# bwconncomp

Find connected components in binary image

## Description

example

CC = bwconncomp(BW) returns the connected components CC found in the binary image BW. bwconncomp uses a default connectivity of 8 for two dimensions, 26 for three dimensions, and conndef(ndims(BW),'maximal') for higher dimensions.

example

CC = bwconncomp(BW,conn) returns the connected components where conn specifies the desired connectivity for the connected components.

## Examples

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Create a small sample 3-D array.

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]);

Find the connected components in the array.

CC = bwconncomp(BW)
CC = struct with fields:
Connectivity: 26
ImageSize: [3 3 3]
NumObjects: 2
PixelIdxList: {[5x1 double]  [3x1 double]}

Calculate centroids of the objects in the array.

S = regionprops(CC,'Centroid')
S=2×1 struct array with fields:
Centroid

Read image into the workspace and display it.

imshow(BW)

Find the number of connected components in the image.

CC = bwconncomp(BW)
CC = struct with fields:
Connectivity: 8
ImageSize: [256 256]
NumObjects: 88
PixelIdxList: {1x88 cell}

Determine which is the largest component in the image and erase it (set all the pixels to 0).

numPixels = cellfun(@numel,CC.PixelIdxList);
[biggest,idx] = max(numPixels);
BW(CC.PixelIdxList{idx}) = 0;

Display the image, noting that the largest component happens to be the two consecutive f's in the word different.

figure
imshow(BW)

## Input Arguments

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

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-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.

Three-Dimensional Connectivities

6-connected

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

18-connected

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

26-connected

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

For higher dimensions, bwconncomp 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

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Connected components, returned as a structure with four fields.

FieldDescription
ConnectivityConnectivity of the connected components (objects)
ImageSizeSize of BW
NumObjectsNumber of connected components (objects) in BW
PixelIdxList1-by-NumObjects cell array where the k-th element in the cell array is a vector containing the linear indices of the pixels in the k-th object.

## 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).

• To compute a label matrix having more memory-efficient data type (for instance, uint8 versus double), use the labelmatrix function on the output of bwconncomp. See the documentation for each function for more information.

## Algorithms

The basic steps in finding the connected components are:

1. Search for the next unlabeled pixel, p.

2. Use a flood-fill algorithm to label all the pixels in the connected component containing p.

3. Repeat steps 1 and 2 until all the pixels are labeled.