Should Spacingd update the spacing field in the meta dict?

[mattwarkentin]

Hi,

I am wondering if the Spacingd and similar transforms which modify the spacing should update the 'spacing' field? The affine matrix is updated, but spacing is not.

from monai.transforms import (
  Compose,
  LoadImaged,
  AddChanneld,
  Spacingd
)

sub = {img: "image.nrrd"}

LoadImaged('img')(sub)['img_meta_dict']['spacing']
# array([0.652344, 0.652344, 2.5     ])

trans = Compose([
  LoadImaged('img'),
  AddChanneld('img'),
  Spacingd('img', pxidim=(1,1,1))
])

sub_t = trans(sub)

sub_t['img_meta_dict']['affine']
# array([[   1.        ,    0.        ,    0.        , -165.347784  ],
#       [   0.        ,    1.        ,    0.        , -166.347784  ],
#       [   0.        ,    0.        ,    1.        , -347.20999146],
#       [   0.        ,    0.        ,    0.        ,    1.        ]])

sub_t['img_meta_dict']['spacing']
# array([0.652344, 0.652344, 2.5     ])

It seems to me like the spacing should be updated alongside the affine matrix? What do you think?

[mattwarkentin]

Maybe update the meta dict with 'old_spacing' and 'spacing', similar to affine?

[mattwarkentin]

Relevant:

MONAI/monai/transforms/spatial/dictionary.py

Line 265 in 9ec1d14

meta_data["affine"] = new_affine

[mattwarkentin]

This also leads to a perhaps larger question of whether the affine should be updated when cropping or padding to update the origin???