[Insight-users] Use of advection image to guide geodesic level set

[Amardeep Singh]

Dear ITK Users,

first of all, thank you very much, Luis, for answering my questions on 
the gradient vector flow (GVF). This
really helped to lift some confusion. Unfortunately, I am stuck again 
with my experiments on (at the moment) synthetic images and so I would 
like to ask for the help of the mailing list.
My scenario is the following:
I am dealing with 2D images and consider a bright ring on a dark 
background. I use the ring as  an edge map and
calculate the gradient vector flow from it. I start with a circle that 
is inside of the ring and want to propagate it to the center of the ring 
by using the gradient vector flow as an advection term by using 
SetAdvectionImage.
My speed image is 1 inside of the ring, 0 in the area of the ring and -1 
outside so that the level set is simply guided by the advection term on 
the ring.
My problem is that the level set locks to the outer border of the ring, 
not to its center, which is really confusing me.
I know that deformable models are very sensitive to the parameter 
settings, but in my case I would think that this sensitivity should be a 
little less important as the speed image is 0 on the ring and only the 
advection term is important. I checked that the 0 isoline of the GVF is 
where I want it to be, in the middle of the ring.
All images have direction cosines of a unit matrix.
My parameters:
GVF:
noise level: 20000 (in order to get a very small time step)
iterations: 500
LevelSet:
iterations: 1000
RMS error: 0.01
propagation scaling: 1
curvature scaling: 0
advection scaling: 1

I would be very happy if anyone could shed light on the question why the 
circle does not lock to inside of the ring.
I have attached my source code and all images.
Thank you very much!

Best regards
Amar
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