Questions tagged [watershed]

A grey-level image may be seen as a topographic water relief, where the grey level of a pixel in the image is interpreted as the height within the relief. A drop of water falling on a topographic relief flows along a path to finally reach a local minimum. The watershed of a relief corresponds to the limits of the nearby catchment basins of the drops of water.

In image processing, different types of watershed lines may be computed. There are also many different algorithms to compute watersheds. Watershed algorithm is used in image processing primarily for image segmentation purposes.

Below is a visualization of the MRI of a heart visualized in a topographical way.

http://upload.wikimedia.org/wikipedia/commons/d/db/Relief_of_gradient_of_heart_MRI.png

^{Source: Wikipedia}

These roughly correspond to the edge strength of the image, and the gradient of the image is shown below.

http://upload.wikimedia.org/wikipedia/commons/e/ea/Gradient_of_MRI_heart_image.png

^{Source: Wikipedia}

Watersheds essentially calculate the final edges that are essentially the basins of where the water collects, and an example is shown below.

http://upload.wikimedia.org/wikipedia/commons/0/0d/Watershed_of_gradient_of_MRI_heart_image.png

^{Source: Wikipedia}

Finally, the topographical visualization of the gradient (or the watershed) is shown below. The previous image is inevitably the result that is being sought, but the output of the watershed algorithm is what is shown below.

http://upload.wikimedia.org/wikipedia/commons/0/0f/Relief_view_of_the_watershed_of_the_gradient_of_an_MRI_heart_image.png

^{Source: Wikipedia}

For more information about the different watershed algorithms that exist, check out the following links:

• Watershed algorithms - CV Online
• Grouping and Segmentation Course Notes - University of Illinois
• The watershed concept and its use in segmentation: a brief history
• Advanced Morphological Segmentation