| Image Processing Toolbox | |
Label connected components in a binary image
Syntax
L = bwlabel(BW,n) [L,num] = bwlabel(BW,n)
Description
L = bwlabel(BW,n) returns a matrix L, of the same size as BW, containing labels for the connected objects in BW. n can have a value of either 4 or 8, where 4 specifies 4-connected objects and 8 specifies 8-connected objects; if the argument is omitted, it defaults to 8.
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.
[L,num] = bwlabel(BW,n) returns in num the number of connected objects found in BW.
Class Support
The input image BW can be of class double or uint8. The output matrix L is of class double.
Remarks
You can use the MATLAB find function in conjunction with bwlabel to return vectors of indices for the pixels that make up a specific object. For example, to return the coordinates for the pixels in object 2
[r,c] = find(bwlabel(BW)==2)
You can display the output matrix as a pseudocolor indexed image. Each object appears in a different color, so the objects are easier to distinguish than in the original image. To do this, you must first add 1 to each element in the output matrix, so that the data is in the proper range. Also, it is good idea to use a colormap in which the first few colors are very distinct.
Example
This example illustrates using 4-connected objects. Notice objects 2 and 3; with 8-connected labeling, bwlabel would consider these a single object rather than two separate objects.
BW = [1 1 1 0 0 0 0 0
1 1 1 0 1 1 0 0
1 1 1 0 1 1 0 0
1 1 1 0 0 0 1 0
1 1 1 0 0 0 1 0
1 1 1 0 0 0 1 0
1 1 1 0 0 1 1 0
1 1 1 0 0 0 0 0]
L = bwlabel(BW,4)
L =
1 1 1 0 0 0 0 0
1 1 1 0 2 2 0 0
1 1 1 0 2 2 0 0
1 1 1 0 0 0 3 0
1 1 1 0 0 0 3 0
1 1 1 0 0 0 3 0
1 1 1 0 0 3 3 0
1 1 1 0 0 0 0 0
[r,c] = find(L==2);
rc = [r c]
rc =
2 5
3 5
2 6
3 6
See Also
Reference
Haralick, Robert M., and Linda G. Shapiro. Computer and Robot Vision, Volume I. Addison-Wesley, 1992. pp. 28-48.
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