[vtk-developers] The ImageStencil Issue

Karthik Krishnan Karthik.Krishnan at kitware.com
Mon Aug 28 11:41:49 EDT 2006

Karthik Krishnan wrote:

> Hi David,
> I find stencils very useful. I've been using them as a data structure 
> for representing segmentations.
> One of the things I would like to contribute to stencils is minkowski 
> operators or "set" operations on stencils. For instance union of two 
> stencils, stencilA-stencilB, intersection of stencils etc.. The need 
> for these arises when a 3D volume has a few disconnected hand-drawn 
> segmentations and as a user, I would like to discard a segment etc.. 
> I've been using these separately in a derived class, but I could move 
> them to the vtkImageStencilData class itself.

They've have been added to the vtkImageStencilData class.


> Another addition, I would hope to see would be grayscale stencils for 
> fuzzy segmentations. Imagine a brush widget dabbing a fuzzy / dithered 
> stroke and you would like to use the stencildata to represent opacity. 
> It would be great if the stencils could store a DataArray in addition 
> to maintaining the ExtentLists. This could possibly be done in a 
> sublcass.
> Thanks
> -karthik
> 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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