Why disk structuring element of disk is 4? It's usually 3 or 21

SE = strel("disk",4);

Answer 1

[DGM]

The size of the structuring element can be whatever is suitable for the task at hand. That's why it's user-specified. While strel() accepts this parameter as an integer-valued radius, bear in mind that the actual structuring element will always have odd diameter. In that sense, a radius of 4 shouldn't seem strange in that it's even. The array size is still odd.

st = strel('disk',4);
st.Neighborhood

ans = 7×7 logical array
   0   0   1   1   1   0   0
   0   1   1   1   1   1   0
   1   1   1   1   1   1   1
   1   1   1   1   1   1   1
   1   1   1   1   1   1   1
   0   1   1   1   1   1   0
   0   0   1   1   1   0   0

[Hoang]

Even I change radius to 1 or example: 50, it still works, right?

[DGM]

It's still odd-sized. Whether that "works" for any given task is something that can't be known until the task is defined.

st = strel('disk',1);
size(st.Neighborhood)

ans = 1×2

3 3

st = strel('disk',50);
size(st.Neighborhood)

ans = 1×2

99 99

[Hoang]

I try st.Neighborhood. It says "No appropriate method, property, or field 'Neighborhood' for class 'strel'."

[Hoang]

Oh never mind. I did it. Thanks for your help