Scaling stl mesh object in Python

Question

I imported a .stl model into Python and plotted it in a 3D plot. However, the mesh model is too small compared to the rest of the figure, which contains various other lines and objects. How can I scale a stl.mesh object in Python?

(I am not using Blender or any other software. This is purely a Python plot that I am trying to create)

Code:

from stl import mesh
import matplotlib.pyplot as plt
from mpl_toolkits import mplot3d
from mpl_toolkits.mplot3d import axes3d
from mpl_toolkits.mplot3d import proj3d

fig = plt.figure()
ax = plt.axes(projection="3d")

my_mesh = mesh.Mesh.from_file('model.stl')
ax.add_collection3d(mplot3d.art3d.Poly3DCollection(my_mesh.vectors))

Answer 1

Short answer

Just add the scaling factor with multiplication to the mesh.vectors, like:

collection1 = Poly3DCollection(street.vectors * SCALE, linewidths=1, alpha=0.2)

And it will "inflate" the model. Surely numbers between 0 and 1 will "deflate" the object.

Long answer

After pushing here, I discussed this problem with a colleague and found the (very simple) solution.

First the regular way to use a mesh stl file, from numpy-stl:

# load mesh file
street = mesh.Mesh.from_file('../../stls_files/strasen.stl')

figure = plt.figure()
ax = mplot3d.Axes3D(figure)    
    
collection1 = Poly3DCollection(street.vectors, linewidths=1, alpha=0.2)

street_color = [0.2, 0.5, 0.7]
collection1.set_facecolor(street_color)

ax.add_collection3d(collection1)

# Auto scale to the mesh size
scale1 = street.points.flatten()

ax.auto_scale_xyz(scale1, scale1, scale1)    
    
plt.show()

The next box shows the modification:

# load mesh file
street = mesh.Mesh.from_file('../../stls_files/strasen.stl')

figure = plt.figure()
ax = mplot3d.Axes3D(figure)

scale_factor = 4
collection1 = Poly3DCollection(street.vectors * scale_factor, linewidths=1, alpha=0.2)

street_color = [0.2, 0.5, 0.7]
collection1.set_facecolor(street_color)

ax.add_collection3d(collection1)

# Auto scale to the mesh size
scale1 = street.points.flatten()

ax.auto_scale_xyz(scale1, scale1, scale1)    
    
plt.show()

Please note I have just added a factor and multiplication with the mesh.vectors of the stl.Mesh.

With subtraction/addition you can also move the object.

Answer 2

If you have the ParaView library in your python, you can move, rotate, and scale an STL file and save it in another file. The below code is for scaling:

from paraview.simple import *

def ScaleStl(inputFileName, scaledFileName, scale):
    
    # create a new 'STL Reader'
    myStl = STLReader(registrationName='myStl.stl', FileNames=[inputFileName])

    # create a new 'Transform'
    transform1 = Transform(registrationName='Transform1', Input=myStl)
    transform1.Transform = 'Transform'

    # Properties modified on transform1.Transform
    transform1.Transform.Scale = scale

    # save data
    SaveData(scaledFileName, proxy=transform1, CellDataArrays=['STLSolidLabeling'], FileType='Ascii')

Use it like:

ScaleStl("body.stl","scaledbody.stl",[0.5,0.5,0.5])

If installing the ParaView library is complicated, the Paraview package comes with its own Python binary, called pvpython which runs the above code.