# bwlabeln

Label connected components in binary image

## Description

example

L = bwlabeln(BW) returns a label matrix, L, containing labels for the connected components in BW.

L = bwlabeln(BW,conn) returns a label matrix, where conn specifies the connectivity.

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

## Examples

collapse all

Create simple sample 3-D binary image.

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])
BW =
BW(:,:,1) =

1     1     0
0     0     0
1     0     0

BW(:,:,2) =

0     1     0
0     0     0
0     1     0

BW(:,:,3) =

0     1     1
0     0     0
0     0     1

Label connected components in the image.

bwlabeln(BW)
ans =
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

## Input Arguments

collapse all

Binary image, specified as a numeric or logical array of any dimension. For numeric input, any nonzero pixels are considered to be on.

Example: BW = imread('text.png');

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | logical

Pixel connectivity, specified as one of the values in this table. The default connectivity is 8 for 2-D images, and 26 for 3-D images.

Value

Meaning

Two-Dimensional Connectivities

4

Pixels are connected if their edges touch. Two adjoining pixels are part of the same object if they are both on and are connected along the horizontal or vertical direction.

Current pixel is shown in gray.

8

Pixels are connected if their edges or corners touch. Two adjoining pixels are part of the same object if they are both on and are connected along the horizontal, vertical, or diagonal direction.

Current pixel is shown in gray.

Three-Dimensional Connectivities

6

Pixels are connected if their faces touch. Two adjoining pixels are part of the same object if they are both on and are connected in:

• One of these directions: in, out, left, right, up, and down

Current pixel is center of cube.

18

Pixels are connected if their faces or edges touch. Two adjoining pixels are part of the same object if they are both on and are connected in:

• One of these directions: in, out, left, right, up, and down

• A combination of two directions, such as right-down or in-up

Current pixel is center of cube.

26

Pixels are connected if their faces, edges, or corners touch. Two adjoining pixels are part of the same object if they are both on and are connected in:

• One of these directions: in, out, left, right, up, and down

• A combination of two directions, such as right-down or in-up

• A combination of three directions, such as in-right-up or in-left-down

Current pixel is center of cube.

For higher dimensions, bwlabeln uses the default value conndef(ndims(BW),'maximal').

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

Data Types: double | logical

## Output Arguments

collapse all

Label matrix, returned as an array of nonnegative integers with the same size as BW. 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.

Data Types: double

Number of connected objects in BW, returned as a nonnegative integer.

Data Types: double

## Tips

• The functions bwlabel, bwlabeln, and bwconncomp all compute connected components for binary images. bwconncomp replaces the use of bwlabel and bwlabeln. It uses significantly less memory and is sometimes faster than the other functions.

FunctionInput DimensionOutput FormMemory UseConnectivity
bwlabel2-DLabel matrix with double-precisionHigh4 or 8
bwlabelnN-DDouble-precision label matrixHighAny
bwconncompN-DCC structLowAny
• To extract features from a binary image using regionprops with default connectivity, just pass BW directly into regionprops using the command regionprops(BW).

## Algorithms

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.

## References

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

## Version History

Introduced before R2006a

expand all