get_voxel_spacing utility function

[cancan101]

From here:

nilearn/nilearn/image/resampling.py

Lines 659 to 663 in f6c5d3e

	# Identify the voxel size using a QR decomposition of the
	# affine
	Q, R = np.linalg.qr(affine[:3, :3])
	target_affine = np.diag(np.abs(np.diag(R))[
	np.abs(Q).argmax(axis=1)])

factor out:

def get_voxel_spacing(affine):
    Q, R = np.linalg.qr(affine[:3, :3])
    return np.abs(np.diag(R))[np.abs(Q).argmax(axis=1)]

This function is useful for example when resizing to isotropic you can use the smallest dimension in the new affine.

[KamalakerDadi]

Isnt't this simple to grab the diagonal of the affine ? Why do we need function ?

You mean we can use this function multiple places in the code base ?

[cancan101]

It isn't quite the (raw) diagonal of the affine. It is doing a QR decomposition.

[KamalakerDadi]

yeah but the result will be the same. isn't it ?

[cancan101]

Take this as an example:

array([[4, 1, 3],
       [1, 0, 0],
       [0, 0, 2]])

> get_voxel_spacing(arr)
array([4.12310563, 0.24253563, 2.        ])

[bthirion]

Well, can you issue a PR ?

[bthirion]

Stalled.