Finding the radius of a particular center voxel in 3D cylinder-like structure

From the image above, assume that it is a 3D image that is of m*n*o size.

Each voxel can only be either 0 or 1. White is 1. Black is 0.

In this 3D environment, there is one cylinder-like structure, but the whole cylinder does not have consistent radius - from one to the other end, the radius changes around. The whole structure (as in "any edge of the structure to the whole thing inside) consists of white voxels or 1 only. All other voxels outside the structure are 0.

Red line is the computed skeleton of the 3D image - this red skeleton line is always right in the middle of the whole structure.

This implies for any particular voxel of the red line, we can draw the blue circle of respective radius for any particular voxel.

The radius or these green lines are what I am looking for.

If we are to compute for the radius of any particular voxel of the red line, how could that be accomplished?

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Would the radius of the largest sphere centered at the red point and entirely contained in the white voxels be what you are looking for? In other words, the distance from the red point to the nearest black voxel. Some related concepts: medial axis, local feature size. – Rahul Jan 27 '12 at 14:46
@Rahul: I don't know the correct technical term for this, but from your comment, I have to say that I am not looking for the distance of any red point to its nearest black voxel. Let's put it this way: I am to place a 2D circle at a perpendicular angle somewhere on the red line, and I am to find the radius of this 2D circle. Does this make things a bit clearer? – Karl Jan 27 '12 at 14:59
@RahulNarain When he says "circle at a perpendicular angle", he probebly means perpedicular to the curve's tangent vector. – Christian Rau Jan 27 '12 at 16:28
@Christian: Ah, the phrase "at a perpendicular angle" wasn't there when I started writing my reply (now deleted). – Rahul Jan 27 '12 at 16:37