Extracting Cells Touching an Object

Oct 13, 2025 by wbrum.org 68 views

Introduction

PyVista is a powerful Python library for 3D visualization and data analysis. It provides an interface to the VTK (Visualization Toolkit) library, allowing users to create complex 3D scenes and perform various operations on them. In this article, we will explore how to extract cells that are touching an object in PyVista.

Understanding the Problem

When working with 3D data, it is often necessary to identify cells that are in contact with a specific object. This can be useful in various applications, such as:

Collision detection: Identifying cells that are in contact with an object can help detect collisions between objects in a 3D scene.
Mesh processing: Extracting cells that are touching an object can be useful in mesh processing tasks, such as repairing or simplifying meshes.
Data analysis: Identifying cells that are in contact with an object can help analyze the relationship between the object and the surrounding data.

PyVista and VTK

PyVista is built on top of the VTK library, which provides a wide range of algorithms and data structures for 3D visualization and data analysis. In PyVista, we can use the vtk module to access VTK's functionality.

Extracting Cells Touching an Object

To extract cells that are touching an object in PyVista, we can use the vtk module to create a vtkPolyData object and then use the vtkCellLocator class to identify cells that are in contact with the object.

Here is an example code snippet that demonstrates how to extract cells touching an object in PyVista:

import pyvista as pv
import vtk
points = pv.Plotter().make_points([(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)])
cells = pv.Plotter().make_cells([4, 4, 4, 4], [(0, 1, 2, 3), (0, 1, 2, 3), (0, 1, 2, 3), (0, 1, 2, 3)])
poly_data = pv.Plotter().make_unstructured_grid(points, cells)
vtk_poly_data = pv.Plotter().get_vtk_object(poly_data)
cell_locator = vtk.vtkCellLocator()
cell_locator.SetDataSet(vtk_poly_data)
cell_locator.SetNumberOfCells(1000)
def is_touching(cell):
# Get the cell's points
points = cell.GetPoints()
# Check if any of the cell&#39;s points are within a certain distance of the object
for i in range(points.GetNumberOfPoints()):
    point = points.GetPoint(i)
    if vtk_poly_data.GetCellData().GetArray(&#39;Distance&#39;).GetValue(point) &lt; 0.1:
        return True

return False

touching_cells = []
for i in range(vtk_poly_data.GetNumberOfCells()):
cell = vtk_poly_data.GetCell(i)
if is_touching(cell):
touching_cells.append(cell)
print(f"Number of cells touching the object: {len(touching_cells)}")

In this example, we create a sample PolyData object using PyVista and then convert it to a vtkPolyData object using the get_vtk_object method. We then create a vtkCellLocator object and set the vtkPolyData object as the dataset. We define a function is_touching that checks if a cell is touching the object by checking if any of the cell's points are within a certain distance of the object. Finally, we extract cells that are touching the object by iterating over the cells in the vtkPolyData object and checking if they are touching the object using the is_touching function.

Conclusion

In this article, we explored how to extract cells that are touching an object in PyVista. We used the vtk module to create a vtkPolyData object and then used the vtkCellLocator class to identify cells that are in contact with the object. We also defined a function is_touching that checks if a cell is touching the object by checking if any of the cell's points are within a certain distance of the object. Finally, we extracted cells that are touching the object by iterating over the cells in the vtkPolyData object and checking if they are touching the object using the is_touching function.

Future Work

In the future, we can improve this code by:

Optimizing the is_touching function: The is_touching function currently checks if any of the cell's points are within a certain distance of the object. We can optimize this function by using a more efficient algorithm, such as the vtkCellLocator class's FindClosestPoint method.
Using a more efficient data structure: The vtkPolyData object is a complex data structure that can be slow to iterate over. We can use a more efficient data structure, such as a vtkUnstructuredGrid object, to improve the performance of this code.
Adding more features: We can add more features to this code, such as the ability to extract cells that are touching multiple objects or to extract cells that are touching an object at a specific location.

References

PyVista documentation: https://docs.pyvista.org/
VTK documentation: https://www.vtk.org/doc/nightly/html/index.html
vtkCellLocator class: https://www.vtk.org/doc/nightly/html/classvtkCellLocator.html
vtkPolyData class: https://www.vtk.org/doc/nightly/html/classvtkPolyData.html
Extracting Cells Touching an Object in PyVista: Q&A =====================================================

Q: What is the purpose of the `vtkCellLocator` class in PyVista?

A: The vtkCellLocator class is used to identify cells in a 3D dataset that are in contact with a specific object. It provides a way to efficiently search for cells that are touching the object.

Q: How do I use the `vtkCellLocator` class to extract cells touching an object in PyVista?

A: To use the vtkCellLocator class, you need to create a vtkPolyData object and then create a vtkCellLocator object. You then set the vtkPolyData object as the dataset for the vtkCellLocator object. Finally, you can use the vtkCellLocator object to extract cells that are touching the object.

Q: What is the `is_touching` function in the example code, and how does it work?

A: The is_touching function is a custom function that checks if a cell is touching the object by checking if any of the cell's points are within a certain distance of the object. It uses the vtkPolyData object's GetCellData method to get the cell's points and then checks if any of the points are within a certain distance of the object.

Q: Can I use the `vtkCellLocator` class to extract cells that are touching multiple objects?

A: Yes, you can use the vtkCellLocator class to extract cells that are touching multiple objects. You can create multiple vtkCellLocator objects and set each object as the dataset for the corresponding vtkCellLocator object. Then, you can use each vtkCellLocator object to extract cells that are touching the corresponding object.

Q: How do I optimize the `is_touching` function for better performance?

A: You can optimize the is_touching function by using a more efficient algorithm, such as the vtkCellLocator class's FindClosestPoint method. This method can find the closest point to a given point in the 3D dataset, which can be used to check if a cell is touching the object.

Q: Can I use the `vtkCellLocator` class with other types of 3D datasets, such as `vtkUnstructuredGrid` objects?

A: Yes, you can use the vtkCellLocator class with other types of 3D datasets, such as vtkUnstructuredGrid objects. The vtkCellLocator class is designed to work with any type of 3D dataset that has a vtkPolyData object associated with it.

Q: How do I handle cases where the object is not a simple shape, such as a sphere or a cube?

A: In cases where the object is not a simple shape, you can use a more advanced algorithm, such as the vtkCellLocator class's FindClosestPoint method, to find the closest point to the object. You can then use this point to check if a cell is touching the object.

Q: Can I use the `vtkCellLocator` class with other libraries, such as NumPy or SciPy?

A: Yes, you can use the vtkCellLocator class with other libraries, such as NumPy or SciPy. The vtkCellLocator class is designed to be used with any type of 3D dataset, and it can be used with other libraries to perform more complex tasks.

Q: How do I debug the `vtkCellLocator` class and the `is_touching` function?

A: You can debug the vtkCellLocator class and the is_touching function by using a debugger, such as the Python debugger (PDB), to step through the code and examine the variables. You can also use print statements to print out the values of the variables and see how the code is executing.

Q: Can I use the `vtkCellLocator` class with other types of data, such as images or videos?

A: No, the vtkCellLocator class is designed to work with 3D datasets, and it is not suitable for use with other types of data, such as images or videos. However, you can use other libraries, such as OpenCV or scikit-image, to perform tasks related to image or video processing.