Watershed algorithm

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The watershed algorithm is an image processing segmentation algorithm 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.