Pointcloud: Normals are lost when flipping axis and freezing transformation

[ChristopherRemde]

First of all, many thanks for your work on Meshlab! It's an amazing tool I couldn't live without!

I recently played around with pointclouds that have normals. I need to flip the pointclouds X-axis to adapt it to Unitys coordinate system.
While the flipping itself works as expected, the normals are then lost when freezing the matrix. This is true for Meshlab and PyMeshlab. In PyMeshlab, it can be seen that the normals arrays are still intact after freezing, but they show 'NaN' values, instead of a valid floating point number.

Steps to reproduce:

• Open any pointcloud with normals in Meshlab (Or open one without normals and then use 'Filters > Point Set > Compute Normals for point sets')
• Open 'Filters > Normals, Curvatures and Orientation > Transform: Flip and or Swap axis'
• Choose any axis to flip and enable 'Freeze Matrix' in this dialog directly
• Or: Deactivate 'Freeze Matrix' in this dialog. Then Flip the pointcloud, and apply 'Freeze Matrix' afterwards
• Normals are now gone

Tested in Meshlab 2023 and 2025

[ChristopherRemde]

As a workaround for anyone having the same problem, you can flip the pointcloud and it's normals properly in pymeshlab with this little script:

import pymeshlab as ml
import numpy as np

#Load the mesh
ms = ml.MeshSet()
ms.load_new_mesh('Path/To/Your/Pointcloud.ply')

#Get the vertex positions, normals and colors as numpy arrays:
positions = ms.current_mesh().vertex_matrix().astype(np.float64)
normals = ms.current_mesh().vertex_normal_matrix().astype(np.float64)
colors = ms.current_mesh().vertex_color_matrix().astype(np.float64)

#Multiply the positions and normals x,y or z axis by -1 to flip it
#Use [:,0] for X-axis, [:,1] for Y-axis and [:,2] for Z-Axis
#Here we flip the X-Axis as an example
positions[:,0] *= -1
normals[:,0] *= -1

#Recreate a mesh from the position, normals and colors arrays
flipped_PC = ml.Mesh(vertex_matrix = positions, v_normals_matrix = normals, v_color_matrix = colors)
ms.add_mesh(flipped_PC, "flipped_PC")

#Set the flipped mesh as current and save it
ms.set_current_mesh(1)
ms.save_current_mesh('Path/To/Your/Flipped_Pointcloud.ply', save_vertex_color = True, save_vertex_normal = True)