[ITK Community] [Insight-users] Efficient distance to a sparse level set in ITK v4

[Danielle Pace]

Hi all,

Is there an efficient way to compute the distance from a point to a
sparse level set (Whitaker/Shi/Malcolm), where that point lies outside
the sparse layers?

More details:

I am working on a coupled level sets implementation, where there are
two level sets and a distance constraint between them.  I would like
to do this in v4 so that I can take advantage of the better setup for
multiple level sets, and ideally keep a sparse representation for
efficiency.

I need to create a new equation term that incorporates the distance,
so that the speed function = 0 when the distance between the level
sets is outside [minDistance, maxDistance].  Practically, this means
that the level sets should never be much more than maxDistance apart
at any time in the evolution.

In ITK v3 I could do this efficiently by increasing the number of
layers in the SparseFieldLevelSetImageFilter to correspond to
maxDistance.  Then I would always be able to grab the distance from
one level set to the other, without losing the sparse representation.
But, the option to change the number of layers doesn't seem to exist
in v4.

Any suggestions, or should I cave and use the dense representation?

One idea would be, on each iteration, to get a label map
representation of the (sparse) level set and then run the Maurer
distance map filter.  Perhaps this would still be more efficient than
using the dense level set representation, because the level set
evolution part would still be computed on a small fraction of the
pixels?  (I don't think this would be more overhead compared to the
dense representation, since the Maurer distance map filter runs on
every iteration anyways in
itkLevelSetEvolution::ReinitializeToSignedDistance).

Thanks,

Danielle
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