[Insight-users] Behavior of FastMarchingImageFilter

[Amardeep Singh]

Hello everybody:
Dear Dan:

Thank you very much for your help!

I have attached some sample code and a binary image of a circle for testing.
The program can be called like the following:
fast_marching_itk_mailing_list ./circle.nii.gz 
./fast_marching_output.nii.gz 200

The last number is the stopping value which I usually set to large 
values. I use
the fast marching filter in this example with a constant speed of 1 so 
that the result should be some kind of a distance map.

Certainly, one important point is my calculation of the alive and trial 
points. All the points of the circle that have a background voxel within 
their
6-neighborhood are considered trial points (so the last layer around the 
circle becomes a layer of trial points). All other points of the circle 
are alive points.
I have attached a screenshot of the 0 isoline of the result (the 
complete output image was too large for the mailing list, 
unfortunately). When you look at it, you can see the jagged edge again. 
But still: Shouldn't it propagate evenly from the trial points?
The intialization (0 isoline of the result) looks jagged already.

Concerning my previous snapshot:
The output image was of type float. Just for the snapshopt, I mapped all 
values > 0 to 1 in order to visualize the jagged edge.

Best regards
Amar


Dan Mueller wrote:
> Hi Amar,
>
> Perhaps you can post a minimal example (code, input image, output
> image) demonstrating the issue.
>
> I have not experienced this issue myself, but the
> FastMarchingImageFilter does propagate the front using
> 4-/6-connectivity, so after a small timestep it may be possible to
> experience the "jagged" edge you are seeing (though not the checked
> pattern inside the hole). How many iterations are you letting the
> filter run for? (ie. what is the stopping value?)
>
> Also, the FastMarchingImageFilter requires a real (float, double)
> output image. What is the pixel type of your output image?
>
> Cheers, Dan
>
> 2009/5/7 Amardeep Singh <amar.singh at gmx.de>:
>   
>> Hello everyone
>>
>> I am wondering about the behavior of the FastMarchingImageFilter.
>> I want to let the algorithm march from a certain surface which is
>> represented by a binary image. I
>> pass the points of the object as alive points to the marcher. The layer
>> outside of the object (in my case: a brain)
>> is passed as trial points. When I look at the output of the fast
>> marching filter, there is something strange -> see attached screenshot.
>>
>> The area within the yellow line is my initial object. So they are all
>> alive points whose value is 0 in the output.  But when you look at the layer
>> outside of the inner object, you see that it is jagged, black (value 0)
>> and white (value != 0) voxels take turns. Why is this the case?
>> What do I need to do, so that the marcher marches equally away from the
>> alive points. The speed image values on the layer around the alive
>> points do have the same value, so this is not the reason.
>>
>> Thanks a lot in advance!
>>
>> Best Regards
>> Amar
>>
>>
>>
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