[Insight-users] region growing from local maxima

[Richard Beare]

Hi,

My first reply had some results attached that resulted in the message
bouncing. Here's the text. If anyone is interested in seeing results
I'll forward them individually, or perhaps Ara can put them on his
website and let everyone know.

I think you'll find that the necessary tools are currently missing.
The closest thing I've heard mentioned (in the InsightJournal, I
think), is a local maximum filter, which finds pixels that are larger
than all of their neighbors. This might help you get started, but
isn't what you really want (which is regional maxima (sets of pixels
surrounded by pixels of lower values).

The itk watershed implementation will find the extrema as part of the
processing, but I don't think they are available externally.

I'm one of what I'm sure is a number of people who are very interested
by itk, but frustrated that it is missing many of the tools that we're
used to, so I think you can be sure that the filters you are
interested in will be along fairly soon.

Your problem looks like a classical blob finding one - very similar to
lots of problems in a variety of fields. It also appears that you have
the luxury of a very flat and well defined background as well as flat
peaks in the lighting. This is a situation that is well tackled using
marker based watershed approaches (available in matlab with some
fiddling).

The procedure could be

find regional maxima (and optionally discard dark ones).
threshold to find background

OR the results together and use a markers for a watershed. The control
surface of the watershed should be a gradient - the watershed lines
will end up at the peak of the gradients, which may or may not be what
you want. We can fill in more details on that later.

If selecting the background threshold is a problem then you might
think about using 2 stages of watershed. The first stage is on the raw
image (possibly inverted) and serves to tesselate the image into
neighborhoods with each neighborhood containing a regional maxima. You
then use the watershed lines as a marker for the background and
combine them with the regional maxima and carry out the watershed on
the image gradient.

Anyway, an example of the first method is attached - regional max are
in green, borders of the watershed regions are in red. This wasn't
generated with itk, but hopefully it will help.

On 10/5/05, Ara.T.Howard <Ara.T.Howard at noaa.gov> wrote:
>
> i'm working with night-time lights satelite imagery
>
>    http://fortytwo.merseine.nu/india_shrunk/india_1995_stable_lights.small.png
>
> although it's hard to tell from that stretch - all of the features have
> considerable 'over-glow' caused by light scattering in the atmousphere.  i've
> been working with several approaches to 'shrinking' the features that boils
> down to finding local maxima and growing them 'downwards' from there.  ala
>
>    http://fortytwo.merseine.nu/india_shrunk/india_1995_stable_lights.small.em.15.png
>
> this is working o.k. but the toolkit i've been using has a flaw/bug/feature
> that has become a glass ceiling for me.  i'm looking at moving to itk.
> skimming the docs it would seem there is not builtin support for finding
> extrema - but maybe i've missed it, the docs are large! ;-)
>
> has anyone out there used itk to
>
>    - local extrema, maxes in particular
>
>    - grow regions from these seeds up to some tolerance (in my case i'm using
>      percent difference within the total region as a limiter)
>
> i'm also looking at the watershed segmentation method and it's depth feature.
>
> and hints or pointers to docs appreciated.
>
> kind regards.
>
> -a
> --
> ===============================================================================
> | email :: ara [dot] t [dot] howard [at] noaa [dot] gov
> | phone :: 303.497.6469
> | Your life dwells amoung the causes of death
> | Like a lamp standing in a strong breeze.  --Nagarjuna
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