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bwconncomp

Find and count connected components in binary image

Description

example

CC = bwconncomp(BW) finds and counts the connected components CC in the binary image BW. The CC output structure contains the total number of connected components, such as regions of interest (ROIs), in the image and the pixel indices assigned to each component. bwconncomp uses a default connectivity of 8 for two dimensions and 26 for three dimensions.

example

CC = bwconncomp(BW,conn) specifies the desired connectivity conn 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.

BW = imread('text.png');
imshow(BW)

Figure contains an axes object. The axes object contains an object of type image.

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)

Figure contains an axes object. The axes object contains an object of type image.

Input Arguments

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Binary image, specified as a numeric or logical array of any dimension. For numeric input, any nonzero pixels are considered to be 1 (true).

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.

Center pixel connected to four pixels

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.

Center pixel connected to eight pixels

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

Center pixel connected to the faces of 6 pixels

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

Center pixel connected to the faces of 6 pixels and the edges of 12 pixels

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

Center pixel connected to the faces of 6 pixels, the edges of 12 pixels, and the corners of 8 pixels

Current pixel is center of cube.

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.

Extended Capabilities

Version History

Introduced in R2009a

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