[Insight-users] Gradient filter problems

[Bjorn Hanch Sollie]

Hi all,

I find that when I use the GradientMagnitudeImageFilter I obtain
double edges in the filtered picture.  For example single line
(typically about one pixel wide in the data I'm working with) turns
into two lines in the output image, one line on either side of where
the actual line is.

The result of this is that when I do edge detection on a volume of a
rather thin pipe-shaped structure, the filtered image contains one
pipe within the other, and this makes it significantly harder to
segment the picture afterwards.

I assume the filter "sees" the pixel-wide line as two transitions,
from dark to light to dark, for example, thus producing two edges
rather than one, but this makes little sense for my application,
unfortunately, and I've seen other similar filters producing only one
edge in this instance.

So, what I want is an edge-detection filter where the detected edges
are actually in the same place as the boundaries I'm interested in.
It isn't really obvious to me why the GradientMagnitudeImageFilter
doesn't do this in the first place, so if I'm really looking for
another filter, I'd be happy if someone can point me in the right
direction.

-Bjorn
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