Regions extracted using same bounding box from BigBrain histology and isocortex segmentation are not aligned

Given a bounding box, I tried to extract a region from the BigBrain histological space and the corresponding isocortex segmentation. Since I used the same bounding box for both, I would expect the resulting volumes to be aligned (up to the precision of the cortex segmentation), but they are not. This is the code I used:

import numpy as np
import brainscapes as bs
from nilearn import plotting
from cloudvolume import Bbox

bb_img = bs.bigbrain.BigBrainVolume(bs.spaces.BIG_BRAIN_HISTOLOGY.url)

# Define two points in BigBrain space
p0 = np.array((-3.979, -61.256, 3.906))
p1 = np.array((5.863, -55.356, -2.487))

def mm_to_vox(p0, p1, img, mip):
    # Bounding box needs to be defined in voxel space, so we need to apply the inverse affine matrix to the points
    inv_aff = np.linalg.inv(img.affine(mip))
    p0_vox = np.dot(inv_aff[:3, :3], p0) + inv_aff[:3, -1]
    p1_vox = np.dot(inv_aff[:3, :3], p1) + inv_aff[:3, -1]
    return p0_vox, p1_vox

# Read region from BigBrain
mip = 0
p0_vox, p1_vox = mm_to_vox(p0, p1, img=bb_img, mip=mip)
# Define bounding box
bbox = Bbox(p0_vox, p1_vox)
img = bb_img.Image(clip=bbox, mip=mip, force=True)

mask_url = "https://neuroglancer.humanbrainproject.eu/precomputed/BigBrainRelease.2015/classif/"
mask_img = bs.bigbrain.BigBrainVolume(mask_url)

# Bounding box needs to be redefined, as mask_img has different voxel space
p0_vox, p1_vox = mm_to_vox(p0, p1, img=mask_img, mip=mip)
# Define bounding box
bbox = Bbox(p0_vox, p1_vox)
mask = mask_img.Image(clip=bbox, mip=mip, force=True)

# Plot on top of each other. I would expect these images to be aligned, since I used the same bounding box, but they are not.
plotting.view_img(mask,
                  bg_img=img,
                  cmap="gray",
                  vim=0,
                  vmax=255,
                  resampling_interpolation="nearest",
                  symmetric_cmap=False)

I could imagine that there is an issue with how the bounding box coordiantes are rounded internally to match to the voxel space.

FZJ-INM1-BDA / siibra-python

Regions extracted using same bounding box from BigBrain histology and isocortex segmentation are not aligned #11