Re: [Teem-users] gage boundary conditions -- how to set

[Gordon L. K. <gl...@uc...>]

There are few fields in the gageContext that are used as output, set  
by gageProbe.  One of them is edgeFrac:

   /* what fraction of the values in the kernel support had to be  
invented
      (by bleeding out the margin) in order to satisfy a probe that  
was near
      the edge (any axis, either high or low) of the volume */

One complication is that this is not just a matter of whether the  
probe location was inside or outside the volume, its a matter of how  
many of the data values that fell within the support of the  
convolution kernel were inside or outside the volume.  That's what  
edgeFrac records.  This is new in Teem v1.11; it wasn't in v1.10.

Right now, gage does what I thought was the safest thing, which is  
"bleeding" (copying) the values at the margin outwards; this is a C0  
continuous thing, but nothing fancier (probably not Neumann; can you  
give me a reference for that?).  One could argue that using value 0 is  
the safest thing, but part of what's great about gage is its  
flexibility for other value kinds, described by the gageKind struct,  
which might even come from outside Teem.  Using all 0s for a vector or  
tensor might be suddenly much more problematic or undefined than just  
re-using whatever values were already stored inside the volume (say,  
for quaternions, or unit-length vectors).  Bleeding values seemed to  
be the best way to "tread lightly".

I have long been meaning to increase the flexibility of gage's  
behaviors at edges, and this list is the place for us to throw around  
ideas.  For me the challenge has been figuring how to specify  
information about values at the margin while maintaining the  
flexibility of working for different gageKinds.  Do you make some "pad  
value" be a field in the gageKind?  You could, but then you have to  
dynamically allocate the gageKind, instead of using "gageKind *const  
gageKindScl", which is very handy.

Ideas?

Gordon

On Nov 24, 2009, at 10:05 AM, Daniel Blezek wrote:

> I’m doing a series of gage queries on a boundary (to resample  
> oblique-ly).  How do I set gage’s boundary conditions?  It currently  
> extends (Neumann?) the last voxel in the volume.  This is great for  
> interpolation, but I’d like the display to be zero.  If I can’t set  
> the value, is there some quick way to tell if my query was outside  
> the volume (‘cause I know gage calculates it).
>
> Thanks,
> -dan
>
> -- 
> Daniel Blezek, PhD
> Medical Imaging Informatics Innovation Center
>
> P 127 or (77) 8 8886
> T 507 538 8886
> E ble...@ma...
>
> Mayo Clinic
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> Harwick SL-44
> Rochester, MN 55905
> mayoclinic.org
>
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