The following code:
strel('ball',3,3)
returns a 2-dimensional nonflat structuring element. Nonflat structuring elements are usually used for gray-scale dilation. Gray-scale dilation for the position (x,y) is defined as:
max{ f(x-x',y-y') + b(x',y') | (x',y') in b}
where "b" is the structuring element, and "(x',y')" are the relative locations in "b"s neighborhood. The values "b(x',y')" correspond to the strel object's "height". In this context, the term "ball" refers to the interpretation of a 2-dimensional grayscale image as a surface, and passing a ball "under" the surface.
For 3-dimensional binary dilation, you will need to create a sphere-shaped structuring element:
[x,y,z] = ndgrid(-3:3);
se = strel(sqrt(x.^2 + y.^2 + z.^2) <=3);
