[vtkusers] pipeline update

[Amy Squillacote]

Hi Adrian,

A call to Update() in VTK does propagate back to earlier filters in the 
pipeline. However, the pipeline checks whether the a filter is 
up-to-date before calling Update() on it, so as long as you don't make 
any changes to the instance of vtkImageGradientMagnitude between calling 
Update() on it and calling Update() on the instance of vtkImageGradient, 
each filter should perform its computation only once.

- Amy

Adrian Albert wrote:
>
> Dear All,
>
>
> I have a question about the pipeline update mechanism in VTK: If I 
> call Update () on an algorithm that has as input the output of another 
> algorithm, does the Update command propagate down the pipeline and 
> causes a recomputation for all algorithms involved?
> For example, this code should calculate the gradient of the gradient 
> magnitude (a measure of a second order derivative):
>
>   // Gradient magnitude
>   vtkImageGradientMagnitude* grad = vtkImageGradientMagnitude::New();
>   grad->SetDimensionality( 3 );
>   grad->SetInput (this->GetMultivariateField (whichField)); 
>   grad->Update ();
>
>   // Gradient of gradient magnitude
>   vtkImageGradient* gradGrad = vtkImageGradient::New();
>   gradGrad->SetDimensionality( 3 );
>   gradGrad->SetInput (grad->GetOutput ());
>   gradGrad->Update ();
>  
> Specifically, would I cause two computations of the gradient magnitude 
> (first 4 lines of code) by calling Update twice?
>
> Thanks!
> Adrian
>
> -- 
> ---------------------------------------------------
> Adrian Albert
> School of Engineering and Science
> Jacobs University Bremen
> Germany
> Tel.: 00494212003258
> ------------------------------------------------------------------------
>
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-- 
Amy Squillacote                    Phone: (256) 726-4839
Computer Scientist                 Fax: (256) 726-4806
CFD Research Corporation           Web: http://www.cfdrc.com
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Huntsville, AL  35805