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Watershed segmentation

segm_watershed.Rd

Applies the watershed segmentation algorithm to an image file or a SpatRaster

Usage

segm_watershed(
  image,
  otb,
  thresh = 0.01,
  level = 0.1,
  mode = "vector",
  vector_neighbor = FALSE,
  vector_stitch = TRUE,
  vector_minsize = 1L,
  vector_simplify = 0.1,
  vector_tilesize = 1024L,
  mask = NULL
)

Arguments

image

path or SpatRaster

otb

output of link2GI::linkOTB()

thresh

depth threshold units in percentage of the maximum depth in the image

level

flood level for generating the merge tree from the initial segmentation (from 0 to 1)

mode

processing mode, either 'vector' or 'raster'. See details

vector_neighbor

logical. If FALSE (the default) a 4-neighborhood connectivity is activated. If TRUE, a 8-neighborhood connectivity is used

vector_stitch

logical. If TRUE (the default), scans polygons on each side of tiles and stitch polygons which connect by more than one pixel

vector_minsize

integer. Objects whose size in pixels is below the minimum object size will be ignored during vectorization

vector_simplify

simplify polygons according to a given tolerance (in pixel). This option allows reducing the size of the output file or database.

vector_tilesize

integer. User defined tiles size for tile-based segmentation. Optimal tile size is selected according to available RAM if NULL

mask

an optional raster used for masking the segmentation. Only pixels whose mask is strictly positive will be segmented

Value

sf or SpatRaster

Details

The watershed segmentation algorithm is a region-based image segmentation technique inspired by topography. It treats the grayscale intensity of an image as a topographic surface, where brighter pixels represent peaks and darker pixels represent valleys. The algorithm simulates flooding of this surface to separate distinct regions. Steps:

  1. Topographic Interpretation: The input image is treated as a 3D landscape, where pixel intensity corresponds to elevation.

  2. Flooding Process: Starting from local minima, the algorithm simulates water flooding the surface. As the water rises, distinct regions (basins) are formed.

  3. Watershed Lines: When two basins meet, a boundary (watershed line) is formed to prevent merging.

  4. Region Labeling: Each basin is assigned a unique label, producing a segmented image where boundaries are clearly defined.

The processing mode 'vector' will output a vector file, and process the input image piecewise. This allows performing segmentation of very large images. IN contrast, 'raster' mode will output a labeled raster, and it cannot handle large data. If mode is 'raster', all the 'vector_*' arguments are ignored.

Examples

if (FALSE) { # \dontrun{
## load packages
library(link2GI)
library(OTBsegm)
library(terra)

## load sample image
image_sr <- rast(system.file("raster/pnoa.tiff", package = "OTBsegm"))

## connect to OTB (change to your directory)
otblink <- link2GI::linkOTB(searchLocation = "C:/OTB/")

## apply segmentation
results_ms_sf <- segm_watershed(
    image  = image_sr,
    otb    = otblink,
    thresh = .1,
    level  = .2
)
} # }