[Insight-developers] CannySegmentationLevelSetImageFilter

[Bill Lorensen]

Josh,
The ReverseExpansionDirection is dangerous since it acts when it is called 
and does not set a mode. If you call it twice, it reverse itself. You can 
change the behavior by changing the signs of the coefficients. I think the 
best thing is clear documentation.

Bill

At 04:22 PM 11/21/2003, Joshua Cates wrote:
>Hi all,
>
>Sorry I missed this discussion on the TCON.  When the level-set section in
>the SoftwareGuide was first written there was still some inconsistency
>among level-set filters, so we left the description of inside/outside to
>up to documentation in specific classes.  Now everything is standardized
>and correctly described in the doxygen pages for
>SegmentationLevelSetImageFilter.
>
>Let me see if I understand correctly: The confusion on the initialization
>is that you expected "inside" to correspond to the intuitive notion of the
>region enclosed by the implicit surface, when actually "inside" is
>determined using values relative (lt) the isosurface you describe in the
>input?  So using a binary initialization 0, 255 and choosing isosurface
>value 127, for example, results in negative values external to the region
>enclosed by the surface.
>
>Note that the opposite sign convention (positive-outside) produces your
>intuitive result from a binary initialization.  A perfectly valid way to
>work with these filters is to turn on "ReverseExpansionDirection" which
>effectively reverses the convention during calculations and gives the
>expansion/contraction of the opposite sign convention.
>
>There may also be some use in another method that allows you to flip the
>sign of the values in your initialization?
>
>I'll add some clarifications to the SoftwareGuide on these points.
>Probably be a week or so before I can get to it.  Let me know if there are
>other points that need clarification.
>
>Josh.
>
>______________________________
>  Josh Cates
>  Scientific Computing and Imaging Institute
>  University of Utah
>  Email: cates at sci . utah . edu
>  Phone: (801) 587-7697
>  URL:   http://www . sci . utah . edu/~cates
>
>
>On Fri, 21 Nov 2003, Miller, James V (Research) wrote:
>
> > I mentioned on the tcon this afternoon that we were having some problems
> > with the CannySegmentationLevelSetImageFilter.  If we used a positive 
> weight
> > for the advection weight, our segmentations would contract whereas if we
> > used a negative weight for the advection weight our segmentations would
> > expand (like we wanted).
> >
> > Lydia mentioned that it may be due to how the seed image is being used.
> > Once again, Lydia is right. We were passing in a binary image (the 
> output of
> > another segmentation algorithm) as the seed image.  The seed image had 255
> > for the object and 0 for the background.  The 
> SparseFieldLevelSetImageFilter
> > takes this seed image and generates an initial level set by assigning 
> pixels
> > above a specified IsoValue as being outside the object and pixels below a
> > specified IsoValue as being inside the object.  So while I thought my
> > segmentation was shrinking, it was actually growing.  It was just that my
> > "object" was actually my background.
> >
> > So.  Here are my suggestions.  One, we should be very clear that the inside
> > of objects have negative level set values and the outside of objects have
> > positive level set values.  Two, we should change the documentation for
> > *SegmentationLevelSetFilters so that it is clear that the seed image 
> follows
> > the same conventions for objectness as the levelsets, i.e. values less than
> > the specified IsoValue are interior to the object, values greater than the
> > specified IsoValue are exterior to the object.  I am not sure whether we
> > want to go so far as to say the seed image is an initial level set (which
> > does not really have to be) or whether we can just say that it follows the
> > same conventions as level sets.
> >
> > Finally, I am not sure when we changed the levelsets to be consistent with
> > negative interiors and positive exteriors but I regardless of when it was
> > done, it looks like the User Guide is out of date.  We'll need to propagate
> > this into the User Guide and perhaps put out a list of "errata" on the web
> > site.
> >
> > Any comments?
> >
> >
> >
> > Jim Miller
> > _____________________________________
> > Visualization & Computer Vision
> > GE Research
> > Bldg. KW, Room C218B
> > P.O. Box 8, Schenectady NY 12301
> >
> > millerjv at research . ge . com <mailto:millerjv at research . ge . com>
> >
> > james . miller at research . ge . com
> > (518) 387-4005, Dial Comm: 8*833-4005,
> > Cell: (518) 505-7065, Fax: (518) 387-6981
> >
> >
> >
>
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