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Label connected components in 2-D binary image

`L = bwlabel(BW)`

`L = bwlabel(BW,conn)`

```
[L,n]
= bwlabel(___)
```

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.

`bwlabel`

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

Run-length encode the input image.

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

Resolve the equivalence classes.

Relabel the runs based on the resolved equivalence classes.

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

`bwconncomp`

| `bwlabeln`

| `bwselect`

| `label2rgb`

| `labelmatrix`

| `regionprops`