[Insight-users] is the ITK consistent regarding signed distance functions ?

[Amardeep Singh]

Hi Oliver

The convention is that signed distance functions for level sets are 
negative inside the object and positive outside the object. The Software 
Guide is not correct here which made me suffer a lot at some point.
If you look at the header files of the level set code - there it is 
written correctly in the comments.
In general, it is always good if you check the Doxygen documentation, as 
well, before you start using a new function/class/...

HTH
Amardeep

Oliver Gloger wrote:
> Hello,
>
> I would like to use the GeodesicActiveContourLevelSetImageFilter and refine its result with the CannySegmentationLevelSetImageFilter. (2D-images first!!!)
>
> Hence I combined the Example-Code for those 2 filters:
>
> I passed the output of the GeodesicActiveContourLevelSetImageFilter as initialModel (initial signed distance function) to the CannySegmentationLevelSetImageFilter.
>
> However, I wondered If I look at the ouputs (I visualized them via png-images) of the FastMarchingImageFilter and the GeodesicActiveContourLevelSetImageFilter: they have lower (I assume:negative) values inside and higher values (I assume positive) outside. Contrarily to the convention that is used in the ITKSoftwareGuide (Fig.9.13)!!!
>
> Hence, the ouputs (after BinaryThresholding given in the example-code) of the GeodesicActiveContours are quite well (inside =="white", outside=="dark").  But the subsequent binary-outputs of the CannySegmentationLevelSetImageFilter are different (inside =="dark", outside=="white")! And the results are worse than the results of the GeodesicActiveContourLevelSetImageFilter.
>
> So, my question is: 
>
> What is the convention in the ITK for signed distance functions? Inside the zero level set >0 like in Fig. 9.13 or the opposite? Is the ITK not consistent here??????
>
> Or did I make another mistake? 
>
> Best regards
>
> Oliver
>
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