[dajiaohuang]

Problem

The documented points DataFrame selection workflow preserves row labels, but the nearest and highest voxelgrid samplers use those labels as .iloc positions. A valid point selection can therefore raise IndexError: positional indexers are out-of-bounds instead of returning a sample.

For example, for either sampler:

import pandas as pd
from pyntcloud import PyntCloud

coordinates = [0.0, 0.1, 0.2, 0.5, 0.9, 1.0]
cloud = PyntCloud(pd.DataFrame({axis: coordinates for axis in "xyz"}))
cloud.points = cloud.points.loc[cloud.points["x"] > 0.3]
voxelgrid_id = cloud.add_structure("voxelgrid")
sample = cloud.get_sample("voxelgrid_nearest", voxelgrid_id=voxelgrid_id)
# "voxelgrid_highest" fails for the same reason.

Change

Use a locally reset index for the grouping/index selection in both samplers, so the selected indices are positions. This does not reset the caller's point index or change the sampling algorithm. Add a public-API regression comparing the selected cloud with the same cloud indexed from zero.

Validation

Windows, Python 3.12.13, NumPy 2.2.6, pandas 2.2.3, SciPy 1.15.3:

• Both public-API cases reproduce the out-of-bounds failure on 8368c6a; the two regression cases pass with this change.
• Ruff 0.9.10 check and format check pass for the changed Python files; git diff --check passes.
• Unit suite: 105 passed, 11 failed, unchanged from base. Selected integration coverage (core class, scalar fields, structures, filters, point/voxel samplers): 83 passed, 15 failed versus 81 passed, 15 failed on base. Existing failures concern voxel boundary expectations (the area of Stabilize voxel boundary binning across platforms in VoxelGrid #369) and four tests using sample is DataFrame; those are not changed here.
• Synthetic PLY/NPZ/XYZ/OBJ round trips and the README HSV/voxelgrid/nearest/NPZ workflow pass. Matplotlib Agg rendering also runs.
• Repository-wide flake8 still reports existing findings; the unchanged long docstring in this module also fails its default line-length check. No full CI/platform matrix or optional Open3D/PyVista/widget-rendering claim.