MATLAB: Behavior of imclose() function

Question

The morphological close operation is a dilation followed by an erosion, using the same structuring element for both operations. Using a logical matrix a as input with a square structure element,

a = [0   0   1   0   0;
    1   1   1   1   1;
    0   1   0   1   0;
    1   0   0   0   1;
    0   0   0   0   0];
se = strel('square', 3);

The result I obtained after performing a dilation, imdilate() followed by an erosion, imerode() on a:

a = imdilate(a, se);
a = imerode(a, se);
% output

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

The result I obtained when applying imclose() directly on a:

a = imclose(a, se);
% output
   0   0   1   0   0
   1   1   1   1   1
   1   1   1   1   1
   1   0   0   0   1
   0   0   0   0   0

Did I miss out something? Why the outputs are different?

Answer 1

Looks like imclose() assumes that the "missing" pixels outside the image are 0 while imerode() assumes they're 1.