CropForeground pads asymmetric

[razorx89]

Describe the bug
In CropForeground/CropForegroundd the user can request a margin around the foreground voxels. However, currently the margin is only correctly padded on the upper side of a dimension.

See:

MONAI/monai/transforms/utils.py

Lines 924 to 926 in c3db337

	arg_max = where(dt == dt.max())[0]
	min_d = max(arg_max[0] - margin[di], 0)
	max_d = arg_max[-1] + margin[di] + 1

To Reproduce

from monai.transforms import CropForeground
import numpy as np

transform = CropForeground(margin=2)
arr = np.eye(3)[np.newaxis, ...]
for i in range(3):
    inp = arr[:, i]
    res = transform(inp)
    print(inp, res.shape)
# [[1. 0. 0.]] (1, 3)
# [[0. 1. 0.]] (1, 4)
# [[0. 0. 1.]] (1, 5)

Expected behavior
The specified margin should be enforced by cropping or padding on both ends of a dimension.

Environment
Monai 0.8.0, but see Permalink above

[Nic-Ma]

Hi @razorx89 ,

Thanks for your feedback, let me verify locally and get back to you soon.

[wyli]

the docstring is also incorrect

MONAI/monai/transforms/utils.py

Line 895 in b0f70e2

This function returns [-1, -1, ...], [-1, -1, ...] if there's no positive intensity.

[Nic-Ma]

The root cause is that: when we developed this transform, the margin was defined to align with image edges and actually the logic is wrong, it only checks left side.
After adding crop_pad and k_divisible support, the margin can support not to align with image edges.
So I clearly enhanced the logic to fix it in PR #3837.

Thanks.