[Insight-developers] binary object and label definitions in ITK

[Gaetan Lehmann]

Hi,

While talking about types usable to manipulate binary objects and labels with 
Zach, we found that they don't seem to be clearly defined in ITK.

For example, ConnectedComponentImageFilter consider that binary objects are 
connected set of non-zero pixels, while BinaryDilateImageFilter use a value 
setted by the user to manipulate the binary object.
Also, while we can consider as label any value allowed by a type, some filters 
may fail if those values are negative.

I think we should clearly define those 2 notions to avoid inconsistency in the 
toolkit. I would like to propose the following ones - please correct and 
comment them:
- The binary objects are defined with a foreground value, by default 
itk::NumericTraits<PixelType>::max(), so we can manipulate several objects 
(labels). If a background value is needed, by default we should have always 
the same: itk::NumericTraits<PixelType>::NonnegativeMin() (or 
itk::NumericTraits<PixelType>::Zero, it just need to be chosen). That's what 
BinaryDilateImageFilter does, but it would break 
ConnectedComponentImageFilter behavior.
- labels are any values allowed by a type. If a type need to exclude some 
values, to define a background for example, the user should be able to set 
those values - it would break the RelabelComponentImageFilter behavior.

Comments are welcome !

Gaetan
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