Hi,

I have generated the shape of default disk-shape structuring elements one obtains with Matlab, for radius 1, 2, 3 4, 5 and 6.

For structuring elements with radius >= 3, different results may be obtained depending on the choice of approximation for decomposition of structuring elements. The displayed image correspond to the default value of 4.

Script:

```
img = zeros(15, 15);
img(8, 8) = 1;
imgDil1 = imdilate(img, strel('disk', 1));
imgDil2 = imdilate(img, strel('disk', 2));
imgDil3 = imdilate(img, strel('disk', 3));
imgDil4 = imdilate(img, strel('disk', 4));
imgDil5 = imdilate(img, strel('disk', 5));
imgDil6 = imdilate(img, strel('disk', 6));
res = [imgDil1 imgDil2 imgDil3 ; imgDil4 imgDil5 imgDil6];
figure; imshow(res);
```

In practice, I sometimes use structuring elements defined directly from the Euclidean distance to central pixel/voxel:

```
[x, y] = meshgrid(-5:5, -5:5);
R = 3; se = hypot(x, y) < R + .5;
```

This results in structuring elements with size equal to 2*R+1 in each dimension, that can be convenient for later processing or interpretation.