![Screen Shot 2022-05-29 at 9.29.13 PM](https://discourse.vtk.org/uploads/default/original/2X/0/07fafa23b0d93410b8cced22ca119b43e8576e15.jpeg)