Any grayscale image can be considered as a topographic surface. If this surface is flooded from its minima, and the merging of waters from different sources are prevented, and the image will be partitioned into two different sets, namely the catchment basins and the watershed lines. If this transform is applied to the image gradient, the catchment basins should theoretically correspond to the homogeneous gray-level regions (segments) of this image.
However, the image gets over-segmented using this transform in practice, because of noise or local irregularities in the gradient image. To prevent over-segmentation, a pre-defined set of markers are used and the flooding of the surface starts from these markers. ...
