- Watershed (algorithm)
The watershed algorithm is an

image processing segmentationalgorithm that splits an image into areas, based on the topology of the image. The length of the gradients is interpreted as elevation information. During the successive flooding of the grey value relief, watersheds with adjacent catchment basins are constructed. This flooding process is performed on the gradient image, i.e. the basins should emerge along the edges. Normally this will lead to an over-segmentation of the image, especially for noisy image material, e.g. medical CT data. Either the image must be pre-processed or the regions must be merged on the basis of a similarity criterion afterwards.A hierarchic watershed transformation converts the result into a graph display (i.e. the neighbor relationships of the segmented regions are determined) and applies further watershed transformations recursively. A problem is that the watersheds will increase in width.

The marker based watershed transformation performs flooding starting from specific marker positions which have been either explicitly defined by the user or determined with morphological operators.

Interactive watershed transformations allow to determine include and exclude points to construct artificial watersheds. This can enhance the result of segmentation.

**Bibliography*** Serge Beucher and Christian Lantuéjoul. Use of watersheds in contour detection. In "International workshop on image processing, real-time edge and motion detection" (1979).

* Serge Beucher and Fernand Meyer. The morphological approach to segmentation: the watershed transformation. In "Mathematical Morphology in Image Processing" (Ed. E.R. Dougherty), pages 433-481 (1993).

* Fernand Meyer. Un algorithme optimal pour la ligne de partage des eaux. Dans "8e congrès de reconnaissance des formes et intelligence artificielle", Vol. 2 (1991), pages 847-857, Lyon, France.

* Luc Vincent and Pierre Soille. Watersheds in digital spaces: an efficient algorithm based on immersion simulations. In "IEEE Transactions on Pattern Annalysis and Machine Intelligence, Vol. 13, Num. 6 (1991), pages 583-598 .

* L. Najman and M. Schmitt. Watershed of a continuous function. In "Signal Processing (Special issue on Mathematical Morphology.)", Vol. 38 (1994), pages 99-112.

* L. Najman and M. Schmitt. Geodesic saliency of watershed contours and hierarchical segmentation. In "IEEE Transactions on Pattern Analysis and Machine Intelligence", Vol. 18, Num. 12 (1996), pages 1163-1173.

* J.B.T.M. Roerdink and A. Meijster. The watershed transform: definitions, algorithms, and parallelization strategies. In "Fundamenta Informaticae" 41 (2000), pp. 187-228.

* Laurent Najman, Michel Couprie and Gilles Bertrand. Watersheds, mosaics, and the emergence paradigm. In "Discrete Applied Mathematics", Vol. 147, Num. 2-3(2005), Pages 301-324 .

**External links*** [

*http://cmm.ensmp.fr/~beucher/wtshed.html The Watershed Transformation*] with animations of the watershed algorithm.

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