The question has been posted on StackExchange. https://mathematica.stackexchange.com/questions/160304/why-this-simple-case-of-watershed-segmentation-fails.
I tried a simple experiment today with WatershedComponents. I took this image from: https://fr.mathworks.com/help/images/ref/watershed.html (readers should use the image from the link above and crop the image themselves, rather than using the image directly below)
then I binarized the image
bin = Binarize[img];
the distance transform of the image yields dist:
dist = DistanceTransform[bin]//ImageAdjust;
Now to determine the seeds:
seeds = MaxDetect[dist]; (* notice two nice seeds in the image *)
finally using WatershedComponents i get this:
WatershedComponents[bin, seeds] // Colorize
Note: This is clearly how they should not be segmented. In contrast please see the segmentation performed by Matlab (link mentioned above). I tried using different methods but could not get a proper segmentation. What am i doing wrong?
Try with applying the watershed segmentation directly to the ridge lines:
ridgelines = RidgeFilter[bin, 3] // ImageAdjust
wsc = WatershedComponents[ridgelines, seeds];
wsc // Colorize
HighlightImage[img, wsc // Image // Binarize // ColorNegate]