bwconncomp

Find connected components in binary image

Syntax

CC = bwconncomp(BW)

CC = bwconncomp(BW,conn)

Description

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.

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

Examples

Calculate Centroids of 3-D Objects

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

Erase Largest Component from Image

Read image into the workspace and display it.

BW = imread('text.png');
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

BW — Input binary image

numeric array | logical array

Input 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

conn — Pixel connectivity

4 | 8 | 6 | 18 | 26 | 3-by-3-by- ... -by-3 matrix of 0s and 1s

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

CC — Connected components

struct

Connected components, returned as a structure with four fields.

Field	Description
Connectivity	Connectivity of the connected components (objects)
ImageSize	Size of BW
NumObjects	Number of connected components (objects) in BW
PixelIdxList	1-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.

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

• 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.

Extended Capabilities

C/C++ Code Generation

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

Usage notes and limitations:

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

• bwconncomp only supports 2-D inputs.

• The conn arguments must be a compile-time constant and the only connectivities supported are 4 or 8. You can also specify connectivity as a 3-by-3 matrix, but it can only be [0 1 0;1 1 1;0 1 0] or ones(3)

• The PixelIdxList field in the CC struct return value is not supported.

GPU Code Generation

Generate CUDA® code for NVIDIA® GPUs using GPU Coder™.

Usage notes and limitations:

• bwconncomp only supports 2-D inputs.

• The conn arguments must be a compile-time constant and the only connectivities supported are 4 or 8. You can also specify connectivity as a 3-by-3 matrix, but it can only be [0 1 0;1 1 1;0 1 0] or ones(3)

• The PixelIdxList field in the CC struct return value is not supported.