[Octave-bug-tracker] [bug #44396] bwmorph shrink does not preserve Euler number

[Cory Bloor]

URL:
  <http://savannah.gnu.org/bugs/?44396>

                 Summary: bwmorph shrink does not preserve Euler number
                 Project: GNU Octave
            Submitted by: cbloor
            Submitted on: Sun 01 Mar 2015 11:03:33 AM GMT
                Category: Octave Forge Package
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: Cory Bloor
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 3.8.1
        Operating System: GNU/Linux

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Details:

The documentation for bwmorph shrink states that it preserves the Euler
number, but I've found a case where that doesn't seem to be true.


octave:1> version
ans = 3.8.1
octave:2> in= [0, 0, 0, 1, 0;
>   1, 1, 1, 1, 0;
>   0, 0, 1, 1, 0;
>   0, 0, 1, 1, 0;
>   0, 0, 0, 1, 0]
in =

   0   0   0   1   0
   1   1   1   1   0
   0   0   1   1   0
   0   0   1   1   0
   0   0   0   1   0

octave:3> out=bwmorph(in, 'shrink')
out =

   0   0   0   0   0
   0   1   1   1   0
   0   0   0   0   0
   0   0   0   1   0
   0   0   0   1   0

octave:4> bweuler(in)
ans =  1
octave:5> bweuler(out)
ans =  2


I discovered this while implementing shrink in C++ from the book "Digital
Image Processing" by William K. Pratt. I was surprised by my result, so I
checked it against Octave's result. I was then very surprised when Octave's
result matched mine. I think we both might be making the same mistake.




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