[Insight-users] Exactness of Maurer distance transform

[Amardeep Singh]

Dear ITK Users,

I have a question concerning the exactness that can be achieved with the
ITK Maurer distance transform. I have run the distance transform on
the attached synthetic image of a circle (signed distance transform with
the real distances, NOT with the squared ones).
Now, I look at the borders of the circle (from the origin of the circle
in any direction - north, south, east, west or foreground, background).
I find that at the last voxel of the circle, the distance is 0. The
first voxel on the outside has distance 1. This suggests that the
boundary of the circle is exactly at the voxel centre of the voxel with
0 distance. However, it is in between the two aforementioned voxels. So,
actually, I would expect them to have the distances -0.5 for the one on
the inside and 0.5 for the one on the outside.
I am a little confused and I would appreciate any help on this matter.
Is there a bug or do I misunderstand something? Or is this just the
behaviour/result that can be achieved?

Thank you very much!

Best regards
Amar

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