[Insight-users] Finding Connctedness

[Ross Whitaker]

"Connectedness" can be used for things other than tissue segmentation. 
 For instance, when tracing edges or tracking features, it is sometimes 
desirable to have a notion of connectedness that is asymmetrical.  There 
is nothing wrong with providing such functionality, as long as it is 
documented.  I.e. I don't think we should not get pedagogical about the 
algorithms we offer.

Is 18 connectedness symmetric?

Ross

Luis Ibanez wrote:

>
> Hi Suresh,
>
> Just for the record: Both 6 and 26 connectedness are a
> ill-defined digital topologies since they lead to an
> asymmetrical definition of object and background.
>
> 26 connected objects require  6 connected backgrounds and
>  6 connected objects require 26 connected backgrounds
>
> When it comes to fully segment a medical image there is
> no such thing as "background". We only have tissue classes.
> That means that some tissues should be considered 26
> connected while others are 6 connected. The choice is totally
> arbitrary and there is no way to get it right.
>
> All this is the consequence of the poor choice made early in
> the image processing field for using rectilinear grids a
> sampling grids.
>
> We should be using hexagonal grids in 2D, and BCC grids in 3D
> as has been continously recommended by the mathematical morphology
> community.
>
>
> but...
> given that we are now stuck with rectilinear grids here is what
> you could do for obtaining 26-connected regions:
>
>
> The current connected filters in ITK are based on the powerful
> itkFloodFillFunctionConditionalIterator.
>
> This iterator visits all the pixels that are connected to a
> seed provided by the user.
>
> The neighborhood relationship used in this iterator in 3D is
> six connectedness. Note that this is a N-D iterator, so in
> general it only looks for the 2N nearest neighbors of a pixel.
>
> It should be relatively easy to modify this iterator in order
> to make it visit another neighborhood structure (e.g. 26 in 3D).
>
> You will find the neighborhood related code in lines 98 to 143
> of the file:
>
> Insight/Code/Common/itkFloodFilleFunctionConditionalIterator.txx
>
> By creating a variant of this iterator, it will be pretty straight
> forward to rebuild any of the region growing filters. (it will be
> basically a typedef change since all the walking process is encoded
> inside the iterator )
>
> Please let us know if you would like more details in how to
> implement this.
>
>
> Thanks
>
>    Luis
>
>
> ============================================================
>
> suresh kumar buddhavarapu wrote:
>
>> Hi friends,
>>
>> I'm working on finding on all connected components within avolume for 
>> a specific connectedness 26 or 6 .
>> I looked at RegionGrowingFilters. I unable find any clas or function 
>> to do it.
>> Can you help me...
>>
>> Thanks
>>
>> suresh
>>
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>
>
>
>
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-- 
Ross T. Whitaker, Assistant Professor
50 S. Central Campus Drive, Rm. 3190
University of Utah
Salt Lake City, UT  84112-9205
voice: 801/587-9549, fax: 801/581-5843
web: www.cs.utah.edu/~whitaker