[Insight-users] Question about Fast Marching Minimal Path Extraction in ITK

[Dan Mueller]

Hi Olga,

Seems you have made good progress.

> if ( m_WayTerminationValue==NULL )
If WayTerminationValue is a double or float, I don't think this line
will work as you expect. I think you will need a separate private bool
member m_HasUserSetWayTerminationValue which flags the state of the
value.

> marching->SetTargetOffset( 2.0 * Superclass::m_TerminationValue );
This line of code specifies how far passed the termination value the
front should expand. This ensures that the front totally surrounds the
point so that we can back track with gradient descent (i.e. the point
needs neighbouring arrival values to compute the gradient). You should
set this to the 2 * max(TerminationValue, WayTerminateValue).

HTH

Cheers, Dan

On 19 July 2011 01:16, Olga C Avila-Montes <ocamcaro at gmail.com> wrote:
> Hi Dan!
> Thank you very much for your fast reply, the explanation was very helpful. .
> .
> I *think* I was able to make the changes you suggested by doing the
> following:
> I changed the second if statement at SpeedFunctionToPathFilter::Execute
> from:
>
>       else if ( currentValue >= Superclass::m_TerminationValue )
>
> to:
>
>       else if ( (currentValue >= Superclass::m_TerminationValue &&
>                m_Info[Superclass::m_CurrentOutput].HasNextFront())
>                || (currentValue >= m_EndTerminationValue &&
>                !m_Info[Superclass::m_CurrentOutput].HasNextFront()) )
>
> If I understood correctly, the flag
> m_Info[Superclass::m_CurrentOutput].HasNextFront() tells me if we are
> tracing a segment to a way point (when it is "true") or if we are tracing to
> the end point (when it is "false")
> So, I used it to decide if we use the normal Termination Value or the End
> Termination Value.
>
> I also added an if in the function SpeedFunctionToPathFilter::GenerateData
> that checks whether o not the user input an EndTerminationValue, and if
> there is no input, the EndTerminationValue would take the value of
> TerminationValue:
>
>       if ( m_WayTerminationValue==NULL )
>         {
>         m_EndTerminationValue = Superclass::m_TerminationValue;
>         }
>
> Now, I have a question:
> In the function SpeedFunctionToPathFilter::ComputeArrivalFunction there is a
> line that says:
>
>      marching->SetTargetOffset( 2.0 * Superclass::m_TerminationValue );
>
> and with the changes I made, it will always take the normal Termination
> Value. Do you think I need to modify this in any way?
>
> Thanks!
>
> -----
> Olga C Avila-Montes
> Research Assistant, Computational Biomedicine Lab (www.cbl.uh.edu)
> Dept. of Computer Science, University of Houston, Houston, TX 77204-3010
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