Image Processing Toolbox User's Guide

bwlabeln

Label connected components in N-D binary image

Syntax

• L = bwlabeln(BW)
  [L,NUM] = bwlabeln(BW)
  [L,NUM] = bwlabeln(BW,CONN)
  

Description

L = bwlabeln(BW) returns a label matrix L containing labels for the connected components in BW. BW can have any dimension; L is the same size as BW. The elements of L are integer values greater than or equal to 0. The pixels labeled 0 are the background. The pixels labeled 1 make up one object, the pixels labeled 2 make up a second object, and so on. The default connectivity is 8 for two dimensions, 26 for three dimensions, and conndef(ndims(BW), 'maximal') for higher dimensions.

[L,NUM] = bwlabeln(BW) returns in NUM the number of connected objects found in BW.

[L,NUM] = bwlabeln(BW,CONN) specifies the desired connectivity. CONN can have any of the following scalar values.

Value	Meaning
Two-dimensional connectivities
4	4-connected neighborhood
8	8-connected neighborhood
Three-dimensional connectivities
6	6-connected neighborhood
18	18-connected neighborhood
26	26-connected neighborhood

Connectivity can also be defined in a more general way for any dimension by using for CONN a 3-by-3-by- ...-by-3 matrix of 0's and 1's. The 1-valued elements define neighborhood locations relative to the center element of CONN. Note that CONN must be symmetric about its center element.

Remarks

bwlabel supports 2-D inputs only; bwlabeln supports inputs of any dimension. In some cases, you might prefer to use bwlabeln even for 2-D problems because it can be faster. If you have a 2-D input whose objects are relatively thick in the vertical direction, bwlabel is probably faster; otherwise bwlabeln is probably faster.

Class Support

BW can be numeric or logical, and it must be real and nonsparse. L is of class double.

Example

• BW = cat(3,[1 1 0; 0 0 0; 1 0 0],...
  [0 1 0; 0 0 0; 0 1 0],...
  [0 1 1; 0 0 0; 0 0 1])
  
  bwlabeln(BW)
  
  ans(:,:,1) =
  
       1     1     0
       0     0     0
       2     0     0
  
  
  ans(:,:,2) =
  
       0     1     0
       0     0     0
       0     2     0
  
  
  ans(:,:,3) =
  
       0     1     1
       0     0     0
       0     0     2
  

Algorithm

bwlabeln uses the following general procedure:

1. Scan all image pixels, assigning preliminary labels to nonzero pixels and recording label equivalences in a union-find table.
2. Resolve the equivalence classes using the union-find algorithm [1].
3. Relabel the pixels based on the resolved equivalence classes.

See Also

bwlabel, label2rgb

Reference

[1]  Sedgewick, Robert, Algorithms in C, 3rd Ed., Addison-Wesley, 1998, pp. 11-20.

bwlabel

bwmorph

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