Image Processing Toolbox

bwareaopen

Binary area open; 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 may 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 may 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:

1. Determine the connected components.

• L = bwlabeln(BW, CONN);
  

2. Compute the area of each component.

• S = regionprops(L, 'Area');
  

3. Remove small objects.

• bw2 = ismember(L, find([S.Area] >= P));
  

Example

Remove all objects containing fewer than 40 pixels in an image.

1. Read in the image and display it.

• bw = imread('text.tif');
  imshow(bw)
  
  
  

2. Remove all objects smaller than 40 pixels. Note the missing letters.

• bw2 = bwareaopen(bw,40);
  figure, imshow(bw2)
  
  
  

See Also

bwlabel, bwlabeln, conndef, regionprops.

bwarea

bwdist