Image Processing Toolbox™

bwareaopen - Morphologically open binary image (remove small objects)

Syntax

BW2 = bwareaopen(BW,P) BW2 = bwareaopen(BW,P,conn)

Description

BW2 = bwareaopen(BW,P) removes from a binary image all connected components (objects) that have fewer than P pixels, producing another binary image, BW2. The default connectivity is 8 for two dimensions, 26 for three dimensions, and conndef(ndims(BW),'maximal') for higher dimensions.

BW2 = bwareaopen(BW,P,conn) specifies the desired connectivity. conn can have any of the following scalar values.

Value	Meaning
Two-dimensional connectivities
4	4-connected neighborhood
8	8-connected neighborhood
Three-dimensional connectivities
6	6-connected neighborhood
18	18-connected neighborhood
26	26-connected neighborhood

Connectivity can be defined in a more general way for any dimension by using for conn a 3-by-3-by-...-by-3 matrix of 0's and 1's. The 1-valued elements define neighborhood locations relative to the center element of conn. Note that conn must be symmetric about its center element.

Class Support

BW can be a logical or numeric array of any dimension, and it must be nonsparse. The return value BW2 is of class logical.

Algorithm

The basic steps are

Determine the connected components.
```
L = bwlabeln(BW, conn);
```
Compute the area of each component.
```
S = regionprops(L, 'Area');
```
Remove small objects.
```
bw2 = ismember(L, find([S.Area] >= P));
```

Examples

Read in the image and display it.

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

Remove all objects smaller than 50 pixels. Note the missing letters.

bwAreaOpenBW = bwareaopen(originalBW,50);
figure, imshow(bwAreaOpenBW)