[vtk-developers] The ImageStencil Issue

David Gobbi dgobbi at atamai.com
Wed Aug 23 12:19:53 EDT 2006

Hi David,

Am I allowed to object? (Just kidding!).

The changes are only a few hours work, Kitware already did most of the 
work and only a few touch-ups remain.

After the changes, people will have to call 
SetInformationInputConnection() on whichever vtkImageStencilDataSource 
they use so there will be some deprecation.

 - David

David Cole wrote:
> I'd like to rephrase the main question of this discussion:
> Would anybody object to David Gobbi making stencils use the new 
> pipeline properly?
> And would anybody object to those changes being propagated into the 
> VTK-5-0 branch if possible?
> David Gobbi,
> Would it be a lot of work to make stencils use the new pipeline properly?
> Do you foresee any difficulty merging those changes into VTK-5-0...?
> I'm all for you making these changes: conforming to the new pipeline 
> is good in general and you are obviously the man for this particular job.
> If you're going to redress ugliness, you may as well redress it in the 
> 5.0 branch, too... I'd be happy to help with merging the changes into 
> the 5.0 branch after you commit them to CVS HEAD.
> Thanks,
> David Cole
> David Gobbi wrote:
>> Hi All,
>> I'd like to redress some ugliness that I introduced to VTK a few 
>> years back.
>> The vtkImageStencilSource class requires the Spacing and Origin of 
>> whichever image eventually uses the stencil to be propagated the 
>> wrong way along the pipeline.  Ouch!
>> Obviously the vtkImageStencilSource should actually take an Image as 
>> an input and should then use that image to get the Spacing and Origin 
>> to use for the stencil.  This is a fairly minor change.  For VTK 5.2, 
>> I can then make VTK produce a deprecation warning if people don't set 
>> an input.
>> I would call the method SetInformationInput() to make it clear that 
>> only the information (the Spacing, Origin, and WholeExtent) of the 
>> data would be used.  Also, vtkPolyDataToImageStencil() needs it 
>> SetInput() method for the polydata input.
>> For example:
>> stencilSource = vtkImplicitFunctionToImageStencil()
>> stencilSource.SetInput(vtkSphere())
>> stencilSource.SetInformationInput(input)
>> histogram = vtkImageAccumulate()
>> histogram.SetStencil(stencilSource.GetOutput())
>> histogram.SetInput(input)
>> Do people think this is a good change?  Are there many people out 
>> there who even use stencils in VTK?  How many people know what a 
>> stencil is?  (Hint: it is an image mask stored in an efficient 
>> run-length format instead of vtkImageData format).
>> Also, I'm not sure what the methods should be named in light of the 
>> VTK 5 pipeline changes.   Maybe I should name them as follows:
>> SetInformationInputConnection(vtkAlgorithmInput *) instead of 
>> SetInformationInput(vtkImageData *)
>> SetStencilInputConnection(vtkAlgorithmInput *) instead of 
>> SetStencil(vtkImageStencilData *)
>> I'd like to get this right, and I think that only small changes are 
>> necessary to do so.
>> Also, of course, I'd like to see more of the filters in Imaging 
>> utilizing stencils.  Right now only a handful do (Blend, Reslice, 
>> Stencil, Accumulate).  The potential here is very high... a stencil 
>> allows you to perform an operation only within a specified region of 
>> interest, where the shape of the region can be whatever you desire.  
>> The ROI can even be described by a vtkPolyData mesh or by a 
>> vtkImplicitFunction, since these types can be converted efficiently 
>> into vtkImageStencilData.
>> - David
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