bwconncomp

Find and count connected components in binary image

Syntax

CC = bwconncomp(BW)

CC = bwconncomp(BW,conn)

Description

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.

CC = bwconncomp(BW,conn) specifies the desired connectivity conn 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 — Binary image

numeric array | logical array

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

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 array 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	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.	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.	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	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	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	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

CC — Connected components

structure

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 connected component.

Tips

• This function sorts the connected components from left to right based on the top-left extremum of each component. When multiple components have the same horizontal position, the function then sorts those components from top to bottom, and again along any higher dimensions. This figure illustrates the extrema of two different regions.

  

• The functions bwlabel, bwlabeln, and bwconncomp all compute connected components for binary images. bwconncomp 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, 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

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.

• The conn argument must be a compile-time constant.

GPU Code Generation

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

Usage notes and limitations:

• bwconncomp only supports 2-D inputs.

• The conn argument 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 structure return value is not supported.

Thread-Based Environment

Run code in the background using MATLAB® backgroundPool or accelerate code with Parallel Computing Toolbox™ ThreadPool.

This function fully supports thread-based environments. For more information, see Run MATLAB Functions in Thread-Based Environment.

Version History

Introduced in R2009a

R2023a: Additional support for C code generation

You can now generate C code for input images of any dimension. Before, bwconncomp supported code generation only for 2-D inputs. Also, you can now specify any connectivity that is a compile-time constant. Before, bwconncomp supported code generation only for connectivities of 4, 8, [0 1 0;1 1 1;0 1 0], or ones(3).

The function can now generate code that returns the PixelIdxList field in the CC structure.

R2022b: Support for thread-based environments

bwconncomp now supports thread-based environments.