Watershed algorithm: why is my 4-connected watershed 8-connected?

Hello,

I did watershed segmentation of my image using the distance transform. I wonder why I get a 8-connected watershed when specifying a 4-connectivty and vice versa?

i_ws4 = watershed(im, 4);
imshow(i_ws4)

i_ws8 = watershed(im);
imshow(i_ws8)

Thanks

Answer 1

[Image Analyst]

With your top image, the four white blobs are separated according to 4-connectivity. That is, they touch at the pixel corners but that is not considered to be a connection like they're the same blob.

However with 8 connectivity they WOULD be connected, so it had to make the black lines larger so that the four white blobs don't touch each other, even at the pixel corners.