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Watershed

lastmod 2023-10-19created 2022-07-127 edits

In the study of image processing, 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.

About

An image processing algorithm used to quickly segment an image into homogeneous regions.

  • Based on the idea that an image shows topographic relief.
  • Homogeneous regions correspond to flat basins delimited by relatively sharp edges.
  • That is, the original version of the watershed algorithm over-segments the image, producing many small regions.
  • Uses a predefined set of markers that guide the definition of the image segmentation.

Watershed is one of the image segmentation algorithms. It first appeared in 1979 in the paper "Use of Watershed in Contour Detection". It treats the image's pixel values as 2D terrain and fills it with water in the vertical direction.

Methods for segmenting images fall into two broad categories.

Edge-based (edge-based image segmentation)
A method that extracts meaningful regions based on boundaries. Because it focuses on detecting boundaries, it is sensitive to noise and has the drawback that boundaries come out broken, i.e., it is hard to obtain boundaries in closed-curve form.
Region-based (region-based image segmentation)
A method based on the similarity of intensity between pixels, suitable for segmenting objects in noisy environments. However, it is not suitable for images with blurring.

Watershed is a region-based algorithm.

  • Pixels with high values are called peaks (Peak) or watershed lines (WaterShed Line),
  • and pixels with low values are called valleys (Valley) or regional minima (Regionalminimum). It treats the set of pixels in the image as terrain and analyzes the elevations.

It is divided into two main methods.

  • The first is the Rainfall method, which drops (?) water droplets from high points of the terrain to form pools of water in low valleys,
  • The second is the Flooding method, which, without dropping water, fills water starting from the low valleys to form pools of water.

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