[Insight-developers] writing filters to support scalar and vector image types

[Nicholas Tustison]

Hi Kris,

For what it's worth, I was faced with the same conundrum in writing the
itkBSplineScatteredDataPointSetToImageFilter.  I opted for your first
choice since it was the easiest.  

Nick



On Sep 1, 2011, at 1:59 PM, Kris Zygmunt wrote:

> Hello,
>   I am currently writing a filter that will need to support both scalar pixels and vector pixels.  What is the recommended approach for handling this?  Are there any best-of-class examples to look at for guidance?  I will be assuming that the components of the vector are independent and thus be performing component-wise math such as addition, subtraction, multiplication, division, minimization, and maximization on each channel separately.  I see a few different approaches possible:
> 
> 1.  I could easily write a filter that only works with vector pixels, but this would require the user to take any scalar image (greyscale, T1 MR, T2 MR, etc) and convert it to a vector (On the input, they could use a scalar image as vector image accessor.  For the output image, they would also have to convert it back to a scalar image.)  This approach seems overly complicated for users with scalar images.
> 
> 2.  I can provide both a scalar and vector implementation of the filter, but this will have a ton of redundancy, making maintenance more complicated and error-prone.
> 
> 3.  I could provide (maybe through numeric_traits??) a definition of operator*, operator/, etc that would do the component-wise operation.  If this is undesirable due to ambiguity, I could also provide ComponentMultiply(val1, val2), ComponentDivide(val1, val2), etc that would have the appropriate specializations for both scalar data (would just default to operator* in this case) and vector data.  And then call these functions as appropriate.  Then for the special cases like finding min/max for each channel, I could call the appropriate specialization (for scalar just use MinimumMaximumImageFilter, for vector, loop over the components, calling MinimumMaximumImageFilter for each channel independently using an accessor/filter that extracts the components).
> 
> 
> Also, my approach for supporting RGB pixels is going to be to treat each color component as an independent channel (I'm writing a nonlocal means denoising filter).  Is the ITK-preferred approach to have the user provide the data as a VectorImage in this case, or should I also apply the chosen strategy (1, 2, or 3 above) for RGB pixels as a third class to consider, separate from scalar and vector?
> 
> Thanks!
> Kris
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