[Insight-users] SparseFieldLevelSetImageFilter Boundary Conditions

[Brady McCary]

insight-users,

To add to the discussion, when I disable all code that computes and
checks the ``on-the-boundary'' status, the level set evolution is
exactly what I expect it to be.

Brady

On Fri, Feb 6, 2009 at 2:01 PM, Brady McCary <brady.mccary+ITK at gmail.com> wrote:
> insight-users,
>
> I have found evidence that makes me strongly believe that this is in
> fact a bug in ITK.
>
> In order to understand this discussion, note that the Sparse Field
> algorithm has to keep track of the STATUS of every pixel (whether it
> is moving between layers, etc.) so that it can properly track and
> construct the layers used in the algorithm. In particular, there is a
> status value which signifies that a particular pixel is on the
> boundary of the image. This status should (logically) be constant
> throughout the entire iteration. However, if the initialization of
> level set is such that the zero level set intersects the boundary of
> the image, then the ``on-the-boundary'' status is lost.
>
> I.e., the status of a pixel being on a boundary is assumed to be
> constant, but in fact this status depends on the initialization of the
> level set, because if the zero level set intersects the boundary of
> the image, the status values associated with the layers close to the
> zero level set overwrite the ``on-the-boundary'' status value.
>
> I have been viewing the the source code and I am not clear why the
> ``on-the-image-boundary'' status is significant, but I am looking. If
> the author of this class could comment, it would be greatly
> appreciated.
>
> I have posted two sequences of images to illustrate my findings. In
> particular, the images named sta_NNNN.nii are the status of each pixel
> at iteration NNNN. In the sequence which has an initialization where
> image boundary pixels are INSIDE, the
>
> http://www.utdallas.edu/~bcm052000/example.tar.bz2
>
> Brady
>
> On Fri, Feb 6, 2009 at 1:00 PM, Koen Vermeer <koen at vermeer.tv> wrote:
>> On Fri, 2009-02-06 at 10:53 -0600, Brady McCary wrote:
>>> As an update to this thread, I have observed the following. To make
>> [...]
>>> dynamics in my problem, it is clear that this is not appropriate.
>>
>> Not very useful, but: I experienced the same issues. In my case, I'm
>> looking at objects that are larger than the image, meaning that the
>> level set at the boundary should move - but often it doesn't. I simply
>> ignored this problem for now, as I'm just exploring possible solutions.
>>
>> So, basically, this is just a 'me too'.
>>
>> Best,
>> Koen
>>
>>
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>