Watershed (image processing)

In the study of , a watershed is a transformation defined on a grayscale image. The name refers metaphorically to a geological watershed, or drainage divide, which separates adjacent drainage basins. The watershed transformation treats the image it operates upon like a topographic map, with the brightness of each point representing its height, and finds the lines that run along the tops of ridges. There are different technical definitions of a watershed. In graphs, watershed lines may be defined on the nodes, on the edges, or hybrid lines on both nodes and edges. Watersheds may also be defined in the continuous domain. There are also many different algorithms to compute watersheds. Watershed algorithms are used in image processing primarily for object purposes, that is, for separating different objects in an image. This allows for counting the objects or for further analysis of the separated objects. Image:Relief of gradient of heart MRI.png|Relief of the gradient magnitude Image:Gradient of MRI heart image.png|Gradient magnitude image Image:Watershed_of_gradient_of_MRI_heart_image.png|Watershed of the gradient Image:Relief view of the watershed of the gradient of an MRI heart image.png|Watershed of the gradient (relief) In geology, a watershed is a divide that separates adjacent catchment basins. The idea was introduced in 1979 by S. Beucher and C. Lantuéjoul.Serge Beucher and Christian Lantuéj workshop on image processing, real-time edge and motion detection (1979). The basic idea consisted of placing a water source in each regional minimum in the relief, to flood the entire relief from sources, and build barriers when different water sources meet. The resulting set of barriers constitutes a watershed by flooding. A number of improvements, collectively called Priority-Flood, have since been made to this algorithm. Intuitively, a drop of water falling on a topographic relief flows towards the "nearest" minimum. The "nearest" minimum is that minimum which lies at the end of the path of steepest descent.