Concise Watershed Algorithm

The watershed algorithm is a classic morphological image segmentation method that divides images into multiple regions by simulating the geographical watershed concept. This algorithm is particularly effective for segmenting touching or overlapping objects. Implementing a streamlined watershed algorithm in MATLAB typically involves three core steps: First is the preprocessing stage, where gradient transformation is applied to the input image to enhance edge features. Common implementation approaches include calculating gradient magnitude using Sobel or Canny operators to form the initial topographic map. In MATLAB code, this can be achieved with functions like `imgradient` or edge detection operators followed by gradient magnitude calculation. Next comes the core watershed transformation process. The algorithm treats the image as a topographic map where gradient values represent elevation, starting from minimum points for "flooding." Watershed lines form when water levels rise to potential merging points. MATLAB's built-in `watershed` function efficiently handles this process through marker-controlled watershed transformation, using regional minima as flooding starting points. Finally, post-processing addresses the common over-segmentation issue. Typical solutions include: 1) Using morphological opening and closing operations to smooth boundaries (implemented with `imopen` and `imclose` functions); 2) Marker-based improvement methods that predefine foreground and background markers using functions like `imregionalmin`; 3) Region merging strategies that combine similar small regions through techniques like region adjacency graph analysis.