bwconncomp
Find connected components in binary image
Description
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:
|
|
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
|
|
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
|
|
For higher dimensions, bwconncomp uses the default
value conndef(ndims(BW),'maximal')
Data Types: double | logical
Output Arguments
Tips
The functions
bwlabel,bwlabeln, andbwconncompall compute connected components for binary images.bwconncompreplaces the use ofbwlabelandbwlabeln. It uses significantly less memory and is sometimes faster than the other functions.To extract features from a binary image using
regionpropswith default connectivity, just passBWdirectly intoregionpropsusing the commandregionprops(BW).To compute a label matrix having more memory-efficient data type (for instance,
uint8versusdouble), use thelabelmatrixfunction on the output ofbwconncomp. See the documentation for each function for more information.
Algorithms
The basic steps in finding the connected components are:
Search for the next unlabeled pixel,
p.Use a flood-fill algorithm to label all the pixels in the connected component containing
p.Repeat steps 1 and 2 until all the pixels are labeled.