Connected components of an image or window

Finds the topologically-connected clumps of pixels in an image or window.

spatial, math
connected(X, background = NA, method="C")
Image (object of class "im") or window (object of class "owin").
Optional. Treat pixels with this value as being part of the background.
String indicating the algorithm to be used. Either "C" or "interpreted". See Details.

This function computes the connected component transform (Rosenfeld and Pfalz, 1966) of a binary image or binary mask. The argument X is first converted into a pixel image with logical values. Then the algorithm identifies the connected components (topologically-connected clumps of pixels) in the foreground.

Two pixels belong to the same connected component if they have the value TRUE and if they are neighbours (in the 8-connected sense). This rule is applied repeatedly until it terminates. Then each connected component contains all the pixels that can be reached by stepping from neighbour to neighbour.

If method="C", the computation is performed by a compiled C language implementation of the classical algorithm of Rosenfeld and Pfalz (1966). If method="interpreted", the computation is performed by an Rimplementation of the algorithm of Park et al (2000).

The result is a factor-valued image, with levels that correspond to the connected components. The Examples show how to extract each connected component as a separate window object.


  • A pixel image (object of class "im") with factor values. The levels of the factor correspond to the connected components.


It may be hard to distinguish different components in the default plot because the colours of nearby components may be very similar. See the Examples for a randomised colour map. The algorithm for method="interpreted" can be very slow for large images (or images where the connected components include a large number of pixels).


Park, J.-M., Looney, C.G. and Chen, H.-C. (2000) Fast connected component labeling algorithm using a divide and conquer technique. Pages 373-376 in S.Y. Shin (ed) Computers and Their Applications: Proceedings of the ISCA 15th International Conference on Computers and Their Applications, March 29-31, 2000, New Orleans, Louisiana USA. ISCA 2000, ISBN 1-880843-32-3.

Rosenfeld, A. and Pfalz, J.L. (1966) Sequential operations in digital processing. Journal of the Association for Computing Machinery 13 471-494.

See Also

im.object, tess

  • connected
  d <- distmap(cells, dimyx=256)
  X <- levelset(d, 0.06)
  Z <- connected(X)

  # number of components
  nc <- length(levels(Z))
  # plot with randomised colour map
  plot(Z, col=hsv(h=sample(seq(0,1,length=nc), nc)))

  # how to extract the components as a list of windows
  W <- tiles(tess(image=Z))
Documentation reproduced from package spatstat, version 1.25-1, License: GPL (>= 2)

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