[Insight-developers] CannySegmentationLevelSetImageFilter

[Joshua Cates]

Yes I agree the best thing is always more documentation.

Note that as implemented in the SegmentationLevelSetImageFilter class, the
ReverseExpansionDirection does in fact set a mode flag.  So you can call
ReverseExp...On to enable and ReverseExp..Off to disable.  It's a
convenience function that flips the signs of the coefficients before
execution (through a call to
SegmentationLevelSetFunction::ReverseExpansionDirection, hence the
confusion).

 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, Bill Lorensen wrote:

> 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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