[Insight-users] Fwd: Seed Value for CollidingFrontsImageFilter

[Luca Antiga]

Bryn, Dan,
  Dan is right, seed values are directly passed on to the two fast  
marching filters,
therefore the arrival times of the two fronts will have an offset  
according to the
seed value you use. However, the filter than takes the gradient  
vectors of the
arrival times of the two fronts (in order to take the dot product  
between them),
so any offset brought in by the seed values is effectively canceled  
out by the
the gradient operator. Hence, you get no change by changing seed values.
Hope this clarifies it.
Best regards

Luca



On Feb 12, 2009, at 11:15 AM, Dan Mueller wrote:

> Forward to Insight-users (with attachments removed to avoid bounce).
>
> ---------- Forwarded message ----------
> From: Dan Mueller <dan.muel at gmail.com>
> Date: 2009/2/12
> Subject: Re: [Insight-users] Seed Value for CollidingFrontsImageFilter
> To: blloyd at vision.ee.ethz.ch
> Cc: Insight-users at itk.org
>
> Hi Bryn,
>
> The seed value is used by the underlying
> FastMarchingUpwindGradientImageFilter. The output of a
> FastMarchingImageFilter is an arrival function, which specifies at
> what "time" the expanding front reaches a given point. The seedValue
> is used as the initial front, so changing this value changes the time
> the front reached the seedPosition. Changing this value will result in
> a (slightly) different arrival function (most likely it will be offset
> by the value you assigned, eg. 500).
>
> From my understanding, the CollidingFrontsImageFilter computes two
> arrival functions (for two different initial seeds). From these the
> gradient is computed, then the dot product, and finally thresholded
> for values when the dot product > 0. Therefore I suspect that you will
> not see any difference in the output of the CollidingFrontsImageFilter
> for different seed values -- the gradient information extracted from
> the arrival functions will be the same, even they are slightly offset.
>
> For your reference, I have attached two images showing what happens
> with different seed values using the FastMarchingImageFilter.
>
> Hope this helps.
>
> Regards, Dan
>
> 2009/2/12 Bryn Lloyd <blloyd at vision.ee.ethz.ch>:
>> Hi,
>>
>> Does the Value at the seed points used in the  
>> CollidingFrontsImageFilter
>> have any meaning or can/should it be ignored?
>> http://www.itk.org/Doxygen/html/classitk_1_1CollidingFrontsImageFilter.html
>>
>> I have modified the CollidingFrontsImageFilterTest to use different  
>> seed
>> values, but cannot find any difference in the result.
>>
>> In the ITK manual the value is set to zero, but no explanation is  
>> given.
>>
>> NodeType node;
>> const double seedValue = 0.0;
>> node.SetValue( seedValue );
>> node.SetIndex( seedPosition );
>>
>> Thanks
>> Bryn
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--
Luca Antiga, PhD
Head, Medical Imaging Unit,
Biomedical Engineering Department,
Mario Negri Institute.
mail: Villa Camozzi, 24020, Ranica (BG), Italy
phone: +39 035 4535-381
email: antiga at marionegri.it
web: http://villacamozzi.marionegri.it/~luca

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