bwlabel

Label connected components in 2-D binary image

Syntax

L = bwlabel(BW)

L = bwlabel(BW,conn)

[L,n] = bwlabel(___)

Description

L = bwlabel(BW) returns the label matrix L that contains labels for the 8-connected objects found in BW.

You optionally can label connected components in a 2-D binary image using a GPU (requires Parallel Computing Toolbox™). For more information, see Image Processing on a GPU.

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

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

Examples

Label Components Using 4-connected Objects

Create a small binary image.

BW = logical ([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]);

Create the label matrix using 4-connected objects.

L = bwlabel(BW,4)

L = 8×8

     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

Use the find command to get the row and column coordinates of the object labeled "2".

[r, c] = find(L==2);
rc = [r c]

rc = 4×2

     2     5
     3     5
     2     6
     3     6

Label Components Using 4-connected Objects on a GPU

Create a small binary image and create a gpuArray object to contain it.

BW = gpuArray(logical([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]));

Create the label matrix using 4-connected objects.

L = bwlabel(BW,4)

Use the find command to get the row and column coordinates of the object labeled "2".

[r,c] = find(L == 2)

Input Arguments

BW — Binary image

2-D numeric matrix | 2-D logical matrix | gpuArray

Binary image, specified as a 2-D numeric or logical matrix. For numeric input, any nonzero pixels are considered to be on.

To label connected components using a GPU, specify BW as a gpuArray that contains a 2-D numeric or logical matrix.

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

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

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

conn — Pixel connectivity

8 (default) | 4

Pixel connectivity, specified as one of these values.

Value	Meaning
Two-Dimensional Connectivities
4-connected	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.
8-connected	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.

Data Types: double | logical

Output Arguments

L — Label matrix

matrix of nonnegative integers | gpuArray

Label matrix of contiguous regions, returned as matrix 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.

If connected components are labeled using a GPU, then L is returned as a gpuArray containing a matrix of nonnegative integers.

Data Types: double

n — Number of connected objects

nonnegative integer

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.

  	Input Dimension	Output Form	Memory Use	Connectivity
  bwlabel	2-D	Double-precision label matrix	High	4 or 8
  bwlabeln	N-D	Double-precision label matrix	High	Any
  bwconncomp	N-D	CC struct	Low	Any

• 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, enter the following:.

  [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. For more information, see label2rgb.

• To compute a label matrix having a more memory-efficient data type (e.g., uint8 versus double), use the labelmatrix function on the output of bwconncomp.

• To extract features from a binary image using regionprops with default connectivity, just pass BW directly into regionprops by using the command regionprops(BW).

• The bwlabel function can take advantage of hardware optimization for data types logical, uint8, and single to run faster. Hardware optimization requires marker and mask to be 2-D images and conn to be either 4 or 8.

Algorithms

bwlabel uses the general procedure outlined in reference [1], pp. 40-48:

1. Run-length encode the input image.

2. Scan the runs, assigning preliminary labels and recording label equivalences in a local equivalence table.

3. Resolve the equivalence classes.

4. Relabel the runs based on the resolved equivalence classes.

References

[1] Haralick, Robert M., and Linda G. Shapiro, Computer and Robot Vision, Volume I, Addison-Wesley, 1992, pp. 28-48.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• bwlabel supports the generation of C code (requires MATLAB Coder™). For more information, see Code Generation for Image Processing.

• When generating code, the parameter n must be a compile-time constant.

GPU Arrays

Accelerate code by running on a graphics processing unit (GPU) using Parallel Computing Toolbox™.

This function fully supports GPU arrays. For more information, see Image Processing on a GPU.