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Anchors are not centered on grid cells #7951

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luukbalkenende opened this issue Jul 25, 2024 · 0 comments

luukbalkenende commented Jul 25, 2024

Describe the bug
Anchors should be placed ideally in the middle of a grid cell.
However, this is not the case here:

MONAI/monai/apps/detection/utils/anchor_utils.py

Lines 253 to 258 in 12d00ce

    
           # compute anchor centers regarding to the image. 
        
           # shifts_centers is [x_center, y_center] or [x_center, y_center, z_center] 
        
           shifts_centers = [ 
        
               torch.arange(0, size[axis], dtype=torch.int32, device=device) * stride[axis] 
        
               for axis in range(self.spatial_dims) 
        
           ]

Expected behavior
The anchors should be placed ideally in the middle of a grid cell.

Possible fix
This could probably be fixed by simply changing these lines to:

shifts_centers = [
    torch.arange(0, size[axis], dtype=torch.int32, device=device) * stride[axis] + int(stride[axis] / 2)
    for axis in range(self.spatial_dims)
]

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