Re: Watershed Algorithm source - bug

Posted by Gabriel Landini on
URL: http://imagej.273.s1.nabble.com/Watershed-Algorithm-source-bug-tp3685192p3685194.html

On Friday 25 Mar 2011 09:58:00 you wrote:
> a side note:
> There is also an alternative to this plugin using Built-in ImageJ
> commands:
> Process>Binary>Watershed, then Process>Binary>Voronoi, and
> thresholding the result at 1-255

Just being picky :-)
The watershed command in ImageJ is not strictly the watershed we are talking
about, it is "watershed separation". Watershed is a greyscale operation, not
binary...

It is useful to perform watersheds on greyscale images with "markers".
Markers (derived from the greyscale image) are used to avoid oversegmentation.
"Impose minima" (which depends of greyscale reconstruction) and "extended
regional minima" can be used for marking the images, so no false basins are
detected.

Going back to the original subject, I tried the suggested fix and indeed it
makes some dam lines thinner, but I do not think this algorithm is correct
anyway. Strangely there is a comment in the source code that this is the
original method, why was the <= added is a mystery to me.

But anyway, a major problem I see is that there are too many horizontal lines.
If you rotate the original image 90 degrees and apply the watershed, the
result is nowhere near to the un-rotated one. Of course I accept that there
might be tiny differences because of the sequential way that the image is
processed, but the observed results cannot be correct.

Daniel Sage's watershed is I think correct, but unfortunately it is slow on
large images and very few times I get some strange patterns (like pixels
isolated inside another basin which should not be there).

Cheers

Gabriel